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LT3844

3844fb

OPERATION (Refer to Functional Diagram)

The LT3844 is a PWM controller with a constant frequency,

current mode control architecture. It is designed for low

to medium power, switching regulator applications. Its

high operating voltage capability allows it to step up

or down input voltages up to 60V without the need for

a transformer. The LT3844 is used in nonsynchronous

applications, meaning that a freewheeling rectiﬁer diode

(D1 of Function Diagram) is used instead of a bottom

side MOSFET. For circuit operation, please refer to the

Functional Diagram of the IC and Typical Application on

the front page of the data sheet. The LT3800 is a similar

part that uses synchronous rectiﬁcation, replacing the

diode with a MOSFET in a step-down application.

Main Control Loop

During normal operation, the external N-channel MOSFET

switch is turned on at the beginning of each cycle. The

switch stays on until the current in the inductor exceeds

a current threshold set by the DC control voltage, VC,

which is the output of the voltage control loop. The voltage

control loop monitors the output voltage, via the VFB pin

voltage, and compares it to an internal 1.231V reference.

It increases the current threshold when the VFB voltage

is below the reference voltage and decreases the current

threshold when the VFB voltage is above the reference

voltage. For instance, when an increase in the load current

occurs, the output voltage drops causing the VFB voltage to

drop relative to the 1.231V reference. The voltage control

loop senses the drop and increases the current threshold.

The peak inductor current is increased until the average

inductor current equals the new load current and the output

voltage returns to regulation.

Current Limit/Short-Circuit

The inductor current is measured with a series sense

resistor (see the Typical Application on the front page).

When the voltage across the sense resistor reaches the

maximum current sense threshold, typically 100mV, the

TG MOSFET driver is disabled for the remainder of that

cycle. If the maximum current sense threshold is still ex-

ceeded at the beginning of the next cycle, the entire cycle

is skipped. Cycle skipping keeps the inductor currents to a

reasonable value during a short-circuit, particularly when

VIN is high. Setting the sense resistor value is discussed

in the “Application Information” section.

VCC/Boosted Supply

An internal VCC regulator provides VIN derived gate-drive

power for start-up under all operating conditions with

MOSFET gate charge loads up to 90nC. The regulator can

operate continuously in applications with VINvoltagesupto

60V, provided the power dissipation of the regulator does

not exceed 250mW. The power dissipation is calculated

as follows:

Pd(REG) = (VIN – 8V) • fSW • QG

where QG is the MOSFET gate charge.

In applications where these conditions are exceeded, VCC

must be derived from an external source after start-up.

Maximum continuous regulator power dissipation may be

exceeded for short duration VIN transients.

For higher converter efﬁciency and less power dissipa-

tion in the IC, VCC can also be supplied from an external

supply such as the converter output. When an external

supply back drives the internal VCC regulator through an

external diode and the VCC voltage is pulled to a diode

above its regulation voltage, the internal regulator is dis-

abled and goes into a low current mode. VCC is the bias

supply for most of the internal IC functions and is also

used to charge the bootstrapped capacitor (CBOOST) via an

external diode. The external MOSFET switch is biased from

the bootstrapped capacitor. While the external MOSFET

switch is off, an internal BJT switch, whose collector is

connected to the SW pin and emitter is connected to the

PGND pin, is turned on to pull the SW node to PGND and

recharge the bootstrap capacitor. The switch stays on until

either the start of the next cycle or until the bootstrapped

capacitor is fully charged.

MOSFET Driver

The LT3844 contains a high speed boosted driver to turn

on and off an external N-channel MOSFET switch. The

MOSFET driver derives its power from the boost capaci-

tor which is referenced to the SW pin and the source of

the MOSFET. The driver provides a large pulse of current

to turn on the MOSFET fast to minimize transition times.

Multiple MOSFETs can be paralleled for higher current

operation.

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