1.25V

Q

Q

S

R

FB

AVIN

SW

GND

6V INTERNAL

SUB

REGULATOR

ON TIMER

DRIVER

CBOOT

LEVEL

SHIFT

RON

THERMAL

SHUTDOWN

4.8A

CURRENT LIMIT

OFF TIMER

Q

START

COMPLETE

Ron

LM2696

UVLO

FB

REGULATION

COMPARATOR

SD

Q

OFF TIMER

SD

PGOOD

94% x Vbg

UNDER-VOLTAGE

COMPARATOR

Shutdown

PVIN

SS

1 PA

1.25V

3.65V

INTERNAL LDO

SD

BUCK

SWITCH

CURRENT

SENSE

1 PA

LM2696

SNVS375B – OCTOBER 2005 – REVISED APRIL 2013

www.ti.com

BLOCK DIAGRAM

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

constant frequency. In continuous conduction mode (CCM) the frequency depends only on duty cycle and on-

time. This is in contrast to hysteretic regulators where the switching frequency is determined by the output

inductor and capacitor. In discontinuous conduction mode (DCM), experienced at light loads, the frequency will

vary according to the load. This leads to high efficiency and excellent transient response.

these conditions are encountered the FET will turn off for a minimum pre-determined time period. This minimum

until the comparator 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. Alternatively, an RC feed forward scheme may be used to eliminate the need for a minimum ESR.

8

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