LTC3703
operaTion (Refer to Functional Diagram)
drop in the feedback voltage relative to the reference. The
COMP voltage then rises, increasing the duty ratio until
the output feedback voltage again matches the reference
voltage. In normal operation, the top MOSFET is turned
on when the RS latch is set by the on-chip oscillator and
is turned off when the PWM comparator trips and resets
the latch. The PWM comparator trips at the proper duty
ratio by comparing the error amplifier output (after being
“compensated” by the line feedforward multiplier) to a
sawtooth waveform generated by the oscillator. When the
top MOSFET is turned off, the bottom MOSFET is turned
on until the next cycle begins or, if pulse-skip mode op-
eration is enabled, until the inductor current reverses as
determined by the reverse current comparator. MAX and
MIN comparators ensure that the output never exceed
V
OUT
50mV/DIV
V
IN
20V/DIV
I
L
2A/DIV
3703 F02
V
LOAD
25V TO 60V V STEP
= 12V
= 1A
20µs/DIV
OUT
I
IN
Figure 2. Line Transient Performance
Strong Gate Drivers
5% of nominal value by monitoring V and forcing the
FB
TheLTC3703containsverylowimpedancedriverscapable
of supplying amps of current to slew large MOSFET gates
quickly. Thisminimizestransitionlossesandallowsparal-
leling MOSFETs for higher current applications. A 100V
floating high side driver drives the topside MOSFET and
a low side driver drives the bottom side MOSFET (see
Figure 3). They can be powered from either a separate
DC supply or a voltage derived from the input or output
voltage(seeMOSFETDriverSuppliessection).Thebottom
outputbackintoregulationquicklybyeitherkeepingthetop
MOSFETofforforcingmaximumdutycycle.Theoperation
ofitsotherfeatures—fasttransientresponse,outstanding
lineregulation,stronggatedrivers,short-circuitprotection
and shutdown/soft-start—are described below.
Fast Transient Response
TheLTC3703usesafast25MHzopampasanerrorampli-
fier.Thisallowsthecompensationnetworktobeoptimized
for better load transient response. The high bandwidth of
the amplifier, along with high switching frequencies and
low value inductors, allow very high loop crossover fre-
quencies. The 800mV internal reference allows regulated
output voltages as low as 800mV without external level
shifting amplifiers.
side driver is supplied directly from the DRV pin. The
CC
top MOSFET drivers are biased from floating bootstrap
capacitor, C , whichnormallyisrechargedduringeachoff
B
cycle through an external diode from DRV when the top
CC
MOSFET turns off. In pulse-skip mode operation, where
it is possible that the bottom MOSFET will be off for an
extended period of time, an internal counter guarantees
that the bottom MOSFET is turned on at least once every
10 cycles for 10% of the period to refresh the bootstrap
capacitor. An undervoltage lockout keeps the LTC3703
shut down unless this voltage is above 8.7V.
Line Feedforward Compensation
TheLTC3703achievesoutstandinglinetransientresponse
using a patented feedforward correction scheme. With
this circuit the duty cycle is adjusted instantaneously to
changes in input voltage, thereby avoiding unacceptable
overshoot or undershoot. It has the added advantage of
making the DC loop gain independent of input voltage.
Figure 2 shows how large transient steps at the input have
little effect on the output voltage.
The bottom driver has an additional feature that helps
minimizethepossibilityofexternalMOSFETshoot-through.
When the top MOSFET turns on, the switch node dV/dt
pulls up the bottom MOSFET’s internal gate through the
Millercapacitance, evenwhenthebottomdriverisholding
the gate terminal at ground. If the gate is pulled up high
enough, shoot-through between the topside and bottom
3703fc
9
Linear Systems [ Linear Systems ]
