LTC3703IGN Datasheet, PDF (26/34 Pages) Linear Integrated Systems – 100V Synchronous Switching Regulator

LTC3703
Applications Information
1. V_CC supply current. The V_CC current is the DC supply
current given in the Electrical Characteristics table which
powers the internal control circuitry of the LTC3703.
Total supply current is typically about 2.5mA and usually
results in a small (<1%) loss which is proportional to
V_CC.
2. DRV_CC current is MOSFET driver current. This current
results from switching the gate capacitance of the power
MOSFETs. Each time a MOSFET gate is switched on
and then off, a packet of gate charge Q_G moves from
DRV_CC to ground. The resulting dQ/dt is a current out
of the DRV_CC supply. In continuous mode, I_DRVCC =
f(Q_G(TOP) + Q_G(BOT)), where Q_G(TOP) and Q_G(BOT) are
the gate charges of the top and bottom MOSFETs.
3. I²R 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
MOSFETs have approximately the same R_DS(ON), then
the resistance of one MOSFET can simply be summed
with the DCR resistance of L to obtain I²R losses. For
example, if each R_DS(ON) = 25mΩ and R_L = 25mΩ, then
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_MAIN equation found in the Power MOSFET
Selection section.
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_RSS at
the expense of higher R_DS(ON).
Other losses including C_IN and C_OUT ESR dissipative
losses, Schottky conduction losses during dead time, and
inductor core losses generally account for less than 2%
total additional loss.
Transient Response
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
load transient, however, the output deviation may be larger
while the feedback loop is responding. Consider a typical
48V input to 5V output application circuit, subjected to a 1A
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
change will generate a (4A) • (R_ESR) voltage step at the
output; with a typical 0.015Ω output capacitor ESR, this
is a 60mV step at the output.
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.
Most loads care only about the maximum deviation from
ideal, which occurs somewhere in the first two cycles after
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
entirely controlled by the ESR of the capacitor and amounts
to most of the total voltage drop. To minimize this drop,
choose a low ESR capacitor and/or parallel multiple capaci-
tors at the output. The capacitance value accounts for the
rest of the voltage drop until the inductor current rises.
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