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LED BRIGHTNESS DIMMING MODE SELECTION

CTRL

low

high

FB

200

mVxdutycycle

Insertbattery

CTRL

low

high

FB

Insertbattery

Programming

code

FBramp

Shutdowndelay

t

EnterESmode

Timingwindow

Programmingcode

50mV

50mV

EnterESmode

PWMsignal

Startup

delay

PWM

mode

Startupdelay

FBramp

Programmedvalue

(ifnotprogrammed, 200mVdefault )

Shutdown

delay

IC

Shutdown

Startupdelay

FBramp

ES

mode

ESdetectdelay

ESdetecttime

PWM BRIGHTNESS DIMMING

V

200 mV

(2)

TPS61160

TPS61161

SLVS791 – NOVEMBER 2007

The CTRL pin is used for the control input for both dimming modes, PWM dimming and 1 wire dimming. The

dimming mode for the TPS61160/1 is selected each time the device is enabled. The default dimming mode is

PWM dimming. To enter the 1 wire mode, the following digital pattern on the CTRL pin must be recognized by

the IC every time the IC starts from the shutdown mode.

1. Pull CTRL pin high to enable the TPS61160/1, and to start the 1 wire detection window.

3. The CTRL pin has to be low for more than EasyScale detection time before the EasyScale detection window

The IC immediately enters the 1 wire mode once the above 3 conditions are met. the EasyScale communication

can start before the detection window expires. Once the dimming mode is programmed, it can not be changed

without another start up. This means the IC needs to be shutdown by pulling the CTRL low for 2.5ms and

restarts. See the Dimming Mode Detection and Soft Start (Figure 13) for a graphical explanation.

Figure 13. Dimming Mode Detection and Soft Start PWM Brightness Dimming

When the CTRL pin is constantly high, the FB voltage is regulated to 200mV typically. However, the CTRL pin

allows a PWM signal to reduce this regulation voltage; therefore, it achieves LED brightness dimming. The

relationship between the duty cycle and FB voltage is given by Equation 2.

Where

Duty = duty cycle of the PWM signal

200 mV = internal reference voltage

As shown in Figure 14, the IC chops up the internal 200mV 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

error 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 scheme which filters the PWM signal for analog dimming, TPS61160/1

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

Copyright © 2007, Texas Instruments Incorporated

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Product Folder Link(s): TPS61160 TPS61161