FN6966 Rev 5.00

Page 21 of 28

May 3, 2011

ZL2103

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for details on how to select specific overvoltage fault response

Output Pre-Bias Protection

An output pre-bias condition exists when an externally applied

voltage is present on a power supply’s output before the power

supply’s control IC is enabled. Certain applications require that

the converter not be allowed to sink current during start up if a

pre-bias condition exists at the output. The ZL2103 provides pre-

bias protection by sampling the output voltage prior to initiating

an output ramp.

If a pre-bias voltage lower than the target voltage exists after the

pre-configured delay period has expired, the target voltage is set

to match the existing pre-bias voltage and both drivers are

enabled. The output voltage is then ramped to the final

regulation value at the ramp rate set by the SS pin.

The actual time the output will take to ramp from the pre-bias

voltage to the target voltage will vary depending on the pre-bias

voltage but the total time elapsed from when the delay period

expires and when the output reaches its target value will match

the pre-configured ramp time (see Figure 17).

If a pre-bias voltage higher than the target voltage exists after the

pre-configured delay period has expired, the target voltage is set

to match the existing pre-bias voltage and both drivers are

enabled with a PWM duty cycle that would ideally create the pre-

bias voltage.

Once the pre-configured soft-start ramp period has expired, the

PG pin will be asserted (assuming the pre-bias voltage is not

higher than the overvoltage limit). The PWM will then adjust its

duty cycle to match the original target voltage and the output will

ramp down to the pre-configured output voltage.

If a pre-bias voltage higher than the overvoltage limit exists, the

device will not initiate a turn-on sequence and will declare an

overvoltage fault condition to exist. In this case, the device will

respond based on the output overvoltage fault response method

that has been selected. See “Output Overvoltage Protection” on

page 20 for response options due to an overvoltage condition.

FIGURE 17. OUTPUT RESPONSES TO PRE-BIAS VOLTAGES