FB

V

Duty 204 mV

u

FB

LED

SET

V

I

R

9

TPS61169

www.ti.com

SNVSA40A – OCTOBER 2014 – REVISED MARCH 2016

Product Folder Links: TPS61169

Submit Documentation Feedback

Copyright © 2014–2016, Texas Instruments Incorporated

Feature Description (continued)

8.3.3 Shutdown

The TPS61169 enters shutdown mode when the CTRL voltage is logic low for more than 2.5 ms. During

shutdown, the input supply current for the device is less than 2

μA (max). Although the internal switch FET does

not switch in shutdown, there is still a DC current path between the input and the LEDs through the inductor and

Schottky diode. The minimum forward voltage of the LED array must exceed the maximum input voltage to

ensure that the LEDs remain off in shutdown.

8.3.4 Current Program

The FB voltage is regulated by a low 204-mV reference voltage. The LED current is programmed externally using

where

•

•

•

(1)

The output current tolerance depends on the FB accuracy and the current sensor resistor accuracy.

8.3.5 LED Brightness Dimming

The TPS61169 receives PWM dimming signal at CTRL pin to control the total output current. When the CTRL

pin is constantly high, the FB voltage is regulated to 204 mV typically. When the duty cycle of the input PWM

signal is low, the regulation voltage at FB pin is reduced, and the total output current is reduced; therefore, it

achieves LED brightness dimming. The relationship between the duty cycle and FB regulation voltage is given

by:

where

•

Duty = Duty cycle of the PWM signal

•

204 mV = internal reference voltage

(2)

Thus, the user can easily control the WLED brightness by controlling the duty cycle of the PWM signal.

As shown in Figure 3, the IC chops up the internal 204-mV reference voltage at the duty cycle of the PWM

signal. The pulse signal is then filtered by an internal low-pass filter. The output of the filter is connected to the

GM amplifier as the reference voltage for the FB pin regulation. Therefore, although a PWM signal is used for

brightness dimming, only the WLED DC current is modulated, which is often referred as analog dimming. This

eliminates the audible noise which often occurs when the LED current is pulsed in replica of the frequency and

duty cycle of PWM control. Unlike other methods which filter the PWM signal for analog dimming, TPS61169

regulation voltage is independent of the PWM logic voltage level which often has large variations.

For optimum performance, use the PWM dimming frequency in the range of 5 kHz to 100 kHz. If the PWM

frequency is lower than 5 kHz, it is out of the low pass filter's filter range, the FB regulation voltage ripple

becomes large, causing large output ripple and may generate audible noise.