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LTC3442EDE-TRPBF

Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation

Linear Technology 

LTC3442

OPERATION

The LTC3442 provides high efficiency, low noise power

for applications such as portable instrumentation. The

LTC proprietary topology allows input voltages above,

below or equal to the output voltage by properly phasing

the output switches. The error amp output voltage on VC

determines the output duty cycle of the switches. Since VC

is a filtered signal, it provides rejection of frequencies from

well below the switching frequency. The low RDS(ON), low

gate charge synchronous switches provide high frequency

pulse width modulation control at high efficiency. Schottky

diodes across the synchronous switch D and synchronous

switch B are not required, but provide a lower voltage

drop during the break-before-make time (typically 15ns).

Schottky diodes will improve peak efficiency by typically

1% to 2%. High efficiency is achieved at light loads when

Burst Mode operation is entered and the IC’s quiescent

current drops to a low 35µA.

LOW NOISE FIXED FREQUENCY OPERATION

Oscillator

The frequency of operation is programmed by an external

resistor from RT to ground, according to the following

equation:

f (kHz )

=

43,300

RT(kΩ)

Error Amp

The error amplifier is a voltage mode amplifier. The loop

compensation components are configured around the

amplifier (from FB to VC) to obtain stability of the converter.

For improved bandwidth, an additional RC feedforward

network can be placed across the upper feedback divider

resistor. The voltage on SHDN/SS clamps the error amp

output, VC, to provide a soft-start function.

Internal Current Limit

There are three different current limit circuits in the

LTC3442. Two have internally fixed thresholds which vary

inversely with VIN, the third is externally programmable,

and does not vary with input voltage.

The first circuit is a high speed peak current limit amplifier

that will shut off switch A if the current exceeds 5A typi-

cal. The delay to output of this amplifier is typically 50ns.

A second amplifier will begin to source current into the FB

pin to drop the output voltage once the peak input current

exceeds 3A typical. This method provides a closed loop

means of clamping the input current. During conditions

where VOUT is near ground, such as during a short-circuit

or during startup, this threshold is cut in half, providing a

foldback feature. For this current limit feature to be most

effective, the Thevenin resistance from FB to ground should

be greater than 100kΩ.

Externally Programmable Current Limit

The third current limit circuit is programmed by an external

resistor on RLIM. This circuit works by mirroring the input

current in switch A, averaging it by means of the external

RC network on RLIM, and comparing the resulting voltage

with an internal reference. If the voltage on RLIM starts

to exceed 0.95V, a Gm amplifier will clamp VC, lowering

VOUT to maintain control of the input current. This allows

the user to program a maximum average input current, for

applications such as USB, where the current draw from the

bus must be limited to 500mA. The resistor and capacitor

values are determined by the following equations:

( ) RLIM(kΩ)

=

70

•





0.86

+

2 • VIN –

40

IIN( AM PS)

VOUT





CLIM(µF )

≥

0.1

RLIM(kΩ)

The programmable current limit feature is disabled in

Burst Mode operation.

3442fb

8

For more information www.linear.com/LTC3442

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