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LTC1702A

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where the charge is going, the fact remains that it all has

to come out of VCC to turn the MOSFET gate on, and when

the MOSFET is turned back off, that charge all ends up at

ground. In the meanwhile, it travels through the LTC1702A’s

gate drivers, heating them up. More power lost!

In this case, the power is lost in little bite-sized chunks, one

chunk per switch per cycle, with the size of the chunk set

by the gate charge of the MOSFET. Every time the MOSFET

switches, another chunk is lost. Clearly, the faster the

clock runs, the more important gate charge becomes as a

loss term. Old-fashioned switchers that ran at 20kHz could

pretty much ignore gate charge as a loss term; in the

550kHz LTC1702A, gate charge loss can be a significant

efficiency penalty. Gate charge loss can be the dominant

loss term at medium load currents, especially with large

MOSFETs. Gate charge loss is also the primary cause of

power dissipation in the LTC1702A itself.

TG Charge Pump

There’s another nuance of MOSFET drive that the LTC1702A

needs to get around. The LTC1702A is designed to use N-

channel MOSFETs for both QT and QB, primarily because

N-channel MOSFETs generally cost less and have lower

RDS(ON) than similar P-channel MOSFETs. Turning QB on

is no big deal since the source of QB is attached to PGND;

the LTC1702A just switches the BG pin between PGND and

VCC. Driving QT is another matter. The source of QT is

connected to SW which rises to VCC when QT is on. To

keep QT on, the LTC1702A must get TG one MOSFET

VGS(ON) above VCC. It does this by utilizing a floating driver

with the negative lead of the driver attached to SW (the

source of QT) and the VCC lead of the driver coming out

separately at BOOST. An external 1

µF capacitor CCP con-

nected between SW and BOOST (Figure 2) supplies power

to BOOST when SW is high, and recharges itself through

DCP when SW is low. This simple charge pump keeps the

TG driver alive even as it swings well above VCC. The value

of the bootstrap capacitor CCP needs to be at least 100

times that of the total input capacitance of the topside

MOSFET(s). For very large external MOSFETs (or multiple

MOSFETs in parallel), CCP may need to be increased over

the 1

µF value.

APPLICATIONS INFORMATION

MOSFET Gate Series Resistors

Traditional switching regulator designs often include re-

sistors in series with the external MOSFET gates to control

gate rise and fall time, usually to control ringing at the

switching node. In LTC1702A applications, such resistors

prevent the onboard drivers from turning off the external

MOSFETs quickly, causing shoot through currents and

erratic operation when both top and bottom MOSFETs are

on simultaneously. Gate resistors are not recommended

in LTC1702A applications. In some cases, long PCB traces

can add enough impedance to the gate to hinder turn off.

Keep gate traces as short and wide as practical.

Switch node ringing can be reduced by adding a low value

resistor (usually 5

Ω to 10Ω) in series with the BOOST pin.

This will slow TG rise time without affecting MOSFET turn

off, allowing the LTC1702A to keep shoot through current

under control. This resistor will typically improve the

waveform at the switch node but it reduces efficiency and

increases power dissipation in the top MOSFET. Use with

caution.

INPUT SUPPLY

The BiCMOS process that allows the LTC1702A to include

large MOSFET drivers on-chip also limits the maximum

input voltage to 7V. This limits the practical maximum

input supply to a loosely regulated 5V or 6V rail. The

LTC1702A will operate properly with input supplies down

to about 3V, so a typical 3.3V supply can also be used if the

external MOSFETs are chosen appropriately (see the Power

MOSFETs section).

At the same time, the input supply needs to supply several

amps of current without excessive voltage drop. The input

supply must have regulation adequate to prevent sudden

load changes from causing the LTC1702A input voltage to

dip. In most typical applications where the LTC1702A is

generating a secondary low voltage logic supply, all of

these input conditions are met by the main system logic

supply when fortified with an input bypass capacitor.

Input Bypass

A typical LTC1702A circuit running from a 5V logic supply

might provide 1.6V at 10A at one of its outputs. 5V to 1.6V

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