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LTC1703

use in LTC1703 circuits running from a 5V supply. As

current flows through this resistance while the MOSFET is

on, it generates I2R watts of heat, where I is the current

flowing (usually equal to the output current) and R is the

MOSFET RDS(ON). This heat is only generated when the

MOSFET is on. When it is off, the current is zero and the

power lost is also zero (and the other MOSFET is busy

losing power).

This lost power does two things: it subtracts from the

power available at the output, costing efficiency, and it

makes the MOSFET hotter—both bad things. The effect is

worst at maximum load when the current in the MOSFETs

and thus the power lost are at a maximum. Lowering

RDS(ON) improves heavy load efficiency at the expense of

additional gate charge (usually) and more cost (usually).

Proper choice of MOSFET RDS(ON) becomes a trade-off

between tolerable efficiency loss, power dissipation and

cost. Note that while the lost power has a significant effect

on system efficiency, it only adds up to a watt or two in a

typical LTC1703 circuit, allowing the use of small, surface

mount MOSFETs without heat sinks.

Gate Charge

Gate charge is amount of charge (essentially, the number

of electrons) that the LTC1703 needs to put into the gate

of an external MOSFET to turn it on. The easiest way to

visualize gate charge is to think of it as a capacitance from

the gate pin of the MOSFET to SW (for QT) or to PGND (for

QB). This capacitance is composed of MOSFET channel

charge, actual parasitic drain-source capacitance and

Miller-multiplied gate-drain capacitance, but can be

approximated 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 LTC1703’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 LTC1703, 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 LTC1703 itself.

TG Charge Pump

There’s another nuance of MOSFET drive that the LTC1703

needs to get around. The LTC1703 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 LTC1703 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 LTC1703 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)

connected 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 beyond the 1

µF value.

INPUT SUPPLY

The BiCMOS process that allows the LTC1703 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

LTC1703 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).

APPLICATIO S I FOR ATIO

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