LTC3429/LTC3429B

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OPERATIO

The LTC3429/LTC3429B are 500kHz, synchronous boost

converters housed in a 6-lead SOT-23 package. Able to

operate from an input voltage below 1V, the device fea-

tures fixed frequency, current mode PWM control for

MOSFET switches enable the device to maintain high

efficiency over a wide range of load current. Detailed

descriptions of the different operating modes follow.

Operation can be best understood by referring to the Block

Diagram.

LOW VOLTAGE START-UP

The LTC3429/LTC3429B include an independent start-up

oscillator designed to start up at input voltages of 0.85V

typically. The frequency and duty cycle of the start-up

oscillator are internally set to 150kHz and 67% respec-

tively. In this mode, the IC operates completely open-loop

and the current limit is also set internally to 850mA. Once

the output voltage exceeds 2.3V, the start-up circuitry is

disabled and normal close-loop PWM operation is initi-

ated. In normal mode, the LTC3429/LTC3429B power

battery voltage to drop to as low as 0.5V without affecting

the circuit operation. The only limiting factor in the appli-

cation becomes the ability of the battery to supply suffi-

cient energy to the output. Soft-start and inrush current

limiting are provided during start-up as well as normal

mode operation.

Soft-Start

The LTC3429/LTC3429B provide soft-start by charging an

internal capacitor with a very weak current source. The

voltage on this capacitor, in turn, slowly ramps the peak

inductor current from zero to a maximum value of 850mA.

The soft-start time is typically 2.5ms, the time it takes to

charge the capacitor from zero to 1.35V. However, this

time varies greatly with load current, output voltage and

input voltage (see Typical Performance Characteristics,

Inrush Current Control and Soft-Start). The soft-start

capacitor is discharged completely in the event of a

commanded shutdown or a thermal shutdown. It is dis-

charged only partially in case of a short circuit at the

output.

LOW NOISE FIXED FREQUENCY OPERATION

Oscillator

The frequency of operation is internally set to 500kHz.

Error Amp

The error amplifier is an internally compensated transcon-

ductance type (current output) with a transconductance

voltage is compared to the voltage at the FB pin to generate

an error signal at the output of the error amplifier. A volt-

voltage via FB from 2.5V to 5V using the equation:

Current Sensing

Lossless current sensing converts the NMOS switch

current signal to a voltage to be summed with the internal

slope compensation. The summed signal is compared to

the error amplifier output to provide a peak current

control command for the PWM. Peak switch current is

limited to approximately 850mA independent of input or

output voltage. The switch current signal is blanked for

60ns to enhance noise rejection.

Zero Current Comparator

The zero current comparator monitors the inductor cur-

rent to the output and shuts off the synchronous rectifier

once this current reduces to approximately 27mA. This

prevents the inductor current from reversing in polarity

thereby improving efficiency at light loads.

Antiringing Control

The antiringing control circuitry prevents high frequency

ringing of the SW pin as the inductor current goes to zero

in discontinuous mode. The damping of the resonant

achieved by placing a 150Ω resistor across the inductor.

Synchronous Rectifier

To prevent the inductor current from running away, the