LTC1705 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1705
Linear Technology
'LTC1705' PDF : 28 Pages View PDF
LTC1705
APPLICATIO S I FOR ATIO
EXTERNAL COMPONENT SELECTION
Power MOSFETs
Getting peak efficiency out of the LTC1705 depends strongly
on the external MOSFETs used. The LTC1705 requires at
least two external MOSFETs per side—more if one or
more of the MOSFETs are paralleled to lower on-resis-
tance. To work efficiently, these MOSFETs must exhibit
low RDS(ON) at 5V VGS to minimize resistive power loss
while they are conducting current. They must also have
low gate charge to minimize transition losses during
switching. On the other hand, voltage breakdown require-
ments in a typical LTC1705 circuit are pretty tame: the 6V
maximum input voltage limits the VDS and VGS the MOSFETs
can see to safe levels for most devices.
Low RDS(ON)
RDS(ON) calculations are pretty straightforward. RDS(ON) is
the resistance from the drain to the source of the
MOSFETwhen the gate is fully on. Many MOSFETs have
RDS(ON) specified at 4.5V gate drive—this is the right
number to use in LTC1705 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 LTC1705 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 LTC1705 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 PVCC 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 LTC1705’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 LTC1705, 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 LTC1705 itself.
TG Charge Pump
There’s another nuance of MOSFET drive that the LTC1705
needs to get around. The LTC1705 is designed to use
N-channel MOSFETs for both QT and QB, primarily be-
cause N-channel MOSFETs generally cost less and have
lower RDS(ON) than similar P-channel MOSFETs. Turning
QB on is simple since the source of QB is attached to
PGND; the LTC1705 just switches the BG pin between
PGND and PVCC . Driving QT is another matter. The source
of QT is connected to SW which rises to PVCC when QT is
on. To keep QT on, the LTC1705 must get TG one MOSFET
VGS(ON) above PVCC . It does this by utilizing a floating
driver with the negative lead of the driver attached to SW
(the source of QT) and the PVCC lead of the driver coming
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