Datasheet
Table Of Contents
- FEATURES
- APPLICATIONS
- DESCRIPTION
- ABSOLUTE MAXIMUM RATINGS
- OPERATING RANGE
- ELECTRICAL CHARACTERISTICS
- TYPICAL PERFORMANCE CHARACTERISTICS
- BLOCK DIAGRAM
- APPLICATION INFORMATION
- CONSTANT ON-TIME CONTROL OVERVIEW
- INTERNAL OPERATION UNDER-VOLTAGE COMPARATOR
- ON-TIME GENERATOR SHUTDOWN
- CURRENT LIMIT
- N-CHANNEL HIGH SIDE SWITCH AND DRIVER
- THERMAL SHUTDOWN
- COMPONENT SELECTION
- FREQUENCY SELECTION
- INDUCTOR SELECTION
- OUTPUT CAPACITOR
- RIPPLE FEED FORWARD
- FEEDBACK RESISTORS
- INPUT CAPACITOR
- AVIN CAPACITOR
- SOFT-START CAPACITOR
- EXTVCC CAPACITOR
- SHUTDOWN
- CBOOT CAPACITOR
- PGOOD RESISTOR
- CATCH DIODE
- BYPASS CAPACITOR
- EXTERNAL OPERATION STARTUP
- UNDER- & OVER-VOLTAGE CONDITIONS
- CURRENT LIMIT
- MODES OF OPERATION
- LINE REGULATION
- TRANSIENT RESPONSE
- EFFICIENCY
- PRE-BIAS LOAD STARTUP
- THERMAL CONSIDERATIONS
- LAYOUT CONSIDERATIONS
- Revision History
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
EXTV
CC
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
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 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.
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 minimum 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 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.
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