6

LTC1430

OVERVIEW

The LTC1430 is a voltage feedback PWM switching regu-

lator controller (see Block Diagram) designed for use in

high power, low voltage step-down (buck) converters. It

includes an onboard PWM generator, a precision refer-

ence trimmed to

±0.5%, two high power MOSFET gate

drivers and all necessary feedback and control circuitry to

form a complete switching regulator circuit. The PWM

loop nominally runs at 200kHz.

The 16-lead versions of the LTC1430 include a current

limit sensing circuit that uses the upper external power

MOSFET as a current sensing element, eliminating the

need for an external sense resistor.

Also included in the 16-lead version is an internal soft-

start feature that requires only a single external capacitor

to operate. In addition, 16-lead parts feature an adjustable

oscillator which can run at frequencies from 50kHz to

beyond 500kHz, allowing added flexibility in external com-

ponent selection. The 8-lead versions do not include

current limit, internal soft-start or frequency adjustability.

THEORY OF OPERATION

Primary Feedback Loop

The LTC1430 senses the output voltage of the circuit at the

feeds this voltage back to the internal transconductance

amplifier FB. FB compares the resistor-divided output

voltage to the internal 1.26V reference and outputs an

error signal to the PWM comparator. This is then com-

pared to a fixed frequency sawtooth waveform generated

by the internal oscillator to generate a pulse width modu-

lated signal. This PWM signal is fed back to the external

MOSFETs through G1 and G2, closing the loop. Loop

compensation is achieved with an external compensation

network at COMP, the output node of the FB transconduc-

tance amplifier.

MIN, MAX Feedback Loops

Two additional comparators in the feedback loop provide

high speed fault correction in situations where the FB

amplifier may not respond quickly enough. MIN compares

the feedback signal to a voltage 40mV (3%) below the

internal reference. At this point, the MIN comparator

overrides the FB amplifier and forces the loop to full duty

cycle, set by the internal oscillator at about 90%. Similarly,

the MAX comparator monitors the output voltage at 3%

above the internal reference and forces the output to 0%

duty cycle when tripped. These two comparators prevent

extreme output perturbations with fast output transients,

while allowing the main feedback loop to be optimally

compensated for stability.

Current Limit Loop

The 16-lead LTC1430 devices include yet another feed-

back loop to control operation in current limit. The current

monitors the voltage drop across external MOSFET M1

As the peak current rises, the drop across M1 due to its

that M1’s drain current has exceeded the maximum level,

capacitor, cutting the duty cycle and controlling the output

generates a signal to disable the MIN comparator to

prevent it from conflicting with the current limit circuit. If

the internal feedback node drops below about 0.8V, indi-

cating a severe output overload, the circuitry will force the

internal oscillator to slow down by a factor of as much as

100. If desired, the turn on time of the current limit loop

can be controlled by adjusting the size of the soft-start

capacitor, allowing the LTC1430 to withstand short over-

current conditions without limiting.

the current limit circuit eliminates the sense resistor that

would otherwise be required and minimizes the number of

components in the external high current path. Because

with temperature, the LTC1430 current limit is not de-

signed to be accurate; it is meant to prevent damage to the

power supply circuitry during fault conditions. The actual

current level where the limiting circuit begins to take effect

may vary from unit to unit, depending on the power

MOSFETs used. See Soft-Start and Current Limit for more

details on current limit operation.

APPLICATIO S I FOR ATIO