FB

SW

L1

VOUT

LM25018

+

VOUT

(low ripple)

OUT

SW

11

ON

V

f

9

10

R

-

=

´

´

LM25018

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SNVS953D – DECEMBER 2012 – REVISED DECEMBER 2014

7.3 Feature Description

7.3.1 Control Overview

The LM25018 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with

the output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the

reference the internal buck switch is turned on for the one-shot timer period, which is a function of the input

falls below the reference, but never before the minimum off-time forced by the minimum off-time one-shot timer.

When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck

switch is turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB

voltage is approximately equal to 1.225 V (typ).

In a synchronous buck converter, the low side (sync) FET is on when the high side (buck) FET is off. The

inductor current ramps up when the high side switch is on and ramps down when the high side switch is off.

There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative

direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of

the output loading. The operating frequency remains relatively constant with load and line variations. The

operating frequency can be calculated as shown in Equation 1.

(1)

as shown in Equation 2.

(2)

This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum

required for the LM25018. In cases where the capacitor ESR is too small, additional series resistance may be

For applications where lower output voltage ripple is required the output can be taken directly from a low ESR

Figure 10. Low Ripple Output Configuration

The LM25018 device contains an internal high voltage linear regulator with a nominal output of 7.6 V. The input

internal circuit blocks including the synchronous MOSFET driver and the logic circuits. When the voltage on the

boot capacitor when SW pin is low.

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 8.55 V and 14 V, the internal regulator is

disabled. This reduces the power dissipation in the IC.

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