LTC1704/LTC1704B
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APPLICATIO S I FOR ATIO
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Gate Charge
Gate charge is amount of charge (essentially, the number
of electrons) that the LTC1704 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 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 PV
CC
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 LTC1704’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 LTC1704, 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 LTC1704 itself.
TG Charge Pump
There’s another nuance of MOSFET drive that the LTC1704
needs to get around. The LTC1704 is designed to use
N-channel MOSFETs for both QT and QB, primarily be-
cause N-channel MOSFETs generally cost less and have
lower R
DS(ON)
than similar P-channel MOSFETs. Turning
QB on is no big deal since the source of QB is attached to
PGND; the LTC1704 just switches the BG pin between
PGND and PV
CC
. Driving QT is another matter. The source
of QT is connected to SW which rises to V
IN
when QT is on.
To keep QT on, the LTC1704 must get TG one MOSFET
V
GS(ON)
above V
IN
. It does this by utilizing a floating driver
with the negative lead of the driver attached to SW (the
source of QT) and the PV
CC
lead of the driver coming out
separately at BOOST. An external 1µF capacitor (C
CP
)
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 V
IN
.
The value of the bootstrap capacitor C
CP
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), C
CP
may need to be in-
creased beyond the 1µF value.
Input Supply
The BiCMOS process that allows the LTC1704 switcher
supply to include large MOSFET drivers on-chip also limits
the maximum input voltage to 6V. This limits the practical
maximum input supply to a loosely regulated 5V or 6V rail.
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 LTC1704 input voltage to
dip. In most typical applications where the LTC1704 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 Capacitor Selection
A typical LTC1704 circuit running from a 5V logic supply
might provide 1.6V at 10A at its switcher output. 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 results in 4.66A
RMS
ripple current. At 550kHz, switch-
ing cycles last about 1.8µs; 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.
Consider our 10A example. The input bypass capacitor
gets exercised in three ways: its ESR must be low enough
