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LTC1649

OVERVIEW

The LTC1649 is a voltage feedback PWM switching regu-

lator controller (see Block Diagram) designed for use in

high power, low input voltage step-down (buck) convert-

ers. It includes an onboard PWM generator, a precision

reference trimmed to

±0.5%, two high power MOSFET

gate drivers and all necessary feedback and control cir-

cuitry to form a complete switching regulator circuit. Also

included is an internal charge pump which provides 5V

gate drive to the external MOSFETs with input supply

voltage as low as 2.7V. The LTC1649 runs at an internally

fixed 200kHz clock frequency and requires an external

resistor divider to set the output voltage.

The LTC1649 includes 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 is an internal soft start

feature that requires only a single external capacitor to

operate.

THEORY OF OPERATION

Primary Feedback Loop

The LTC1649 senses the output voltage of the circuit at the

output capacitor through a resistor divider connected to

the FB pin and 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 compared to a fixed frequency sawtooth waveform

generated by the internal oscillator to generate a pulse

width modulated 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 compen-

sation network at COMP, the output node of the FB

transconductance 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 93%. 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 LTC1649 includes yet another feedback loop to con-

pin during the portion of the cycle when G1 is high. It

to pull current out of the external soft start capacitor,

cutting the duty cycle and controlling the output current

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, indicating a se-

vere 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 LTC1649 to withstand brief overcurrent con-

ditions 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 LTC1649 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.