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LTC3113

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-

ﬁer output (VC) determines the output duty cycle of each

switch. The low RDS(ON), low gate charge, synchronous

power switches provide high frequency pulse width modu-

lation control. High efﬁciency 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:

T ≅

fMHz

()

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 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 ampliﬁer output to decrease until the average

current through switch A decreases approximately to the

current limit value. The average current 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 FB to

ground should exceed 100k.

The speed of the average current limit circuit is limited

by the dynamics of the error ampliﬁer. 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 ﬁxed frequency operation, a reverse-current com-

parator on switch D monitors the current entering VOUT.

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.

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