5

LT3433

3433ia

PI FU CTIO S

ing input of the error amplifier is connected to an internal

divider from the converter output. Values for the resistor

relation:

high value feedback resistors could cause a converter

output that is slightly higher than expected. Bias current

error at the output can be estimated as:

frequency through a “frequency-foldback” function. When

than the 200kHz typical operating frequency. The oscilla-

tor frequency slows with reduced voltage on the pin, down

maximum switch current is reduced to half of the normal

reaches 0.6V when the normal maximum switch current

level is restored. The frequency and current-limit foldback

functions add robustness to short-circuit protection and

help prevent inductor current runaway during start-up.

pin to ground. The output voltage of the LT3433 error

amplifier corresponds to the peak current sense amplifier

output detected before resetting the switch output(s). The

soft-start circuit forces the error amplifier output to a zero

peak current for start-up. A 5

µA current is forced from the

SS pin onto an external capacitor. As the SS pin voltage

ramps up, so does the LT3433 internally sensed peak cur-

rent limit. This forces the converter output current to ramp

from zero until normal output regulation is achieved. This

function reduces output overshoot on converter start-up.

SGND (Pins 1, 8, 9, 16): Low Noise Ground Reference.

ply rail is referenced to the SW_H pin. Supply voltage is

to the SW_H pin. A 1

µF capacitor is generally adequate for

most applications.

The charge on the bootstrap capacitor is refreshed through

a diode, typically connected from the converter output

operation assures that the boost capacitor is refreshed each

ply voltages up to 75V (absolute maximum) as referenced

to ground.

SW_H (Pin 3): Boosted Switch Output. This is the current

return for the boosted switch and corresponds to the emitter

of the switch transistor. The boosted switch shorts the

maximum efficiency. The “ON” resistance of the boosted

switch is 0.8

Ω.

to the boosted switch and corresponds to the collector of

the switch transistor.This pin also supplies power to most

externally. This supply will be subject to high switching

transient currents so this pin requires a high quality bypass

capacitor that meets whatever application-specific input

ripple current requirements exist. See Applications Infor-

mation.

BURST_EN (Pin 5): Burst Mode Enable Pin. Shorting this

pin to SGND enables Burst Mode operation. If Burst Mode

will disable the burst function.

pin corresponds to the maximum switch current per oscil-

lator cycle. The error amplifier is typically configured as an

integrator circuit by connecting an RC network from this

pin to ground. This circuit typically creates the dominant

pole for the converter regulation feedback loop. Specific in-

tegrator characteristics can be configured to optimize tran-

sient response. See Applications Information.