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LM2621Low Input Voltage, Step-Up DC-DC ConverterGeneral DescriptionThe LM2621 is a high efficiency, step-up DC-DC switchingregulator for battery-powered and low input voltage systems.It accepts an input voltage between 1.2V and 14V and con-verts it into a regulated output voltage. The output voltagecan be adjusted between 1.24V and 14V. It has an internal0.17 N-Channel MOSFET power switch. Efficiencies up to90% are achievable using the LM2621.The high switching frequency (adjustable up to 2MHz) of theLM2621 allows for tiny surface mount inductors and capaci-tors. Because of the unique constant-duty-cycle gated oscil-lator topology very high efficiencies are realized over a wide

load range. The supply current is reduced to 80A becauseof the BiCMOS process technology. In the shutdown mode,the supply current is less than 2.5A.The LM2621 is available in a Mini-SO-8 package. This pack-age uses half the board area of a standard 8-pin SO and hasa height of just 1.09 mm.

Featuresn Small Mini-SO8 Package (Half the Footprint of Standard

8-Pin SO Package)n 1.09 mm Package Heightn Up to 2 MHz Switching Frequencyn 1.2V to 14V Input Voltagen 1.24V - 14V Adjustable Output Voltagen Up to 1A Load Currentn 0.17 Internal MOSFETn Up to 90 % Regulator Efficiencyn 80 A Typical Operating Currentn

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Connection Diagram

Ordering Information

Order Number Package Type NSC Package

DrawingPackageMarking

Supplied As

LM2621MMX Mini SO-8 MUA08A S06A 3.5k Units on Tape and ReelLM2621MM Mini SO-8 MUA08A S06A 250 Units on Tape and Reel

Mini SO-8 (MM) Package

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Top View

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Absolute Maximum Ratings (Note 1)If Military/Aerospace specified devices are required,please contact the National Semiconductor Sales Office/Distributors for availability and specifications.

SW Pin Voltage 0.5 V to 14.5VBOOT, V DD , EN and FB Pins 0.5V to 10VFREQ Pin 100A

JA (Note 2) 240C/WTJmax (Note 2) 150CStorage Temperature Range 65C to +150 CLead Temp. (Soldering, 5 sec) 260C

Power Dissipation (T A=25C)(Note 2)

500mW

ESD Rating (Note 3) 2kV

Operating Conditions (Note 1)VDD Pin 2.5V to 5VFB, EN Pins 0 to V DDBOOT Pin 0 to 10VAmbient Temperature (T A) 40C to +85C

Electrical CharacteristicsLimits in standard typeface are for T J = 25C, and limits in boldface type apply over the full operating temperature range of40C to +85C. Unless otherwisespecified:V DD = VOUT = 3.3V.

Symbol Parameter Condition Typ Min Max UnitsVIN_ST Minimum Start-Up Supply

Voltage (Note 4)

ILOAD = 0mA 1.1 1.2 V

VIN_OP Minimum Operating SupplyVoltage (once started)

ILOAD = 0mA 0.65 V

VFB FB Pin Voltage 1.24 1.2028 1.2772 VVOUT_MAX Maximum Output Voltage 14 VVHYST Hysteresis Voltage (Note 7) 30 45 mV

Efficiency V IN = 3.6V; VOUT = 5V; ILOAD= 500mA

87%

VIN = 2.5V; VOUT = 3.3V; ILOAD= 200mA

87

D Switch Duty Cycle 70 60 80 %

IDD Operating Quiescent Current(Note 6)

FB Pin > 1.3V; EN Pin at V DD 80 110 A

ISD Shutdown Quiescent Current(Note 7)

VDD , BOOT and SW Pins at5.0V; EN Pin

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Pin Description

Pin Name Function1 PGND Power Ground

2 EN Active-Low Shutdown Input3 FREQ Frequency Adjust. An external resistor connected between this pin and Pin 6 (V DD ) sets the

switching frequency of the LM2621.4 FB Output Voltage Feedback5 SGND Signal Ground6 VDD Power Supply for Internal Circuitry7 BOOT Bootstrap Supply for the Gate Drive of Internal MOSFET Power Switch8 SW Drain of the Internal MOSFET Power Switch

Typical Performance Characteristics

Efficiency vs Load CurrentVOUT = 5.0V

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Efficiency vs Load CurrentVOUT = 3.3V

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VFB vs Temperature

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IOP vs Temperature

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ISD vs Temperature

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ISD vs V DD

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Typical Performance Characteristics (Continued)

Detailed Description

OPERATING PRINCIPLE

The LM2621 is designed to provide step-up DC-DC voltageregulation in battery-powered and low-input voltage sys-tems. It combines a step-up switching regulator, N-channelpower MOSFET, built-in current limit, thermal limit, and volt-age reference in a single 8-pin MSOP package . The switch-ing DC-DC regulator boosts an input voltage between 1.2Vand 14V to a regulated output voltage between 1.24V and14V. The LM2621 starts from a low 1.1V input and remainsoperational down to 0.65V.

This device is optimized for use in cellular phones and otherapplications requiring a small size, low profile, as well as lowquiescent current for maximum battery life during stand-byand shutdown. A high-efficiency gated-oscillator topology of-fers an output of up to 1A.Additional features include a built-in peak switch currentlimit, and thermal protection circuitry.

IOP vs V DD

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VIN_ST vs Load CurrentV

OUT= 3.3V

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Switching Frequency vs R FQ

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Peak Inductor Current vsLoad Current

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Maximum Load Current vsInput Voltage

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Detailed Description (Continued)

GATED OSCILLATOR CONTROL SCHEMEA unique gated oscillator control scheme enables theLM2621 to have an ultra-low quiescent current and providesa high efficiency over a wide load range. The switching fre-quency of the internal oscillator is programmable using anexternal resistor and can be set between 300 kHz and 2MHz.This control scheme uses a hysteresis window to regulatethe output voltage. When the output voltage is below the up-per threshold of the window, the LM2621 switches continu-ously with a fixed duty cycle of 70 % at the switching fre-quency selected by the user. During the first part of eachswitching cycle, the internal N-channel MOSFET switch isturned on. This causes the current to ramp up in the inductorand store energy. During the second part of each switchingcycle, the MOSFET is turned off. The voltage across the in-

ductor reverses and forces current through the diode to theoutput filter capacitor and the load. Thus when the LM2621switches continuously, the output voltage starts to ramp up.When the output voltage hits the upper threshold of the win-dow, the LM2621 stops switching completely. This causesthe output voltage to droop because the energy stored in theoutput capacitor is depleted by the load. When the outputvoltage hits the lower threshold of the hysteresis window, theLM2621 starts switching continuously again causing the out-

put voltage to ramp up towards the upper threshold. Figure 2 shows the switch voltage and output voltage waveforms.Because of this type of control scheme, the quiescent cur-rent is inherently very low. At light loads the gated oscillatorcontrol scheme offers a much higher efficiency compared tothe conventional PWM control scheme.

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FIGURE 1. Functional Diagram

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Detailed Description (Continued)

LOW VOLTAGE START-UPThe LM2621 can start-up from input voltages as low as 1.1V.On start-up, the control circuitry switches the N-channelMOSFET continuously at 70 % duty cycle until the outputvoltage reaches 2.5V. After this output voltage is reached,the normal step-up regulator feedback and gated oscillatorcontrol scheme take over. Once the device is in regulation itcan operate down to a 0.65V input, since the internal powerfor the IC can be boot-strapped from the output using theVDD pin.

SHUTDOWNThe LM2621 features a shutdown mode that reduces thequiescent current to less than a guaranteed 2.5A over tem-perature. This extends the life of the battery in battery pow-ered applications. During shutdown, all feedback and controlcircuitry is turned off. The regulators output voltage drops toone diode drop below the input voltage. Entry into the shut-down mode is controlled by the active-low logic input pin EN(Pin 2). When the logic input to this pin pulled below0.35V DD , the device goes into shutdown mode. The logic in-put to this pin should be above 0.7V DD for the device to workin normal step-up mode.

OUTPUT VOLTAGE RIPPLE FREQUENCYA major component of the output voltage ripple is due to thehysteresis used in the gated oscillator control scheme. Thefrequency of this voltage ripple is proportional to the loadcurrent. The frequency of this ripple does not necessitate theuse of larger inductors and capacitors however, since thesize of these components is determined by the switching fre-quency of the oscillator which can be set upto 2MHz usingan external resistor.

INTERNAL CURRENT LIMIT AND THERMALPROTECTIONAn internal cycle-by-cycle current limit serves as a protectionfeature. This is set high enough (2.85A typical, approxi-mately 4A maximum) so as not to come into effect duringnormal operating conditions. An internal thermal protection

circuitry disables the MOSFET power switch when the junc-tion temperature (T J ) exceeds about 160C. The switch isre-enabled when T J drops below approximately 135C.

Design Procedure

SETTING THE OUTPUT VOLTAGEThe output voltage of the step-up regulator can be set be-tween 1.24V and 14V by connecting a feedback resistive di-vider made of R F1 and R F2 . The resistor values are selectedas follows:

RF2 = RF1 /[(VOUT / 1.24) 1]A value of 150k is suggested for R F1 . Then, R F2 can be se-lected using the above equation.A 39pF capacitor (C F1 ) con-nected across R F1 helps in feeding back most of the ACripple at V OUT to the FB pin. This helps reduce the

peak-to-peak output voltage ripple as well as improve the ef-ficiency of the step-up regulator, because a set hysteresis of30mV at the FB pin is used for the gated oscillator controlscheme.

BOOTSTRAPPINGWhen the output voltage (V OUT ) is between 2.5V and 5.0V abootstrapped operation is suggested. This is achieved byconnecting the V DD pin (Pin 6) to V OUT . However if the V OUTis outside this range, the V DD pin should be connected to avoltage source whose range is between 2.5V and 5V. Thiscan be the input voltage (V IN ), VOUT stepped down using alinear regulator, or a different voltage source available in thesystem. This is referred to as non-bootstrapped operation.The maximum acceptable voltage at the BOOT pin (Pin 7) is10V.

SETTING THE SWITCHING FREQUENCY

The switching frequency of the oscillator is selected bychoosing an external resistor (R FQ ) connected betweenFREQ and V DD pins. See the graph titled Switching Fre-quency vs R FQ in the Typical Operating Characteristics sec-tion of the datasheet for choosing the R FQ value to achievethe desired switching frequency. A high switching frequency

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FIGURE 2. Typical Step-Up Regulator Waveforms

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Design Procedure (Continued)allows the use of very small surface mount inductors and ca-pacitors and results in a very small solution size. A switchingfrequency between 300kHz and 2MHz is recommended.

INDUCTOR SELECTIONThe LM2621s high switching frequency enables the use of asmall surface mount inductor. A 6.8H shielded inductor issuggested. The inductor should have a saturation currentrating higher than the peak current it will experience duringcircuit operation (see graph titled Peak Inductor Current vs.Load Current in the Typical Performance Characteristicssection). Less than 100m ESR is suggested for high effi-ciency.Open-core inductors cause flux linkage with circuit compo-nents and interfere with the normal operation of the circuit.They should be avoided. For high efficiency, choose an in-ductor with a high frequency core material, such as ferrite, toreduce the core losses. To minimize radiated noise, use atoroid, pot core or shielded core inductor. The inductor

should be connected to the SW pin as close to the IC as pos-sible. See Table 1 for a list of the inductor manufacturers.

OUTPUT DIODE SELECTIONA Schottky diode should be used for the output diode. Theforward current rating of the diode should be higher than theload current, and the reverse voltage rating must be higherthan the output voltage. Do not use ordinary rectifier diodes,since slow switching speeds and long recovery times causethe efficiency and the load regulation to suffer. Table 1 showsa list of the diode manufacturers.

INPUT AND OUTPUT FILTER CAPACITORS SELECTIONTantalum chip capacitors are recommended for the input andoutput filter capacitors. A 22F capacitor is suggested for theinput filter capacitor. It should have a DC working voltage rat-ing higher than the maximum input voltage. A 68F tantalumcapacitor is suggested for the output capacitor. The DCworking voltage rating should be greater than the output volt-age. Very high ESR values ( >3) should be avoided. Table 1 shows a list of the capacitor manufacturers.

TABLE 1. Suggested Manufacturers List

Inductors Capacitors Diodes

CoilcraftTel: (800) 322-2645Fax: (708) 639-1469

Sprague/ VishayTel: (207) 324-4140Fax: (207) 324-7223

MotorolaTel: (800) 521-6274Fax: (602) 244-6609

CoiltronicsTel: (407) 241-7876Fax: (407) 241-9339

KemetTel: (864) 963-6300Fax: (864) 963-6521

International Rectifier (IR)Tel: (310) 322-3331Fax: (310) 322-3332

Pulse EngineeringTel: (619) 674-8100Fax: (619) 674-8262

NichiconTel: (847) 843-7500Fax: (847) 843-2798

General SemiconductorTel: (516) 847-3222Fax: (516) 847-3150

PC BOARD LAYOUTHigh switching frequencies and high peak currents make aproper layout of the PC board an important part of design.Poor design can cause excessive EMI and ground-bounce,both of which can cause malfunction and loss of regulationby corrupting voltage feedback signal and injecting noiseinto the control section.Power components - such as the inductor, input and outputfilter capacitors, and output diode - should be placed asclose to the regulator IC as possible, and their traces shouldbe kept short, direct and wide. The ground pins of the input

and output filter capacitors and the PGND and SGND pins ofLM2621 should be connected using short, direct and widetraces. The voltage feedback network (R F1 , RF2 , and C F1 )should be kept very close to the FB pin. Noisy traces, suchas from the SW pin, should be kept away from the FB andVDD pins. The traces that run between V out and the FB pin ofthe IC should be kept away from the inductor flux. Alwaysprovide sufficient copper area to dissipate the heat due topower loss in the circuitry and prevent the thermal protectioncircuitry in the IC from shutting the IC down.

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APPLICATION EXAMPLES

U1 National LM2621MMC1 Vishay/Sprague 595D226X06R3B2T, TantalumC2 Vishay/Sprague 595D686X0010C2T, TantalumD1 Motorola MBRS140T3L Coilcraft DT1608C-682

U1 National LM2621MMC1 Vishay/Sprague 592D156X06R3B2T, TantalumC2 Vishay/Sprague 592D336X06R3C2T, Tantalum

D1 Motorola MBRS140T3L Vishay/Dale ILS-3825-03

EXAMPLE 1. 5V/0.5A Step-Up Regulator

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EXAMPLE 2. 2mm Tall 5V/0.2A Step-Up Regulator for Low Profile Applications

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APPLICATION EXAMPLES (Continued)

U1 National LM2621MMC1 Vishay/Sprague 595D226X06R3B2T, TantalumC2 Vishay/Sprague 595D686X0010C2T, TantalumD1 Motorola MBRS140T3L1, L2 Coilcraft DT1608C-682CS Vishay/Vitramon VJ1210Y105M , Ceramic

EXAMPLE 3. 3.3V/0.5A SEPIC Regulator

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Physical Dimensions inches (millimeters) unless otherwise noted

LIFE SUPPORT POLICY

NATIONALS PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DE-VICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMI-CONDUCTOR CORPORATION. As used herein:1. Life support devices or systems are devices or sys-

tems which, (a) are intended for surgical implant intothe body, or (b) support or sustain life, and whose fail-ure to perform when properly used in accordancewith instructions for use provided in the labeling, canbe reasonably expected to result in a significant injuryto the user.

2. A critical component in any component of a life supportdevice or system whose failure to perform can be rea-sonably expected to cause the failure of the life supportdevice or system, or to affect its safety or effectiveness.

National SemiconductorCorporationAmericasTel: 1-800-272-9959Fax: 1-800-737-7018Email: support@nsc.com

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8-Lead Mini SO-8 (MM)NS Package Number MUA08A

For Order Numbers, refer to the table in the Ordering Information section of this document.

L M 2 6 2 1 L o w I n p u t V o l t a g e

, S t e p - U p D C

- D C C o n v e r t e r

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.