LTC1702A datasheet(15/24 Pages) LINER

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LT3844

3844fb

APPLICATIONS INFORMATION

VSUPPLY(ON) is the input voltage at which the undervoltage

lockout is disabled and the supply turns on.

Example: Select RB = 49.9kΩ, VSUPPLY(ON) = 14.5V (based

on a 15V minimum input voltage)

A =

⎛

⎝⎜

⎞

⎠⎟

49 9

14 5

135

.•

–

= 486.1k (499k resistor is selected)

If low supply current in standby mode is required, select

a higher value of RB.

The supply turn off voltage is 9% below turn on. In the

example the VSUPPLY(OFF) would be 13.2V.

If additional hysteresis is desired for the enable function,

an external positive feedback resistor can be used from

the LT3844 regulator output.

The shutdown function can be disabled by connecting the

SHDN pin to the VIN through a large value pull-up resistor.

This pin contains a low impedance clamp at 6V, so the SHDN

pin will sink current from the pull-up resistor(RPU):

ISHDN =

–6

Because this arrangement will clamp the SHDN pin to the

6V, it will violate the 5V absolute maximum voltage rating of

the pin. This is permitted, however, as long as the absolute

maximum input current rating of 1mA is not exceeded.

Input SHDN pin currents of <100μA are recommended:

a 1M or greater pull-up resistor is typically used for this

conﬁguration.

Soft-Start

The desired soft-start time (tSS) is programmed via the

CSS capacitor as follows:

1 231

μ •

The amount of time in which the power supply can withstand

a VIN, VCC or VSHDN UVLO fault condition (tFAULT) before

the CSS pin voltage enters its active region is approximated

by the following formula:

FAULT

•.

065

Oscillator SYNC

The oscillator can be synchronized to an external clock.

Set the RSET resistor at least 10% below the desired sync

frequency.

It is recommended that the SYNC pin be driven with a

square wave that has amplitude greater than 2V, pulse

width greater than 1ms and rise time less than 500ns. The

rising edge of the sync wave form triggers the discharge

of the internal oscillator capacitor.

Efﬁciency Considerations

The efﬁciency of a switching regulator is equal to the output

power divided by the input power times 100%. Express

percent efﬁciency as:

% Efﬁciency = 100% - (L1 + L2 + L3 + ...)

where L1, L2, etc. are individual loss terms as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, four main contributors usually account for most

of the losses in LT3844 circuits:

1. LT3844 VIN and VCC current loss

2. I2R conduction losses

3. MOSFET transition loss

4. Schottky diode conduction loss

1. The VIN and VCC currents are the sum of the quiescent

currents of the LT3844 and the MOSFET drive currents.

The quiescent currents are in the LT3844 Electrical Char-

acteristics table. The MOSFET drive current is a result

of charging the gate capacitance of the power MOSFET

each cycle with a packet of charge, QG. QG is found in

the MOSFET data sheet. The average charging current is

calculated as QG • fSW. The power loss term due to these

currents can be reduced by backdriving VCC with a lower

voltage than VIN such as VOUT.

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