10

ON

FB

SW

L1

VOUT

LM34927

+

VOUT

(low ripple)

LM34927

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SNVS799F – APRIL 2012 – REVISED DECEMBER 2013

At high input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall

voltage is driven externally by an alternate voltage source, between 8 V and 14 V, the internal regulator is

disabled. This reduces the power dissipation in the IC.

Figure 13. Low Ripple Output Configuration

Regulation Comparator

The feedback voltage at FB is compared to an internal 1.225 V reference. In normal operation, when the output

voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225 V. The high side

switch will stay on for the on-time, causing the FB voltage to rise above 1.225 V. After the on-time period, the

high side switch will stay off until the FB voltage again falls below 1.225 V. During start-up, the FB voltage will be

below 1.225 V at the end of each on-time causing the high side switch to turn on immediately after the minimum

forced off-time of 144 ns. The high side switch can be turned off before the on-time is over, if peak current in the

inductor reaches the current limit threshold.

Overvoltage Comparator

The feedback voltage at FB is compared to an internal 1.62 V reference. If the voltage at FB rises above 1.62 V

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

changes suddenly. The high side switch will not turn on again until the voltage at FB falls below 1.225 V.

On-Time Generator

LM34927 is:

(4)

operation. This requirement limits the maximum frequency for each application.

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