LTC3703
applicaTions inForMaTion
1. V supply current. The V current is the DC supply
Transient Response
CC
CC
currentgivenintheElectricalCharacteristicstablewhich
powers the internal control circuitry of the LTC3703.
Totalsupplycurrentistypicallyabout2.5mAandusually
results in a small (<1%) loss which is proportional to
Due to the high gain error amplifier and line feedforward
compensation of the LTC3703, the output accuracy due
to DC variations in input voltage and output load current
will be almost negligible. For the few cycles following a
loadtransient,however,theoutputdeviationmaybelarger
while the feedback loop is responding. Consider a typical
48Vinputto5Voutputapplicationcircuit,subjectedtoa1A
to 5A load transient. Initially, the loop is in regulation and
the DC current in the output capacitor is zero. Suddenly,
an extra 4A (= 5A – 1A) flows out of the output capacitor
while the inductor is still supplying only 1A. This sudden
V .
CC
2. DRV current is MOSFET driver current. This current
CC
resultsfromswitchingthegatecapacitanceofthepower
MOSFETs. Each time a MOSFET gate is switched on
and then off, a packet of gate charge Q moves from
G
DRV to ground. The resulting dQ/dt is a current out
CC
of the DRV supply. In continuous mode, I
=
CC
+ Q
DRVCC
f(Q
), where Q
and Q
are
change will generate a (4A) • (R ) voltage step at the
G(TOP)
G(BOT)
G(TOP)
G(BOT)
ESR
the gate charges of the top and bottom MOSFETs.
output; with a typical 0.015Ω output capacitor ESR, this
is a 60mV step at the output.
2
3. I R losses are predicted from the DC resistances of
MOSFETs, the inductor and input and output capacitor
ESR. In continuous mode, the average output current
flows through L but is “chopped” between the topside
MOSFET and the synchronous MOSFET. If the two
The feedback loop will respond and will move at the
bandwidth allowed by the external compensation network
towards a new duty cycle. If the unity-gain crossover
frequency is set to 50kHz, the COMP pin will get to 60%
of the way to 90% duty cycle in 3µs. Now the inductor is
seeing 43V across itself for a large portion of the cycle
and its current will increase from 1A at a rate set by di/
dt = V/L. If the inductor value is 10µH, the peak di/dt
will be 43V/10µH or 4.3A/µs. Sometime in the next few
microseconds after the switch cycle begins, the inductor
current will have risen to the 5A level of the load current
and the output voltage will stop dropping. At this point,
the inductor current will rise somewhat above the level
of the output current to replenish the charge lost from
the output capacitor during the load transient. With a
properly compensated loop, the entire recovery time will
be inside of 10µs.
MOSFETs have approximately the same R
, then
DS(ON)
the resistance of one MOSFET can simply be summed
2
with the DCR resistance of L to obtain I R losses. For
example, if each R
= 25mΩ and R = 25mΩ, then
DS(ON)
L
total resistance is 50mΩ. This results in losses ranging
from 1% to 5% as the output current increases from
1A to 5A for a 5V output.
4. Transition losses apply only to the topside MOSFET in
buck mode and they become significant when operat-
ing at higher input voltages (typically 20V or greater).
Transition losses can be estimated from the second
term of the P
equation found in the Power MOSFET
MAIN
Selection section.
Most loads care only about the maximum deviation from
ideal, whichoccurssomewhereinthefirsttwocyclesafter
the load step hits. During this time, the output capacitor
does all the work until the inductor and control loop regain
control. The initial drop (or rise if the load steps down) is
entirelycontrolledbytheESRofthecapacitorandamounts
to most of the total voltage drop. To minimize this drop,
choosealowESRcapacitorand/orparallelmultiplecapaci-
tors at the output. The capacitance value accounts for the
rest of the voltage drop until the inductor current rises.
The transition losses can become very significant at
the high end of the LTC3703 operating voltage range.
To improve efficiency, one may consider lowering the
frequency and/or using MOSFETs with lower C
at
RSS
the expense of higher R
.
DS(ON)
Other losses including C and C
ESR dissipative
OUT
IN
losses, Schottky conduction losses during dead time, and
inductor core losses generally account for less than 2%
total additional loss.
3703fc
26
Linear Systems [ Linear Systems ]
