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LT3433

3433ia

APPLICATIO S I FOR ATIO

Overview

The LT3433 is a high input voltage range, step-up/step-

down DC/DC converter IC using a 200kHz constant fre-

quency, current mode architecture. Dual internal switches

allow the full input voltage to be imposed across the

switched inductor, such that both step-up and step-down

modes of operation can be realized using the same single

inductor topology.

The LT3433 has provisions for high efficiency, low load

operation for battery-powered applications. Burst Mode

operation reduces average quiescent current to 100

µA in

no load conditions. A low current shutdown mode can also

be activated, reducing total quiescent current to 10

µA.

Much of the LT3433’s internal circuitry is biased from an

internal low voltage linear regulator. The output of this

ing of the internal regulator. The associated internal

circuitry can be powered directly from the output of the

converter, increasing overall converter efficiency. Using

externally derived power also eliminates the IC’s power

regulator.

Theory of Operation (See Block Diagram)

pin. The difference between the voltage on this pin and an

internal 1.231V reference is amplified to generate an error

for the current sense comparator.

During normal operation, the LT3433 internal oscillator

runs at 200kHz. At the beginning of each oscillator cycle,

the switch drive is enabled. The switch drive stays enabled

threshold for the current sense comparator and, in turn,

disables the switch driver. If the current comparator

threshold is not obtained for the entire oscillator cycle, the

switch driver is disabled at the end of the cycle for 250ns.

This minimum off-time mode of operation assures regen-

If the converter input and output voltages are close

together, proper operation in normal buck configuration

would require high duty cycles. The LT3433 senses this

condition as requiring a duty cycle greater than 75%. If

such a condition exists, a second switch is enabled during

the switch on time, which acts to pull the output side of the

inductor to ground. This “bridged” operation allows volt-

Shutdown

The LT3433 incorporates a low current shutdown mode

reduced to 10

µA. Pulling the SHDN pin down to 0.4V or

less activates shutdown mode.

Burst Mode Operation

The LT3433 employs low current Burst Mode functionality

to maximize efficiency during no load and low load condi-

tions. Burst Mode function is disabled by shorting the

function is enabled by shorting BURST_EN to SGND.

voltage, is below 30% of maximum, the Burst Mode

the 30% load level, that level of sense current is latched

into the IC. If the output load requires less than this latched

current level, the converter will overdrive the output slightly

during each switch cycle. This overdrive condition forces

is disabled, and the LT3433 shuts down most of its internal

circuitry, reducing quiescent current to 100

µA. When the

returns to normal operation and switching resumes.

Antislope Compensation

Most current mode switching controllers use slope com-

pensation to prevent current mode instability. The LT3433

is no exception. A slope compensation circuit imposes an

artificial ramp on the sensed current to increase the rising

slope as duty cycle increases. Unfortunately, this addi-

tional ramp corrupts the sensed current value, reducing

the achievable current limit value by the same amount as

the added ramp represents. As such, current limit is

typically reduced as duty cycles increase.