LTC3112_15 Datasheet, PDF (11/32 Pages) Linear Technology – 15V, 2.5A Synchronous Buck-Boost DC/DC Converter

LTC3112
Operation
INTRODUCTION
The LTC3112 is an extended input and output range,
high current synchronous buck-boost DC/DC converter
optimized for a variety of demanding applications. The
LTC3112 utilizes a proprietary switching algorithm, which
allows its output voltage to be regulated above, below or
equal to the input voltage. The error amplifier output on
COMP determines the output duty cycle of the switches.
The low R_DS(ON), low gate charge synchronous switches
provide high efficiency pulse width modulation control.
High efficiency is achieved at light loads when Burst Mode
operation is commanded.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator, Phase-Locked Loop
An internal oscillator circuit sets the normal frequency of
operation to 750kHz. A pulse train applied to the PWM/
SYNC pin allows the operating frequency to be programmed
between 300kHz to 1.5MHz via an internal phase-locked
loop circuit. The pulse train must have a minimum high or
low state of at least 100ns to guarantee operation (Note 7).
Error Amplifier
The error amplifier is a high gain voltage mode ampli-
fier. The loop compensation components are configured
around the amplifier (from FB to COMP and V_OUT to FB)
to obtain stability of the converter and rapid response
to load transients. Refer to the Applications Information
section of this data sheet under Closing the Feedback
Loop for information on selecting compensation type
and components.
Current Limit Operation
The buck-boost converter has two current limit circuits.
The primary current limit is an average current limit circuit
which sources current into the feedback divider network
proportional to the extent that switch A current exceeds
6A typical. Due to the high gain of the feedback loop, the
injected current forces the error amplifier output to decrease
until the average current through switch A decreases ap-
proximately to the current limit value. The average cur-
rent limit utilizes the error amplifier in an active state and
thereby provides a smooth recovery with little overshoot
once the current limit fault condition is removed. Since
the current limit is based on the average current through
switch A, the peak inductor current in current limit will
have a dependency on the duty cycle (i.e. on the input
and output voltages) in the overcurrent condition. For this
current limit feature to be most effective, the Thevenin
resistance from the FB to ground should exceed 100kΩ.
The speed of the average current limit circuit is limited by
the dynamics of the error amplifier. On a hard output short,
it would be possible for the inductor current to increase
substantially beyond current limit before the average cur-
rent limit circuit would react. For this reason, there is a
second current limit circuit which turns off switch A if the
current ever exceeds approximately 160% of the average
current limit value. This provides additional protection in
the case of an instantaneous hard output short.
Should the output become shorted, the average current
limit is reduced to approximately one half of the normal
operating current limit.
Reverse Current Limit
During fixed frequency operation, a reverse current com-
parator on switch D monitors the current entering the
V_OUT pin. When this reverse current exceeds 1A (typical)
switch D will be turned off for the remainder of the switch-
ing cycle. This feature protects the buck-boost converter
from excessive reverse current if the buck-boost output
is above the regulation voltage.
Internal Soft-Start
The LTC3112 buck-boost converter has an independent
internal soft-start circuit with a nominal duration of 2ms.
The converter remains in regulation during soft-start and
will therefore respond to output load transients which
occur during this time. In addition, the output voltage rise
time has minimal dependency on the size of the output
capacitor or load current during start-up.
For more information www.linear.com/LTC3112
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