Ps

10

ON

LM25019

www.ti.com

SNVS952 – DECEMBER 2012

Regulation Comparator

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

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

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

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

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

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

the inductor reaches the current limit threshold.

Overvoltage Comparator

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

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 1.225V.

On-Time Generator

LM25019 is:

(4)

Current Limit

The LM25019 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 240mA the

present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is

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 48V.

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 the following equation:

(5)

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

N-Channel Buck Switch and Driver

The LM25019 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

0.01uF ceramic capacitor connected between the BST pin and the SW pin provides the voltage to the driver

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

recharge the bootstrap capacitor.

Synchronous Rectifier

The LM25019 provides an internal synchronous N-Channel MOSFET rectifier. This MOSFET provides a path for

the inductor current to flow when the high-side MOSFET is turned off.

The synchronous rectifier has no diode emulation mode, and is designed to keep the regulator in continuous

conduction mode even during light loads which would otherwise result in discontinuous operation.

Copyright © 2012, Texas Instruments Incorporated

Submit Documentation Feedback

9

Product Folder Links: LM25019