17
LTC1703
1703fa
use in LTC1703 circuits running from a 5V supply. As
current flows through this resistance while the MOSFET is
on, it generates I
2
R watts of heat, where I is the current
flowing (usually equal to the output current) and R is the
MOSFET R
DS(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
R
DS(ON)
improves heavy load efficiency at the expense of
additional gate charge (usually) and more cost (usually).
Proper choice of MOSFET R
DS(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 V
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 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 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 LTC1703 just switches the BG pin between
PGND and V
CC
. Driving QT is another matter. The source
of QT is connected to SW which rises to V
CC
when QT is
on. To keep QT on, the LTC1703 must get TG one MOSFET
V
GS(ON)
above V
CC
. It does this by utilizing a floating driver
with the negative lead of the driver attached to SW (the
source of QT) and the V
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 D
CP
when SW is low. This simple charge pump
keeps the TG driver alive even as it swings well above V
CC
.
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
increased beyond the 1µF 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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