XTR115 + TPS54062Objective: Increase the available power for sensor and other external circuitry on a remote 2-wire current loop transmitter.

With existing designs, sensors and systems that draw more than ~3.3mA will not be able to meet typical under-scale current or zero-scale levels of 3.8mA or 4mA.

Loop Supply – 16VDesired Iout range = 3.8mA DC – 20.2mA DC with 1mApp HART AC signalRequired Sensor Current = 7.1mA

XTR115 + TPS54062

Vreg

Vref

Iin

Iret

V+

B

E

Iout

Loop Supply

(24VDC)

20k

TPS54062EVM

Iout_TPS = 9mA

Iret = 2.75mA

Iout_XTR

XTR115

Vout (5VDC) Vin

GND

250R

+

-

Sensor

V+

V-

Out

4-20mA

Solution: Use low-current buck topology switching power converter to buck down the loop-supply to the required VREG levels. All SMPS current must sink back through IRET of XTR115.

EfficiencyPout

Pin

Vout Iout_TPS

VLoop IRETIout_TPS

Efficiency IRET VLoop

Vout

XTR115 + TPS54062Setup:

XTR115 + TPS54062EVM with reduced input capacitance of 1uF

Output: 3.8mA DC + 1mAPP 2.2kHz HART signal

Results:

At 2.2kHz, Max IRET increased to 2.6mA before signal degradation occurred

TPS_IOUT = 7.2mA

IRET = 2.6mA

XTR115 + TPS54062Setup:

XTR115 + TPS54062EVM with reduced input capacitance of 1uF

Output: 3.8mA DC + 1mAPP 2.2kHz HART signal

Results:

Showing the +3.3V output with the reduced 1uF capacitance, output looks good

DC Couple AC Couple

XTR115 + TPS54062Setup:

XTR115 + TPS54062EVM with reduced input capacitance of 0.47uF

Output: 3.8mA DC + 1mAPP 2.2kHz HART signal

Results:

At 2.2kHz, Max IRET increased to 2.85mA before signal degradation occurred

IRET = 2.8mA

TPS_IOUT = 9mA

XTR115 + TPS54062Setup:

XTR115 + TPS54062EVM with reduced input capacitance of 0.47uF

Output: 3.8mA DC + 1mAPP 2.2kHz HART signal

Results:

At 2.2kHz, Max IRET increased to 2.85mA before signal degradation occurred

IRET = 2.85mA

TPS_IOUT = 9.25mA

XTR115 + TPS54062Setup:

XTR115 + TPS54062EVM with reduced input capacitance of 0.47uF

Output: 3.8mA DC + 1mAPP 2.2kHz HART signal

Results:

At 2.2kHz, Showing distortion with IRET = 2.9mA

IRET = 2.9mA

TPS_IOUT = 9.5mA

XTR115 + TPS54062Setup:

XTR115 + TPS54062EVM with reduced input capacitance of 0.47uF

Output: 3.8mA DC + 1mAPP 2.2kHz HART signal

Results:

At 2.2kHz, Showing distortion with IRET = 3mA

IRET = 3mA

TPS_IOUT = 10mA

XTR115 + TPS54062Setup:

XTR115 + TPS54062EVM with reduced input capacitance of 0.47uF

Output: 3.8mA DC + 1mAPP 2.2kHz HART signal

Results:

Showing the +3.3V output with the reduced 0.47uF capacitance, output looks good

DC Couple AC Couple

XTR115 + TPS54062Setup:

XTR115 + TPS54062EVM with reduced input capacitance of 0.47uF

Output: 3.8mA DC + 1mAPP 2.2kHz HART signal

Results:

Showing the +16V input ripple with the reduced 0.47uF capacitance, looks similar when compared to original

2.2uF Input cap 0.47uF Input Cap