LTC1702IGN datasheet(17/36 Pages) LINER

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LTC1702

charge, actual parasitic drain-source capacitance and

Miller-multiplied gate-drain capacitance, but can be ap-

proximated as a single capacitance from gate to source.

Regardless of 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 LTC1702’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 LTC1702, 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 LTC1702 itself.

TG Charge Pump

There’s another nuance of MOSFET drive that the LTC1702

needs to get around. The LTC1702 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 LTC1702 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 LTC1702 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

APPLICATIONS INFORMATION

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

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

the 1

µF value.

INPUT SUPPLY

The BiCMOS process that allows the LTC1702 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

LTC1702 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 LTC1702 input voltage to

dip. In most typical applications where the LTC1702 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 LTC1702 circuit running from a 5V logic supply

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

implies a duty cycle of 32%, which means QT is on 32%

of each switching cycle. During QT’s on-time, the current

drawn from the input equals the load current and during

the rest of the cycle, the current drawn from the input is

near zero. This 0A to 10A, 32% duty cycle pulse train adds

up to 4.7ARMS at the input. At 550kHz, switching cycles

last about 1.8

µs—most system logic supplies have no

hope of regulating output current with that kind of speed.

A local input bypass capacitor is required to make up the

difference and prevent the input supply from dropping

drastically when QT kicks on. This capacitor is usually

chosen for RMS ripple current capability and ESR as well

as value.

The input bypass capacitor in an LTC1702 circuit is

common to both channels. Consider our 10A example

case with the other side of the LTC1702 disabled. The input

bypass capacitor gets exercised in three ways: its ESR

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