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3433EFE

High Voltage Step-Up/Step-Down DC/DC Converter

Linear Technology 

LT3433

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 100µA in

no load conditions. A low current shutdown mode can also

be activated, reducing total quiescent current to 10µA.

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

internal low voltage linear regulator. The output of this

regulator is brought out to the VBIAS pin, allowing bypass-

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

dissipation associated with the internal VIN to VBIAS

regulator.

Theory of Operation (See Block Diagram)

The LT3433 senses converter output voltage via the VFB

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

internal 1.231V reference is amplified to generate an error

voltage on the VC pin which is, in turn, used as a threshold

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

until the sensed switch current exceeds the VC-derived

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-

eration of the VBST bootstrapped supply.

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

8

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-

age conversion to continue when VOUT approaches or

exceeds VIN.

Shutdown

The LT3433 incorporates a low current shutdown mode

where all IC functions are disabled and the VIN current is

reduced to 10µA. 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

BURST_EN pin to either VBIAS or VOUT. Burst Mode

function is enabled by shorting BURST_EN to SGND.

In certain wide current range applications, the IC could

enter burst operation during normal load conditions. If the

additional output ripple and noise generated by Burst

Mode operation is not desired for normal operation,

BURST_EN can be biased using an external supply that is

disabled during a no-load condition. This enables Burst

Mode operation only when it is required. The BURST_EN

pin typically draws 35µA when Burst Mode operation is

disabled (VBURST_EN ≥ 2V) and will draw no more than

75µA with VBURST_EN = 2V.

When the required switch current, sensed via the VC pin

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

function is employed. When the voltage on VC drops below

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

the voltage on the VC pin to continue to drop. When the

voltage on VC drops below the 15% load level, switching

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

circuitry, reducing quiescent current to 100µA. When the

voltage on the VC pin climbs back to 20% load level, the IC

returns to normal operation and switching resumes.

3433f

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