3724MPFE datasheet(11/24 Pages) LINER

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LT3724

3724fc

OPERATION

Soft-Start

The soft-start function controls the slew rate of the power

supply output voltage during start-up. A controlled output

voltage ramp minimizes output voltage overshoot, reduces

inrush current from the VIN supply, and facilitates supply

sequencing. A capacitor, CSS, connected between VOUT of

the supply and the CSS pin of the IC, programs the slew

rate. The capacitor provides a current to the CSS pin which

is proportional to the dV/dt of the output voltage. The

soft-start circuit overrides the control loop and adjusts the

inductor current until the output voltage slew rate yields a

2μA current through the soft-start capacitor. If the current is

greater than 2μA, then the current threshold set by the DC

control voltage, VC, is decreased and the inductor current

is lowered. This in turn lowers the output current and the

output voltage slew rate is decreased. If the current is less

than 2μA, then the current threshold set by the DC control

voltage, VC, is increased and the inductor current is raised.

This in turn increases the output current and the output

voltage slew rate is increased. Once the output voltage is

within 5% of its regulation voltage, the soft-start circuit

is disabled and the main control regulates the output. The

soft-start circuit is reactivated when the output voltage

drops below 70% of its regulation voltage.

Slope/Antislope Compensation

The IC incorporates slope compensation to eliminate

potential subharmonic oscillations in the current control

loop. The IC’s slope compensation circuit imposes an

artiﬁcial ramp on the sensed current to increase the rising

slope as duty cycle increases.

Unfortunately, this additional ramp typically affects the

sensed current value, thereby reducing the achievable

current limit value by the same amount as the added ramp

represents. As such, the current limit is typically reduced

as the duty cycle increases. The LT3724, however, contains

antislope compensation circuitry to eliminate the current

limit reduction associated with slope compensation. As the

slope compensation ramp is added to the sensed current,

a similar ramp is added to the current limit threshold. The

end result is that the current limit is not compromised so

the LT3724 can provide full power regardless of required

duty cycle.

Shutdown

The LT3724 includes a shutdown mode where all the

internal IC functions are disabled and the VIN current is

reduced to less than 10μA. The shutdown pin can be used

for undervoltage lockout with hysteresis, micropower shut-

down or as a general purpose on/off control of the converter

output. The shutdown function has two thresholds. The

ﬁrst threshold, a precision 1.23V threshold with 120mV

of hysteresis, disables the converter from switching. The

second threshold, approximately a 0.7V referenced to

SGND, completely disables all internal circuitry and reduces

the VIN current to less than 10μA. See the Application

Information section for more information.

(Refer to Functional Diagram)

3724MPFE Datenblatt(PDF) 11 Page - Linear Technology

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