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OVERVOLTAGE PROTECTION (OVP)

ENABLE PWM DIMMING

DIGITAL BRIGHTNESS CONTROL (ILED)

TPS61060

TPS61061

TPS61062

SLVS538 – NOVEMBER 2004

DETAILED DESCRIPTION (continued)

As with any current source, the output voltage rises when the output gets high impedance or disconnected. To

prevent the output voltage exceeding the maximum switch voltage rating (33 V) of the main switch, an

overvoltage protection circuit is integrated. As soon as the output voltage exceeds the OVP threshold, the

converter stops switching and the output voltage falls down. When the output voltage falls below the OVP

threshold, the converter continues operation until the output voltage exceeds the OVP threshold again. To allow

the use of inexpensive low-voltage output capacitors, the TPS6106x series has different OVP levels that need to

be selected according to the number of external LEDs and their maximum forward voltage.

The enable pin allows disabling and enabling of the device as well as brightness control of the LEDs by applying

a PWM signal up to typically 1 kHz. When a PWM signal is applied, the LED current is turned on when the EN is

high and off when EN is pulled low. Changing the PWM duty cycle therefore changes the LED brightness. To

allow higher PWM frequencies on the enable pin, the device continues operation when a PWM signal is applied.

As shown in the block diagram, the EN pin needs to be pulled low for at least 50 ms to fully turn the device off.

The enable input pin has an internal 300-k

Ω pulldown resistor to disable the device when this pin is floating.

The ILED pin features a simple digital interface to allow digital brightness control. This can save processor power

and battery life. Using the digital interface to control the LED brightness does not required a PWM signal all the

time, and the processor can enter sleep mode if available. To save signal lines, the ILED pin can be connected

to the enable pin to allow digital programming and enable/disable function at the same time with the same signal.

Such a circuit is shown in Figure 22.

mV. When the ILED pin is pulled high, the digital brightness control is enabled starting at its midpoint where the

giving 32 steps equal to a 15.6-mV change in feedback voltage per step. To increase or decrease the internal

reference voltage, the ILED pin needs to be pulled low over time as outlined in Table 1 and specified in the

electrical table. When the internal DAC is programmed to its highest or lowest value, it stays at this value until it

gets programmed in the opposite direction again.

Table 1. Increase/Decrease Internal Reference Voltage

FEEDBACK VOLTAGE

TIME

ILED LOGIC LEVEL

Increase

1 µs to 75 µs

Low

Decrease

180 µs to 300 µs

Low

Brightness control disabled

≥550 µs

Low

Delay between steps

1.5 µs

High

10