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LTC3703

3703f

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 VFB and forcing the output

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.

(Refer to Functional Diagram)

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 DRVCC pin.

The top MOSFET drivers are biased from floating boot-

strap capacitor CB, which normally is recharged during

each off cycle through an external diode from DRVCC when

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

µs/DIV

VOUT

50mV/DIV

VOUT = 12V

ILOAD = 1A

25V TO 60V VIN STEP

VIN

20V/DIV

2A/DIV

3703 F02

Figure 2. Line Transient Performance

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