LTC3429/LTC3429B

7

3429fa

Thermal Shutdown

An internal temperature monitor will start to reduce the

peak current limit if the die temperature exceeds 125°C. If

the die temperature continues to rise and reaches 160°C,

the part will go into thermal shutdown, all switches will be

turned off and the soft-start capacitor will be reset. The

part will be enabled again when the die temperature drops

by about 15°C.

Burst Mode OPERATION (LTC3429 Only)

Portable devices frequently spend extended time in low

power or standby mode, only switching to high power

consumption when specific functions are enabled. To

improve battery life in these types of products, it is

important to maintain a high power conversion efficiency

over a wide output power range. The LTC3429 provides

automatic Burst Mode operation to increase efficiency of

the power converter at light loads. Burst Mode operation

is initiated if the output load current falls below an inter-

nally programmed threshold. This threshold has an in-

verse dependence on the duty cycle of the converter and

also the value of the external inductor (See Typical Perfor-

mance Characteristics, Output Current Burst Mode Thresh-

the circuitry required to monitor the output is kept alive

and the rest of the device is turned off. This is referred to

as the sleep state in which the IC consumes only 20µA

from the output capacitor. When the output voltage droops

by about 1% from its nominal value, the part wakes up and

commences normal PWM operation. The output capacitor

recharges and causes the part to re-enter the sleep state

if the output load remains less than the Burst Mode

threshold. The frequency of this intermittent PWM or

burst operation depends on the load current; that is, as the

load current drops further below the burst threshold, the

LTC3429 turns on less frequently. When the load current

increases above the burst threshold, the LTC3429

seamlessly resumes continuous PWM operation. Thus,

Burst Mode operation maximizes the efficiency at very

light loads by minimizing switching and quiescent losses.

However, the output ripple typically increases to about 2%

peak-to-peak. Burst Mode ripple can be reduced, in some

circumstances, by placing a small phase-lead capacitor

Diagram). However, this may adversely affect the effi-

ciency and the quiescent current requirement at light

OUTPUT DISCONNECT AND INRUSH LIMITING

The LTC3429/LTC3429B are designed to allow true output

disconnect by eliminating body diode conduction of the

volts during shutdown, drawing zero current from the

input source. It also allows for inrush current limiting at

start-up, minimizing surge currents seen by the input

supply. Note that to obtain the advantage of output discon-

nect, there must not be an external Schottky diode con-

Board layout is extremely critical to minimize voltage

overshoot on the SWITCH pin due to stray inductance.

Keep the output filter capacitor as close as possible to the

over 4.3V, a Schottky diode is required to limit the peak

SWITCH voltage to less than 6V unless some form of

external snubbing is employed. This diode must also be

placed very close to the pins to minimize stray inductance.

See the Applications Information.

SHORT CIRCUIT PROTECTION

Unlike most boost converters, the LTC3429/LTC3429B

allow their output to be short circuited due to the output dis-

connect feature. The devices incorporate internal features

such as current limit foldback, thermal regulation and ther-

mal shutdown for protection from an excessive overload

or short circuit. In the event of a short circuit, the internal

soft-start capacitor gets partially discharged. This, in turn,

causes the maximum current limit to foldback to a smaller

value. In addition to this, a thermal regulation circuit starts

to dial back the current limit farther if the die temperature

rises above 125°C. If the die temperature still reaches

160°C, the device shuts off entirely.

The LTC3429/LTC3429B will maintain voltage regulation

even if the input voltage is above the output voltage. This

OPERATIO