LTC3703IGN-5 Linear Technology, LTC3703IGN-5 Datasheet - Page 9

• Manufacturer Part Number: LTC3703IGN-5
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

OPERATIO
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 “com-
pensated” 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 operation
is enabled, until the inductor current reverses as deter-
mined by the reverse current comparator. MAX and MIN
comparators ensure that the output never exceed ±5% of
nominal value by monitoring V
back into regulation quickly by either keeping the top
MOSFET off or forcing maximum duty cycle. The opera-
tion of its other features—fast transient response, out-
standing line regulation, strong gate drivers, short-circuit
protection, and shutdown/soft-start—are described be-
low.
Fast Transient Response
The LTC3703 uses a fast 25MHz op amp as an error
amplifier. This allows the compensation network to be
optimized 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 frequencies. The 800mV internal reference al-
lows regulated output voltages as low as 800mV without
external level shifting amplifiers.
Line Feedforward Compensation
The LTC3703 achieves outstanding line transient response
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.
U
(Refer to Functional Diagram)
FB
and forcing the output
Strong Gate Drivers
The LTC3703 contains very low impedance drivers ca-
pable of supplying amps of current to slew large MOSFET
gates quickly. This minimizes transition losses and allows
paralleling MOSFETs for higher current applications. A
100V floating high side driver drives the top side 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 (see MOSFET Driver Supplies section). The bot-
tom side driver is supplied directly from the DRV
The top MOSFET drivers are biased from floating boot-
strap capacitor C
each off cycle through an external diode from DRV
the top 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 9V.
The bottom driver has an additional feature that helps
minimize the possibility of external MOSFET shoot-thru.
When the top MOSFET turns on, the switch node dV/dt
pulls up the bottom MOSFET’s internal gate through the
Miller capacitance, even when the bottom driver is holding
the gate terminal at ground. If the gate is pulled up high
enough, shoot-thru between the top side and bottom side
50mV/DIV
20V/DIV
2A/DIV
V
OUT
V
IN
I
L
Figure 2. Line Transient Performance
V
I
25V TO 60V V
LOAD
OUT
= 12V
= 1A
B
, which normally is recharged during
IN
STEP
20µs/DIV
LTC3703
3703 F02
CC
CC
when
3703fa
9
pin.