UVLO

7 V

SW Pin

Inductor

Current

Vin

t 2

t1

t 3

0V

LM5008A

SNVS583F – MARCH 2009 – REVISED MARCH 2013

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Figure 10. Startup Sequence

Regulation Comparator

The feedback voltage at FB is compared to an internal 2.5V reference. In normal operation (the output voltage is

regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch will stay on for

the on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch will stay off until

the FB voltage again falls below 2.5V. During start-up, the FB voltage will be below 2.5V at the end of each on-

time, resulting in the minimum off-time of 300 ns. Bias current at the FB pin is nominally 100 nA.

Over-Voltage Comparator

The feedback voltage at FB is compared to an internal 2.875V reference. If the voltage at FB rises above 2.875V

the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load,

change suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5V.

On-Time Generator and Shutdown

(Vin), resulting in a nearly constant frequency as Vin is varied over its range. The on-time equation for the

LM5008A is:

(4)

8

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