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LT3844EFE-PBF

High Voltage, Current Mode Switching Regulator Controller with Programmable Operating Frequency

Linear Technology 

LT3844

U

OPERATIO (Refer to Functional Diagram)

exceeds the CSS pin voltage then the current threshold set

by the DC control voltage, VC, is decreased and the

inductor current is lowered. This in turn decreases the

output voltage slew rate allowing the CSS pin voltage ramp

to catch up to the VFB pin voltage. An internal 100mV offset

is added to the VFB pin voltage relative to the to CSS pin

voltage so that at start-up the soft-start circuit will dis-

charge the VC pin voltage below the DC control voltage

equivalent to zero inductor current. This will reduce the

input supply inrush current. The soft-start circuit is dis-

abled once the CSS pin voltage has been charged to 200mV

above the internal reference of 1.231V.

During a VIN UVLO, VCC UVLO or SHDN UVLO event, the

CSS pin voltage is discharged with a 50μA current source.

In normal operation the CSS pin voltage is clamped to a

diode above the VFB pin voltage. Therefore, the value of the

CSS capacitor is relevant in how long of a fault event will

retrigger a soft-start. In other words, if any of the above

UVLO conditions occur, the CSS pin voltage will be dis-

charged with a 50μA current source. There is a diode worth

of voltage headroom to ride through the fault before the

CSS pin voltage enters its active region and the soft-start

function is enabled.

Also, since the CSS pin voltage is clamped to a diode above

the VFB pin voltage, during a short circuit the CSS pin

voltage is pulled low because the VFB pin voltage is low.

Once the short has been removed the VFB pin voltage starts

to recover. The soft-start circuit takes control of the output

voltage slew rate once the VFB pin voltage has exceeded

the slowly ramping CSS pin voltage, reducing the output

voltage overshoot during a short circuit recovery.

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.

Typically, this additional ramp 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 LT3844, however, contains antislope com-

pensation circuitry to eliminate the current limit reduction

associated with slope compensation. As the slope com-

pensation 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 LT3844

can provide full power regardless of required duty cycle.

Shutdown

The LT3844 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

shutdown or as a general purpose on/off control of the

converter output. The shutdown function has two thresh-

olds. The first threshold, a precision 1.23V threshold with

120mV of hysteresis, disables the converter from switch-

ing. The second threshold, approximately a 0.7V refer-

enced to SGND, completely disables all internal circuitry

and reduces the VIN current to less than 10μA. See the

Application Information section for more information.

APPLICATIO S I FOR ATIO

The basic LT3844 step-down (buck) application, shown in

the Typical Application on the front page, converts a larger

positive input voltage to a lower positive or negative

output voltage. This Application Information section as-

sists selection of external components for the require-

ments of the power supply.

RSENSE Selection

The current sense resistor, RSENSE, monitors the inductor

current of the supply (See Typical Application on front

page). Its value is chosen based on the maximum required

output load current. The LT3844 current sense amplifier

has a maximum voltage threshold of, typically, 100mV.

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