STOP

RUN

Input

Voltage

LM34910

1.3 x 10

ON

LM34910

SNVS297A – OCTOBER 2004 – REVISED FEBRUARY 2005

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Regulation Comparator

The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V). 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

stays on for the on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch

stays off until the FB voltage falls below 2.5V. Bias current at the FB pin is nominally 100 nA.

Over-Voltage Comparator

The 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 changes

suddenly, or if the inductor (L1) saturates. The buck switch remains off until the voltage at FB falls below 2.5V.

ON-Time Timer, and Shutdown

from:

(5)

(6)

Figure 6. Shutdown Implementation

Current Limit

Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling

diode (D1). Referring to Typical Application Circuit and Block Diagram, when the buck switch is turned off the

current exceeds 1.25A the current limit comparator output switches to delay the start of the next on-time period if

voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed 1.25A

during each on-time, that is detected at the beginning of each off-time. The operating frequency may be lower

due to longer-than-normal off-times.

8

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