The TPS923610/1/2 implements PWM controlled analog dimming to adjust the brightness of LED strings during operation. Pulse width modulation (PWM) signal can be applied at ADIM pin to control the total output current as an analog value. When the voltage of the ADIM pin is constantly high, the voltage of the FB pin is regulated to the full value, 200mV typically. When the signal applied to the ADIM pin is PWM, the regulation voltage at the FB pin is reduced to the full value multiplied by PWM duty cycle, and as a result, the total output current is reduced, too. The TPS923610/1/2 supports such dimming method with dimming ratio down to 0.1%.

The relationship between the PWM duty cycle and FB regulation voltage is given by:

where

• Duty = Duty cycle of the input PWM signal
• 200mV = The full value of internal reference voltage

In this way, the user can easily control the LED strings brightness by controlling the duty cycle of the input PWM signal.

As shown in Figure 7-1, the TPS923610/1/2 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 GM amplifier as the reference voltage for the FB pin regulation. Therefore, although a PWM signal is used for brightness dimming, only the LED DC current is modulated, which is often referred as analog dimming. This mechanism 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, TPS923610/1/2 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 10kHz to 200kHz. If the PWM frequency is lower than 10kHz, 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.