7.3.3 Soft Startup

After enabling the device, internal soft startup circuitry ramps up the output voltage which reaches nominal

output voltage during a startup time. Output current rises smoothly and excessive inrush current is avoided. In

battery-power system, the soft startup prevents extra voltage drop on primary power supply with high internal

impedance. The internal soft startup period is xx ms typically.

7.3.4 Low Dropout Operation

The device offers a low input-to-output voltage differential by entering 100% switching duty cycle mode. In

this mode, the high-side MOSFET switch is constantly turned on and the low-side MOSFET is switched off.

The minimum input voltage to maintain output regulation, depending on the load current and output voltage, is

calculated as:

VIN_MIN=VFB+ VF_IOUT+ IOUT x RDS_ON+RL

(1)

where

• VFB is the feedback reference voltage, which is typically 100mV

• VF_IOUT is the LED forward voltage at output current

• IOUT is the output current setting

• RDS_ON is the high-side FET resistance when turning on

• RL is the inductor ohmic resistance DCR

7.3.5 LED Current Setting

The LED current is set by the external resistor between the FB pin and GND, calculated as:

ILED=  VFBRSENSE

(2)

where

• VFB is the feedback reference voltage, which is typically 100mV

• RSENSE is the resistance between FB and GND

7.3.6 Voltage Reference

The feedback reference produces a precise ±5% voltage reference over whole temperature range when

the PWM duty cycle is 100%, which is typically 100mV. In analog dimming mode, the feedback voltage is

proportional to the duty cycle of PWM imput as shown in Figure 7-2.

PWM duty

1%

20%

40%

60%

80%

100%

1

20

40

60

80

100

Figure 7-2. FB voltage with PWM duty cycle

TPS92201, TPS92201A

SLVSH40B – NOVEMBER 2023 – REVISED AUGUST 2024

www.ti.com

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