LM2696MXAX datasheet(7/19 Pages) NSC

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Block Diagram

20153419

Application Information

CONSTANT ON-TIME CONTROL OVERVIEW

The LM2696 buck DC-DC regulator is based on the constant

on-time control scheme. This topology relies on a fixed

switch on-time to regulate the output. The on-time can be set

manually by adjusting the size of an external resistor (R

ON).

The LM2696 automatically adjusts the on-time inversely with

the input voltage (AV

IN) to maintain a constant frequency. In

continuous conduction mode (CCM) the frequency depends

only on duty cycle and on-time. This is in contrast to hyster-

etic regulators where the switching frequency is determined

by the output inductor and capacitor. In discontinuous con-

duction mode (DCM), experienced at light loads, the fre-

quency will vary according to the load. This leads to high

efficiency and excellent transient response.

The on-time will remain constant for a given V

IN unless an

over-current or over-voltage event is encountered. If these

conditions are encountered the FET will turn off for a mini-

mum pre-determined time period. This minimum T

OFF (250

ns) is internally set and cannot be adjusted. After the T

OFF

period has expired the FET will remain off until the compara-

tor trip-point has been reached. Upon passing this trip-point

the FET will turn back on, and the process will repeat.

Switchers that regulate using an internal comparator to

sense the output voltage for switching decisions, such as

hysteretic or constant on-time, require a minimum ESR. A

minimum ESR is required so that the control signal will be

dominated by ripple that is in phase with the switchpin. Using

a control signal dominated by voltage ripple that is in phase

with the switchpin eliminates the need for compensation,

thus reducing parts count and simplifying design. Alterna-

tively, an RC feed forward scheme may be used to eliminate

the need for a minimum ESR.

INTERNAL OPERATION

UNDER-VOLTAGE COMPARATOR

An internal comparator is used to monitor the feedback pin

for sensing under-voltage output events. If the output voltage

drops below the UVP threshold the power-good flag will fall.

ON-TIME GENERATOR SHUTDOWN

The on-time for the LM2696 is inversely proportional to the

input voltage. This scheme of on-time control maintains a

constant frequency over the input voltage range. The on-

time can be adjusted by using an external resistor connected

between the PVIN and RON pins.

CURRENT LIMIT

The LM2696 contains an intelligent current limit off-timer. If

the peak current in the internal FET exceeds 4.9A the

present on-time is terminated; this is a cycle-by-cycle current

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