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1

APPLICATION NOTE

Storing Power with Super Capacitors

Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202377A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 24, 2012

Storing Power with Super CapacitorsPortable system designers have sharply reduced the power requirements of their systems in active and standby mode over the last several years, a growing number of applications pose an entirely different problem. Some of the most attractive features in wireless and portable devices today demand high levels of current for very short durations. Often, this peak current exceeds the capabilities of the system power source. Ironically, as advances in power management have allowed designers to reduce the footprint and cost of their systems by moving to smaller, lower cost Lithium-Ion batteries, they are finding users increasingly demanding applications that require higher levels of peak current than these new power sources can support.

This problem has emerged as designers have increasingly deployed higher current devices in portable systems despite their limited sourcing current. For example, a growing variety of wireless data cards for applications such as WCDMA-HSPA and GSM/EDGE networks, GPRS and WiMAX data communication use TDMA and CDMA techniques which require a peak current during the transmission of signals which can exceed the maximum current specified in the PC card, CF Card and/or USB standards. Similarly, as designers have increased the resolution of camera phones to 3Mpixel and beyond, they have also increased the amount of light required to achieve a high quality image. To reach these high levels, portable systems must drive flash LEDs at currents as high as 4A. Other applications such as GPS readings, music and video also exceed source current availability.

One way to solve this problem is to use a capacitor to store the current and deliver it quickly without draining the main battery. However, conventional capacitor capability would require either a very large case size or multiple devices con-nected in parallel. A more practical solution for space-constrained portable systems is to use very high value or so-called “super” capacitors. These devices offer high levels of capacitance in a relatively small case size. Working with the bat-tery and a DC/DC converter, a super capacitor discharges its power during peak loads and recharges between peaks, providing the power needed to operate systems from battery operated hosts.

By using a super capacitor (SC), designers can deliver the high current levels needed for these short duration events and then recharge from the battery between events. By reducing the current drawn from the battery, this architecture offers users improved talk time and longer battery life. It also allows the designer to reduce the system footprint by optimally sizing the battery and power circuitry to cover just the average power consumption instead of peak levels.

The challenge for designers is determining how to most efficiently interconnect the battery, DC/DC converter and SC in a way that will limit the super capacitor charge current and continually recharge the capacitor between load events. This app note will identify some of the challenges of storing power with super capacitors, identify some of the issues designers must overcome, explore two techniques for delivering high currents without overloading the host supply, and then illustrate potential solutions using implementations of sample applications with test results from a real system.

Defining a Super CapacitorWhat is a super capacitor? Like any capacitor, a super capacitor is basically two parallel conducting plates separated by an insulating material known as a dielectric. The value of the capacitor is directly proportional to the area of the plates and inversely proportional to the thickness of the dielectric. Manufacturers building “super” capacitors achieve higher levels of capacitance while minimizing size by using a porous carbon material for the plates to maximize the surface area and a molecularly thin electrolyte as the dielectric to minimize the distance between the plates. Using this approach they can manufacture capacitors with values from 16mF up to 2.3F. The construction of these devices results in a very low internal resistance allowing them to deliver high peak current pulses with minimal droop in the output voltage.

2

APPLICATION NOTE



Manufacturer Part Number Capacitance (F) Voltage Rating (V)ESR

(mW)Size (mm)

LxWxHTDK EDLC262020-501-2F50 0.5 5.6 50 23x44x1.5

CAP-XX HA230 0.425 5.5 110 20x18x1.5

Benefits ChallengesHigh-Capacity Capacitor Farads rather than micro-Farads Values ranging from 30mF to 2F

Need to control inrush charge current due to low ESR

Recharge in seconds with >500k Cycles Stores energy in an electrostatic field, not a chemical state

Need to recharge Voltage drop/droop is below operational limit of system requirement

Small ESR, low impedance 30 – 185mΩ

Need to disconnect SC from source Short circuit protection Source over-voltage protection Current flow protection

Extends battery life by five times ‘Averages out' high power demandsManufactured in any size and shape Flat and small sizes Maximum voltage, temperature range, ESR

2.75/cell to 5.5V, 70°C to 85°C, 30 to 185mΩAllows smaller, lighter and cheaper batteries. Long life: 10 to 12 yrs

Need cell balancing resistors required for stacked SC for higher oper-ating voltageOpen-circuit (high ESR) failure mode

Carbon, aluminum, organic electrolyte

Figure 1: Example Portable Super Capacitors; Size Continues to Fall as Voltage and Values Rise.

There are both benefits and challenges to overcome when designing with super capacitors.

3

APPLICATION NOTE



Super Capacitor BenefitsConventional capacitor technology requires either a very large case size or multiple devices connected in parallel to achieve high capacitance values. Super capacitors can be manufactured in a smaller size for a given capacitance and shape. A super capacitor can be used to store the required current and deliver it quickly without draining the main battery. Working together with the battery, the super capacitor discharges its power during peak loads and recharges between peaks, providing the power needed to operate systems from battery-operated hosts up to 200% longer while extending the life of the battery. They can be used to extend battery life by five times by ‘averaging out' high power demands and therefore allow designers to use smaller, lighter and cheaper batteries. Super capacitors also offer an operating life as long as 10 to 12 years and their failure mode is an open-circuit (high ESR), rather than a battery’s destructive event. Similarly, if over-voltage is applied to the device, the only consequence will be a slight swelling and a rise in ESR, eventually progressing to an open circuit. Super capacitors recharge in seconds with >500k cycles and store energy in an electrostatic field rather than a chemical state like a battery. Their small ESR and low impedance also helps ensure that the voltage does not droop excessively until heavy load currents when fully charged.

Super Capacitor ChallengesThe challenge for designers is how to efficiently interconnect the battery, DC/DC converter and super capacitor in a way that will limit the super capacitor inrush charge current and continually recharge the capacitor between load events. The low ESR presents designers with an inherent problem during the initial charge cycle. In any system the capacitor is initially discharged; when the supply voltage is then applied, the super capacitor looks like a low value resistor. This results in a huge inrush current if the current is not controlled or limited. Therefore, designers must imple-ment some sort of inrush current limit to ensure the battery does not shut down. In addition, the super capacitor must be recharged when the voltage drops/droops below the operational limit of the load. When the super capacitor is fully charged, it must then be disconnected from the source. Typically any circuit of this type also requires short-circuit, over-voltage and current flow protection.

One simple strategy is to use a series resistor. In a typical PC card circuit the maximum current that can be drawn prior to successful host/card negotiation is 70mA. If we assume that the PC card controller needs half that current to perform the negotiation, then at power-up the super capacitor must be either disconnected from the supply or current-limited using an approximately 100Ω resistor (R = V/I). Given those factors, the capacitor will be fully charged in approxi-mately 6.7 minutes (assuming the capacitor is fully charged in approximately 5 time constants).

A more practical approach would allow the PC card to source more power after the successful negotiation between the host and the card. A lower value resistor can then be used to increase the charging current. As the capacitor starts to charge and the voltage starts to rise, the power dissipation declines and the resistor value can be decreased.

Figure 2 depicts a sample circuit comprised of a series of decreasing resistor values which are switched during the capacitor charging cycle. This architecture requires that the timing of the switching points be closely controlled, which demands very accurate and expensive resistors, or monitored by several additional voltage detectors. Furthermore, when the capacitor is fully charged and the PC card is removed, the energy stored in the capacitor would be sufficient to dam-age the connector pin. Such a scheme is inexpensive, but a control to switch in different accurate R values requires tim-ing and monitoring. There is also a loss across the resistor and a very long charge time if a single R is used.

4

APPLICATION NOTE



SuperCap

LOAD123

VCC From PC Card

GND From PC Card

Sequence:1 - On card plug in2 – Intermediate Limit3 – Final Limit

DC-DC

Figure 2: Simple Charging Scheme.

A third approach is to use a current-limiting SmartSwitch™ to charge the super capacitor. This type of device uses an independent integrated P- or N-channel MOSFET as a load switch and integrates additional monitoring and protection circuitry to limit the amount of output current. Products of this type feature thermal overload protection to ensure that the device turns off if the chip temperature exceeds its rated maximum while in current limit. As the chip cools down, the device will turn back on and thermally oscillate at a low frequency until the period of high dissipation ends. These devices typically feature an independent current limit to avoid violating standards or draining the battery. They also feature reverse blocking to keep the super capacitor charge from going back to the source and to protect the source during a short circuit. They offer a power loop to control the charge rate and low RDS(ON) to eliminate thermal foldback or droop in the output voltage. Moreover, these devices only require 1.4V for the enable control pin to set startup and full power current.

A current-limited SmartSwitch™ adds all the circuitry needed to limit current, protect the PC card connector, continu-ously charge the capacitor, notify the system when it is ready for use, and determine when to start recharging the capacitor.

5

APPLICATION NOTE



Reverse Blocking

Over-Temp Protection

GND

EN IL

EN IU

ISETU

VCC OUT

ISETL

RDYUnder-Voltage Lockout

Current Limit

Control1.2V

Reference

RSETU RSETL

SuperCap

To LoadVCC

100kΩ

To μμCPowerLoop

RHYS

Load

Figure 3: Current Limiting SmartSwitch™ with Independent Current Limits from 75mA to 1200mA and Soft-Start Used to Overcome Initial Charging of the Super Capacitor.

A power loop (thermally activated current reduction) minimizes charge time while controlling power dissipation with over-temperature and short circuit protection. Reverse current blocking prevents discharge of the super capacitor back to the power supply and 50-65mΩ typical RDS(ON) charges the capacitor voltage close to the input supply. A system READY output alerts the system that the super capacitor is charged and ready with programmable hysteresis for adjustable recharge.

In a typical super capacitor application, the SmartSwitch™ turns on and immediately limits current due to the high in-rush current. This large in-rush current drives up temperature and drives the device into thermal shutdown where it thermally oscillates. All SmartSwitch™ devices are designed to operate in this manner. However, during the time the switch is off, the capacitor is not charging, and therefore increasing the time to full charge. Moreover, there is no way to detect when the capacitor is fully charged and ready for transmission without additional circuitry.

Devices such as Skyworks SmartSwitch™ feature two different current limits for host/card negotiation. On power up the device provides a low resistance path between the supply and the super capacitor. If thermal dissipation is low, the device eventually enters current limit and the capacitor continues to charge until it reaches approximately 98% of its final value. At that point a system ready signal changes state alerting the system that transmission can begin. If ther-mal dissipation is high, the chip temperature will rise rapidly. When it reaches an internally programmed limit, the device initiates an integrated digital power loop which reduces the current to a safe value. The power loop regulates the die’s temperature to approximately 100°C by sensing the die temperature at regular intervals and increasing or decreasing the current by 1/32 of the current limit set point. This function protects the device while minimizing charge time by ensuring that the super capacitor is charging at all times. Typically designers size the super capacitor to minimize the voltage droop during transmission and allow recharging during the receive phase. The SmartSwitch™ adds adjustable hysteresis for a Ready (RDYB) signal. The RHYS resistor sets the hysteresis in which the RDYB signal is turned back off. The RDYB signal always turns on when VOUT = 0.98 · VIN. The SmartSwitch™ stops charging the super-cap when VOUT > (VIN - 18mV). It then turns back on when VOUT < (VIN - 18mV-hysteresis).

6

APPLICATION NOTE



GSM/GPRS Application ExampleA GSM/GPRS wireless data card design in a current limited environment is used to demonstrate the advantages of this approach. GPRS type data cards typically must supply relatively large amounts of current to the power amplifier (PA) to send data for short durations. However, successive data transmissions can extend the time needed to supply the PA. The reason that the super capacitor is useful is because the data card is plugged into either a limited supply current PC or CF card or USB slot on a laptop. The supply currents for three different portable standards are listed in Table 1.

GPRS (General Packet Radio Service) is defined in classes as shown in Table 2. The first column is the GPRS multislot class, which is a list of speeds. The second column is the downlink timeslots receiving data from the network. The third column is the uplink timeslots to transmit data to the network. The most common GPRS multislot classes are Class 8, which has 33% faster data download than Classes 4 and 6, and Class 10 which has better data uploading than Class 8 and is used in cell phones and PC/Express cards. Class 12 has maximum upload performance for high-end PC cards.

In the example shown in Figures 4, 5, and 6, when the GPRS card uses one slot @ +33 dBm, the current draw from the super capacitor is 1.1A for 577µs/4.615ms or a 12.5% duty cycle. PA power consumption is equivalent to +33 dBm (50% efficiency) or 2W/50% or 4W. Current during transmit was 4W/3.75V or 1.1A. Average current was 1.1A · 12.5% of the duty cycle or 138mA. A second 2-slot example drew 1.1A from the super capacitor for 1.154 ms every 4.615ms. At 2 slots per frame this represented 25% of the duty cycle. PA power consumption and current during transmit was the same but average current was 1.1A · 25% or 275mA. The actual measured PA current was approximately 2A dur-ing the test.

PC Card

Voltage Current Level 0 (max) Current Level 1 (max)3.3V ± 10% 70mA 1000mA5.0V ± 10% 100mA 1000mA

CF Card

Voltage Current Level 0 (max) Current Level 1 (max)3.3V ± 5% 75mA 500mA5.0V ± 10% 100mA 500mA

USB Port

Voltage Current Level 0 (max) Current Level 1 (max)5.0V ± 10% 100mA 500mA

Table 1: Maximum Supply Currents For Laptop Standard Ports.

Multislot Class Downlink Slots Uplink Slots2 2 1 4 3 16 3 28 4 110 4 212 4 432 5 3

Table 2: GPRS Multislot Classes.

7

APPLICATION NOTE



CLASS 10GPRS Transmit 2-Slot Ch975, 33dBm

-3

-2

-1

0

1

2

3

4

5

Time (1ms/div)

Inpu

t Vol

tage

(V) T

op

Out

put V

olta

ge (V

) Mid

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Ou

tput Current (A) B

ot

VIN = 3.83V

IOUT

VOUT = 3.76V

CLASS 8GPRS Transmit 1-Slot Ch975, 33dBm

-3

-2

-1

0

1

2

3

4

5

Time (1ms/div)

Inpu

t Vol

tage

(V) T

op

Out

put V

olta

ge (V

) Mid

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Output Current (A) Bot

VIN = 3.83V

IOUT

VOUT = 3.80V

GPRS Transmit = PA drive currentClass 8 (4R1T) = 4 receive slots and 1 transmit timeslotClass 10 (3R2T) = 3 receive slots and 2 transmit timeslots

GSM FRAME = 8 TIMESLOTS577µs per Slot/ 4.615mS Total

Transmit 1-Slot

Receive 4-Slots

Transmit 2-Slots

Receive 4-Slots

Figure 4: GSM, GPRS Data Communications Slots and Transmit/Receive Functions and Test Results for Class 8 and Class 10.

Startup waveform no load 800mA current limit

CH 4 -VCC

CH 3 -VOUT CH4: VCC, 1V/div

CH3: VOUT, 1V/ divCH2: IIN, 5 00mA/divCH1: /RDY, 1V/div T IME: 500 ms/div

CH 1 -/RDY

CH 2 -IIN

Startup waveform no load 96mA current limit

CH4 -VCC

CH3 -VOUT

CH1 -/RDY

CH2 -IIN

CH4: VCC, 1V/div CH3: VOUT, 1V/divCH2: IIN, 500mA/divCH1: /RDY, 1V/div TIME: 5s/div

VCC

/RDY

VOUT tr=2.5ms

IIN = 800mA

VCC

/RDY

VOUT tr=20s

IIN = 100mA

Current Limiting Smart Switch

UltraCap

LX

EN/SET

PGND

OUTL=4.7uH

AGND

Cout

Control

FB

RSET

VCC

CT

Power Amp

Battery Input: 3.3V

Boost DC/DC

3.7V System Power

ENL

ENH/RDY

ISETU ISETL

Figure 5: 3.3V Express Card with VOUT = 3.7V, Super Capacitor = Cap-XX GW201G, 0.3F, 85mΩ.

Important features such as charge ramp of the super capacitor, RDS(ON) IR drop, the duration of the current pulse test results. Inrush current for the super capacitor can be limited to under 100mA or up to 800mA.

8

APPLICATION NOTE



VOUT 3.7V VOUT 174mV Drop

IIN 500mA

IUCOUT 2A

VOUT 3.7V VOUT 1.7V Droop

IIN 1A

VIN 3.3V

Wireless GSM Card w/o Ultra Cap Wireless GSM Card with Ultra Cap

Figure 6: Wireless Data Card Application.

The initial IR voltage drop caused by the super capacitor ESR plus the capacitor voltage drop which is VDROP = I x ESR+I x (tms/CmF) = 2A x 85mW + (2 x (1.154ms/300mF) = 174mV.

In this demonstration, the battery supplies input to the boost converter which supplies a constant system supply of 3.7V. The super capacitor supplies approximately 1.5A of peak current during GPRS 2-slot data transmission and the SmartSwitch™ recharges the super capacitor during receive function. The two waveforms in Figure 6 compare a GSM PC card without a super capacitor load (left) with a GSM PC card with a super capacitor load (right). Together they illustrate how designs using a SmartSwitch™ to charge the super capacitor can eliminate droop during transmit. Super capacitors can operate high current devices such as rf PAs during transmit for load leveling, and energy storage between transmits. Super capacitors are getting smaller with improved specs such as temperature range (85°C) and maximum voltage limits (5.5V). A current limiting SmartSwitch™ alleviates the issues with super capacitor inrush cur-rent and provides short-circuit protection, source OV protection, and current flow protection to meet portable standards and run time requirements without taxing the battery.

Flash LED Application ExampleIn a second example, a current limiting SmartSwitch™ with a boost DC/DC LED driver and super capacitor is used in a high current flash LED application. The SmartSwitch™ is integrated with the boost DC/DC LED driver as shown. Many of today’s camera phones now offer resolution of 3MP (megapixel) and above, as shown in Figure 7. These higher levels of image resolution require proportionally higher levels of light to achieve a high quality image. Image sensors need more light for high MP photos and first generation camera phone flash offer limited light intensity. Second generation camera phone flash is still not suitable for >3MP cameras and they also require video capture which needs a movie/torch mode.

9

APPLICATION NOTE



Source: 2005-2010 Strategies Analytics

Figure 7: The Camera Phone Market Continues to Grow.

To achieve these light levels, flash LEDs must be driven at currents of between 1A and 2A, as shown in Figure 8. Complicating the problem, the forward voltage across the LED as these high currents can range up to 4.8V. If we include 200 mV of overhead for the current control circuitry, the total load voltage during a flash event can range up to 5V.

Measured PWM-4 IF vs VF

0

500

1000

1500

2000

2500

3000

3 3.5 4 4.5 5 5.5 6

Forward Voltage (Volts)

Forw

ard

Cur

rent

(m

A)

VF PeakVF Cont.Req. HRSCap Voltage

Figure 8: High Intensity Flash LEDs Require Large Forward Current (IF) and High Forward Voltage (VF).

Since the forward voltage of the flash LED is proportional to IF, flash LEDs require >1A. To achieve 2A, the VF must be >4.8V.

10

APPLICATION NOTE



Current solutions consist of a WLED flash which is charge-pump based and limited to up to 800mA, or an inductive based step-up converter solution which is limited to 1-1.5A due to high input battery current. If the battery voltage is 3.5V, and the boost converter is 90% efficient, the battery would need to supply over 3A for the duration of a 2A flash pulse.

If VBAT = 3.3V and IF = 1.5A @ VF = 4.4V, then:

IBAT = VF · IF

Eff · VBAT

=

= 2.35A

4.4 · 1.50.85 · 3.3

This current can cause a number of issues. Peak current can create a battery voltage drop, causing the camera or phone to reset. If the current exceeds the maximum allowed battery current, the battery disconnects for safety; this will either cause the battery protection circuit to shut the battery down or cause a low voltage shutdown with plenty of energy still remaining in the battery or the battery is damaged.

Another available solution is Xenon flash, but it requires high voltage, 300V with ignition at several kVs. It is bulky and needs a reflector and high voltage capacitor. Also, Xenon flash cannot operate in torch mode and is very fragile, as shown in Figure 9.

Figure 9: Xenon vs. Super Capacitor Flash.

The super capacitor can provide LED flash better than Xenon flash in a solution which is a fraction of the size, and provide torch mode with true DSC performance.

11

APPLICATION NOTE



A super capacitor-powered LED flash unit can drive high-current LEDs to provide light intensity many times greater than standard battery-powered LED flash units or longer than Xenon strobe solutions. The super capacitor solution shown in Figure 10 contains a step-up converter used to boost the 3.2V-to-4.2V battery input voltage up to a constant 5.5V and a SmartSwitch™ to charge a super capacitor. The super capacitor holds the charge for the flash LEDs until needed by the user in either flash or movie mode. The solution controls and regulates the current from a cell phone’s battery source, steps up the battery voltage, and manages the charging of a super cap for the control and supply of high-current to flash LEDs in the end application. This solution also contains flash management capabilities such as movie-mode and red eye reduction.

AAT128x

EN

SuperCap

SDA

SCL

Dig

ital i

nter

face

FLEN

OUT

FLA

FLB

IN

SW

Step-up/BoostConverter withSmart Switch

3.2-4.2V

Figure 10: Flash LED Operation with a Super Capacitor.

1. After enable (EN), step-up converter charges the super capacitor to 5.5V in seconds (Red).2. The step-up converter automatically changes to light load mode. 3. During the flash (FLEN), the boost engine is shut down and the two LEDs connected to current channel

(FLA or FLB) share the output current supplied by the super capacitor equally (Blue).

To better achieve this, the step-up converter features built-in circuitry that prevents excessive inrush current during start-up as well as a fixed input current limiter of 800mA or 500mA for USB and true load disconnect after the super capacitor is charged. The output voltage is limited by internal overvoltage protection circuitry, which prevents damage to the controller and super capacitor from open LED (open circuit) conditions. During an open circuit, the output volt-age rises and reaches 5.5V (typical), and the OVP circuit disables the switching, preventing the output voltage from rising higher. Once the open circuit condition is removed, switching will resume. At this point the controller will return to normal operation and maintain an average output voltage. An industry-standard I2C serial digital input is used to enable and disable LEDs, and set the movie-mode current with up to 16 movie-mode settings for lower light output.

The detailed schematic in Figure 11 illustrates the minimal components needed to implement this flash lighting subsys-tem. A 0.55F, 85mΩ super capacitor delivers 9W LED power-bursts using the AAT1282 high power 2A flash LED driver which has the super capacitor charger integrated with the boost DC/DC LED driver. To achieve high light levels, the flash LEDs are driven at currents of between 1A and 2A. The forward voltage (VF) across the LED at these high currents can range up to 4.8V. If we include 200 mV of overhead for the current control circuitry, it’s easy to see how the total load voltage during a flash event can range up to 5V and require a 5.5V step-up voltage. To achieve these levels of power, designers must use a boost converter. If the battery voltage is 3.5V and the boost converter is 90% efficient, the battery must supply over 3A for the duration of a 2A flash pulse. Typically this requirement will result in the battery protection circuit shutting the battery down or at the very least create a low voltage shutdown with significant amounts of energy left in the battery. This solution combines high-frequency boost converter with fixed input current limiting smart switch and high-capacity super capacitor. Dual output regulated current sinks and I2C control supplies high intensity light

12

APPLICATION NOTE



required for mobile phones using cameras with resolutions of > 3 MP and limits peak current consumed by the battery. Future integrated solutions include the SmartSwitch™ with PMUs to supply portable solid state disk drives.

Smart Switch plus boost converter – super capacitor provides the flash current

SuperCapacitor

2 x WLED FlashWith Lens

Smart Switch integrated with the boost converter Component

Solution

Figure 11: Flash LED Application.

The super capacitor is mounted on the back side of the PCB.

13

APPLICATION NOTE



Figure 12 shows test results using two flash LEDs at 1A each and 1 LED at 2A. As can be seen, the super capacitor can easily supply the necessary current for 100ms while holding up the supply voltage sufficiently above the VF of the LED. Between flash events, the super capacitor is recharged at a slower rate to be ready for the next picture. The time to charge the super capacitor between flashes is set externally and can be optimized for different battery sizes/chemistries.

• Vin = 3.62V (875mA/hr Li-Ion Polymer battery cell)• Flash Time-out set for 120 to 130ms• LEDs = Lumiled PWM-4 • Super Capacitor = Cap-XX HS206F - 055F – 85mΩ ESR

LED1 Current

LED2 Current

Super Cap Voltage

LED1 Current

LED2 Current

Super Cap Voltage

LED1 Current

IFL1(500mA/div)

VOUT(500mV/div)

IFL2(500mA/div)

Super Cap = Vout= 5.5V

130ms

120msIFLX(500mA/div)

VOUT(500mV/div)

Figure 12: Flash LED Application Test Results for High Current Flash LED Application (Two LEDs Powered at 1A Each and One LED Powered at 2A).

Since the super capacitor is the only source for the LED flash current, the duration of the flash is determined by the energy stored in the super capacitor. During flash, the energy of the super capacitor is discharged; at the same time, the voltage of the super capacitor is decreased. Once the super capacitor voltage is lowered to a level (the minimum sink pin voltage + the LED forward voltage), the flash is ended. With a fully charged super capacitor in place, the flash for two 1A LEDs can last for more than 500ms.

To determine the super capacitor size for the flash LED application, the total voltage drop has to be considered. The initial IR voltage drop caused by the super capacitor ESR plus the voltage drop due to time that the LEDs are conduct-ing current.

VDROP = I · ESR + I · t (ms)C (mF)

• VOUT drop to the LED VF Limits Flash time• IR drop due to SC ESR = VOUT - VMINSTEP = (5.5 - 5.4) = 100mV• ESR = 100mV/2A = 50mΩ• Voltage drop on SC = (VMINSTEP - VMINDROP) = (5.4 - 4.5) = 900mV• SCMIN = 2A(150ms)/900mv = 0.33F

For example using a super capacitor with an ESR of 50mΩ and 2A of LED forward current:

VDROP = 2A · 50mΩ + I · = 1V150ms333mF

14

APPLICATION NOTE



The maximum flash current in each FLOUTA and FLOUTB is set by the RSET resistor and can be calculated using the fol-lowing equation:

IFLOUTA = IFLOUTB = = = ~1000mA per channel81kΩ · A

RSET

81kΩ · A80.6kΩ

To prevent excessive power dissipation during higher flash current operation, RSET values smaller than 80.6kΩ are not recommended. An example showing flash time longer than 500ms where the super capacitor voltage droops below the VF required by the flash LEDs is shown in Figures 13 and 14.

IFLx(500mA/div)

ILX(500mA/div)

VOUT(2V/div)

VIN = 3.6V

Super Cap = VOUT = 5.5V

LED Current starts to go out of regulation500ms

LinearMode

Boost Mode

Figure 13: 1A/LED Flash Operation Test Results – Long Duration Flash.

IFLOUTB(500mA/div)

VSW(10V/div)

IFLOUTA(500mA/div)

90ms 90ms230ms

Figure 14: 1A Double Flash Operation Showing Recharge of the Super Capacitor and Turn-On/Off of the Boost Converter Between Flashes as it is Disconnected from the Battery During the Flash Event.

15

APPLICATION NOTE



Movie-ModeMovie or torch (flashlight) mode is when the LEDs are turned on at a lower light level to record a video or movie. The maximum movie-mode current level is set by the maximum, programmed flash current reduced by the programmed flash-to-movie-mode ratio in which the default value is 7.3:

IMOVIE-MODE(A/B) = = = 137mAIFLOUT(A/B)(MAX)

7.31000mA

7.3

To change the configuration or the settings, the AAT1282 can be programmed via the I2C interface. An example of movie mode between flashes is shown in Figure 15. Only LEDs can provide movie mode; a xenon tube cannot be used in this mode.

LED CURRENT

Flash times out

ENABLE

FLASH ENABLE

tCT

Turn on LEDs in Movie Mode

controlled by I2Cinterface

VOUT = SuperCap voltage

5.5V

I2C Control for Movie Mode Enable

Figure 15: Double Flash Operation (Super Capacitor Recharges Between Flashes).

This solution includes a timer circuit that enables the flash current for a programmed period of time. This feature eliminates the need for an external, housekeeping baseband controller to contain a safety delay routine. It also serves as a protection feature to minimize thermal issues with the flash LEDs in the event an external controller’s flash soft-ware routine experiences hang-up or freeze. The flash safety timeout, T can be calculated by the following equation:

T = 13.5s/μF • CT

Where T is in seconds and CT is the capacitance of the timer capacitor in μF.

For example, using a 74nF capacitor for CT sets the flash timeout to:

Flash Safety Timeout = 13.5s/μF • 0.074μF = 1s

16

APPLICATION NOTE



Mode 1 Mode 2 Mode 3Time Duration LED Current Time Duration LED Current Time Duration LED Current

2 sec 50mA 2 sec 50mA 2 sec 50mA140ms 1A 70ms 1.5A 40ms 2A100ms OFF 100ms OFF 100ms OFF140ms 1A 70ms 1.5A 40ms 2A

Figure 16: Flash Reference Design with 3 Modes of Operation Depending on the Light Intensity Required for the Flash.

Xenon Super Capacitor

Light Machine Xe filled quartz tube 15x5x3mm = 0.225cc Si LED 2x 1.6x0.7mm = 0.00448cc

Energy Storage Electrolytic capacitor 2x 70x18mm = 1.766cc eff Super capacitor 1x or 2x 17x28.5x1mm = 0.48 - 0.96cc

FPC Circuit 10x30x0.2mm = 0.06cc 10x30x0.2mm = 0.06cc

Control Chip 4x4x1 = 0.016cc 4x4x1 = 0.016cc

Transformer 5x5x4mm = 0.1cc n/a

Light Sensor 3x3x1mm = 0.009cc n/a

IGBT 1.5x1.5x0.8mm = 0.0018cc n/a

Total ~2.2cc ~0.56-1.04cc

Table 3: Flash Reference Design Size Comparison to Equivalent Xenon Flash Solution.

17

APPLICATION NOTE



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ConclusionUntil recently, super capacitors have been rarely used in portable systems. Typically they have been limited to back-up or standby functions that use relatively low currents and offer fairly long charge times. But a new generation of SmartSwitch™ devices which integrate all the functions needed to limit current, protect the battery or source supply, continuously charge the capacitor and notify the system when the capacitor is ready for use are changing that sce-nario. By combining these new smart switches with super capacitors, designers can now create compact solutions which supply high levels of current for short durations and, in the process, extend battery life or allow the use of smaller, lighter and less expensive power sources. Super capacitors can operate high current devices such as rf PAs during transmit for load-leveling and energy storage. The super capacitors are getting smaller with improved tem-perature and working voltage specifications up to 5.5V and wider temperature range (85°C). Battery life and system operation is improved to meet portable standards and run time requirements. Future integrated solutions include the SmartSwitch™ with PMUs to supply portable solid state disk drives that plug into laptop ports.

A current limiting smart switch alleviates the issues with super/super capacitors, inrush control, short circuit protection, source OV protection and cCurrent flow protection. Using a super capacitor makes it possible to drive very high LED currents for a super bright WLED flash with two flash LEDs driven at 2A to deliver more light than a K800i xenon strobe. The super capacitor flash solution is < 2mm thick and can enhance other features in a phone for longer talk time and better audio while limiting peak current consumed by the battery. A current limiting smart switch with boost converter eliminates limitations with super capacitors and flash LEDs.

ReferencesComparison of xenon flash and high current LEDs for photo flash in camera phones Use of super capacitors to improve performance of GPRS mobile stations

Pierre Mars CAP-XX Ltd.9/12 Mars RoadLane Cove NSW 2066 Australiahttp://www.cap-xx.com

storing power with super capacitors - · pdf filestoring power with super capacitors...

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