TPS62902-Q1 datasheet(39/54 Pages) TI

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9.4 System Examples

9.4.1 LED Power Supply

The TPS62902-Q1 can be used as a power supply for power LEDs. The FB pin can be easily set to lower

values than nominal by using the SS/TR pin. With that, the voltage drop on the sense resistor (R2) is reduced to

avoid excessive power loss. Because the SS/TR pin provides 2.5 µA (typical), the feedback pin voltage can be

adjusted by an external resistor per Equation 19. This drop, proportional to the LED current, is used to regulate

the output voltage (anode voltage) to a proper level to drive the LED. Both analog and PWM dimming are

supported with the TPS62902-Q1. Figure 9-64 shows an application circuit, tested with analog dimming.

VIN

SS/TR

VIN

MODE/

S-CONF

VOS

FB/

VSET

TPS629xx-Q1

VOUT

GND

10 μF

22 μF

Figure 9-64. Single Power LED Supply

The resistor at SS/TR defines the FB voltage. The resistor at SS/TR is set to 304 mV by RSS/TR = R4 = 162 kΩ

using Equation 19. This cuts the losses on R2 to half from the nominal 0.6 V of feedback voltage while it still

provides good accuracy.

VFB = 0.75 x 2.5μA x RSS/TR

(19)

The device now supplies a constant current set by resistor R2 from FB/VSET to GND. The minimum input

voltage has to be rated according the forward voltage needed by the LED used. More information is available in

the Step-Down LED Driver With Dimming With the TPS621-Family and TPS821-Family application report.

9.4.2 Powering Multiple Loads

In applications where the TPS62902-Q1 is used to power multiple load circuits, the total capacitance on the

output can be very large. To properly regulate the output voltage, there must be an appropriate AC signal level

on the VOS pin. Tantalum capacitors have a large enough ESR to keep output voltage ripple sufficiently high

on the VOS pin. With low-ESR ceramic capacitors, the output voltage ripple can get very low, so TI does not

recommend to use a large capacitance directly on the output of the device. If there are several load circuits

with their associated input capacitor on a PCB, these loads are typically distributed across the board. This adds

enough trace resistance (Rtrace) to keep a large enough AC signal on the VOS pin for proper regulation.

The minimum total trace resistance on the distributed load is 10 mΩ. The total capacitance n × CIN in the use

case below was 32 × 47 μF of ceramic X7R capacitors.

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TPS62902-Q1

SLVSG70 – APRIL 2023

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