LTC1702A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1702A' PDF : 36 Pages View PDF
LTC1702A
APPLICATIONS INFORMATION
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 LTC1702A’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 LTC1702A, 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 LTC1702A itself.
TG Charge Pump
There’s another nuance of MOSFET drive that the LTC1702A
needs to get around. The LTC1702A 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 LTC1702A 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 LTC1702A 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 1µF 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
MOSFET(s). For very large external MOSFETs (or multiple
MOSFETs in parallel), CCP may need to be increased over
the 1µF value.
MOSFET Gate Series Resistors
Traditional switching regulator designs often include re-
sistors in series with the external MOSFET gates to control
gate rise and fall time, usually to control ringing at the
switching node. In LTC1702A applications, such resistors
prevent the onboard drivers from turning off the external
MOSFETs quickly, causing shoot through currents and
erratic operation when both top and bottom MOSFETs are
on simultaneously. Gate resistors are not recommended
in LTC1702A applications. In some cases, long PCB traces
can add enough impedance to the gate to hinder turn off.
Keep gate traces as short and wide as practical.
Switch node ringing can be reduced by adding a low value
resistor (usually 5Ω to 10Ω) in series with the BOOST pin.
This will slow TG rise time without affecting MOSFET turn
off, allowing the LTC1702A to keep shoot through current
under control. This resistor will typically improve the
waveform at the switch node but it reduces efficiency and
increases power dissipation in the top MOSFET. Use with
caution.
INPUT SUPPLY
The BiCMOS process that allows the LTC1702A 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
LTC1702A 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 LTC1702A input voltage to
dip. In most typical applications where the LTC1702A 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 LTC1702A circuit running from a 5V logic supply
might provide 1.6V at 10A at one of its outputs. 5V to 1.6V
1702afa
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