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LTC3713

3713fa

converter. To ensure that the ripple current doesn’t exceed

a specified amount, the inductance can be chosen accord-

ing to the following equation:

IN MIN

IN MAX

OUT BOOST













∆

()

–

•

Diode D3 Selection

A Schottky diode is recommended for use in the boost

converter section. The Motorola MBR0520 is a very good

choice.

Boost Converter Output Capacitor

Because the LTC3713’s boost converter is internally com-

pensated, loop stability must be carefully considered when

choosing its output capacitor. Small, low cost tantalum

capacitors have some ESR, which aids stability. However,

ceramic capacitors are becoming more popular, having

attractive characteristics such as near-zero ESR, small size

and reasonable cost. Simply replacing a tantalum output

capacitor with a ceramic unit will decrease the phase margin,

in some cases to unacceptable levels. With the addition of

a phase-lead capacitor and isolating resistor, the boost

converter portion of the LTC3713 can be used success-

fully with ceramic output capacitors.

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Although all dissipative

elements in the circuit produce losses, four main sources

account for most of the losses in LTC3713 circuits:

1. DC I2R losses. These arise from the resistances of the

MOSFETs, inductor and PC board traces and cause the

efficiency to drop at high output currents. In continuous

mode the average output current flows through L, but is

chopped between the top and bottom MOSFETs. If the two

MOSFETs have approximately the same RDS(ON), then the

resistance of one MOSFET can simply be summed with the

resistances of L and the board traces to obtain the DC I2R

be estimated from:

CSS > COUT • VOUT • RSENSE (10–4 [F/V s])

Generally 0.1

µF is more than sufficient.

Overcurrent latchoff operation is not always needed or

desired. Load current is already limited during a short-

circuit by the current foldback circuitry and latchoff

operation can prove annoying during troubleshooting.

The feature can be overridden by adding a pull-up current

greater than 5

µA to the RUN/SS pin. The additional

current prevents the discharge of CSS during a fault and

also shortens the soft-start period. Using a resistor to VIN

as shown in Figure 6a is simple, but slightly increases

shutdown current. Connecting a resistor to INTVCC as

shown in Figure 6b eliminates the additional shutdown

current, but requires a diode to isolate CSS. Any pull-up

network must be able to pull RUN/SS above the 4.2V

maximum threshold of the latchoff circuit and overcome

the 4

µA maximum discharge current.

INTVCC Supply

The 5V supply that powers the drivers and internal cir-

cuitry within the LTC3713 can be supplied by either an

internal P-channel low dropout regulator if VIN is greater

than 5V or the internal boost regulator if VIN is less than 5V.

The INTVCC pin can supply up to 50mA RMS and must be

bypassed to ground with a minimum of 4.7

µF tantalum or

other low ESR capacitor. Good bypassing is necessary to

supply the high transient currents required by the MOSFET

gate drivers. Applications using large MOSFETs with a

high input voltage and high frequency of operation may

cause the LTC3713 to exceed its maximum junction tem-

perature rating or RMS current rating. In continuous mode

operation, this current is IGATECHG = f(Qg(TOP) + Qg(BOT)).

The junction temperature can be estimated from the

equations given in Note 2 of the Electrical Characteristics.

Inductor Selection for Boost Converter

For the boost converter, the inductance should be 4.7

µH

for input voltages less then 3.3V and 10

µH for inputs

above 3.3V. The inductor should have a saturation current

rating of approximately 0.5A or greater. A guide for select-

ing an inductor for the boost converter is to choose a ripple

current that is 40% of the current supplied by the boost

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