PWM BRIGHTNESS DIMMING

V

200 mV

(2)

VBG

200mV

Error

Amplifier

FB

CTRL

UNDERVOLTAGE LOCKOUT

THERMAL SHUTDOWN

TPS61160A

TPS61161A

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 12, 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,

TPS61160A/61A 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 5kHz to 100kHz. The requirement of

minimum dimming frequency comes from the output ripple. Low frequency causes high output ripple. Since the

CTRL pin is logic only pin, applying an external RC filter to the pin does not work.

Figure 12. Block Diagram of Programmable FB Voltage Using PWM Signal

To use lower PWM dimming, add an external RC network connected to the FB pin as shown in the additional

typical application (Figure 15).

An undervoltage lockout prevents operation of the device at input voltages below typical 2.2V. When the input

voltage is below the undervoltage threshold, the device is shutdown and the internal switch FET is turned off. If

the input voltage rises by undervoltage lockout hysteresis, the IC restarts.

An internal thermal shutdown turns off the device when the typical junction temperature of 160°C is exceeded.

The device is released from shutdown automatically when the junction temperature decreases by 15°C.

Copyright © 2009, Texas Instruments Incorporated

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Product Folder Link(s): TPS61160A TPS61161A