LTC3113
9
3113f
OPERATION
INTRODUCTION
The LTC3113 is a low noise, high power synchronous
buck-boost DC/DC converter optimized for demanding
applications. The LTC3113 utilizes a proprietary switching
algorithm, which allows its output voltage to be regulated
above, below or equal to the input voltage. The error ampli-
fi er output (VC) determines the output duty cycle of each
switch. The low R
DS(ON)
, low gate charge, synchronous
power switches provide high frequency pulse width modu-
lation control. High effi ciency is achieved at light loads
when Burst Mode operation is commanded.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation can be programmed between
300kHz and 2MHz by an external resistor from the RT pin
to ground, according to the following equation:
R
T
≅
90
fMHz
()
kΩ
()
Error Amplifi er
The error amplifi er is a high gain voltage mode ampli-
fi er. The loop compensation components are confi gured
around the amplifi er (from FB to VC) to obtain stable
converter operation. For improved bandwidth, an addi-
tional RC feedforward network can be placed across the
upper feedback divider resistor. 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 cir-
cuit which sources current into FB to reduce the output
voltage, should the input current exceed 7.8A. Due to the
high gain of the feedback loop, the injected current forces
the error amplifi er output to decrease until the average
current through switch A decreases approximately to the
current limit value. The average current limit utilizes the
error amplifi er 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 FB to
ground should exceed 100k.
The speed of the average current limit circuit is limited
by the dynamics of the error amplifi er. On a hard output
short, it is 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 142% of the average
current limit value. This provides additional protection in
the case of an instantaneous hard output short.
Should the output voltage become less then 1.2V nomi-
nally, both the current limits are reduced compared to the
normal operating current limits.
Reverse Current Limit
During fi xed frequency operation, a reverse-current com-
parator on switch D monitors the current entering V
OUT
.
When this current exceeds 1A (typical) switch D will be
turned off for the remainder of the switching cycle. This
feature protects the buck-boost converter from excessive
reverse current if the buck-boost output is held above the
regulation voltage by an external source.
In applications where the oscillator frequency is pro-
grammed above 1MHz and the output voltage is held above
its programmed regulation value, reverse currents greater
than 1A (typical) may be observed. In conjunction with
oscillator frequencies higher than 1MHz, higher output
voltages will also increase the magnitude of observed
reverse current. Refer to the Negative Inductor Current
vs Oscillator Frequency graph in the Typical Performance
Characteristics section for typical variations.
