(

)

IN

OFF ILIM

FB

0.07

V

T

s

V

0.2 V

´

=

m

+

10

ON

LM34927

www.ti.com

SNVS799G – APRIL 2012 – REVISED DECEMBER 2014

Feature Description (continued)

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, from 8.55 V to 14 V, the internal regulator is disabled.

This reduces the power dissipation in the IC.

7.3.3 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.

7.3.4 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.

7.3.5 On-Time Generator

LM34927 device is shown in Equation 3.

(3)

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

7.3.6 Current Limit

The LM34927 device contains an intelligent current limit off-timer. If the current in the buck switch exceeds

1.02 A, the present cycle is immediately terminated, and a nonresetable off-timer is initiated. The length of off-

maximum off-time is set to 16

μs. This condition occurs when the output is shorted, and during the initial part of

start up. This amount of time ensures safe short circuit operation up to the maximum input voltage of

100 V.

In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is

reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and

start-up time. The off-time is calculated from Equation 4.

(4)

The current limit protection feature is peak limited, the maximum average output will be less than the peak.

7.3.7 N-Channel Buck Switch and Driver

The LM34927 device integrates an N-Channel Buck switch and associated floating high-voltage gate driver. The

gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high-voltage diode. A

0.01-µF ceramic capacitor connected between the BST pin and SW pin provides the voltage to the driver during

the on-time. During each off-time, the SW pin is at approximately 0 V and the bootstrap capacitor charges from

recharge the bootstrap capacitor.

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