6

LTC1504

OVERVIEW

The LTC1504 is a complete synchronous switching regu-

lator controller (see Block Diagram). It includes two

on-chip 1.5

Ω power MOSFETs, eliminating the need for

external power devices and minimizing external parts

count. The internal switches are set up as a synchronous

buck converter with a P-channel device (Q1) from the

input supply to the switching node and an N-channel

device (Q2) as the synchronous rectifier device from the

switching node to ground. An external inductor, input and

output bypass capacitors and the compensation network

complete the control loop. The LTC1504 adjustable output

parts require an additional pair of resistors to set the

output voltage. The LTC1504-3.3 parts include an onboard

resistor divider preset to a 3.3V output voltage. A func-

tional 3.3V output regulator can be constructed with an

LTC1504-3.3 and as few as four external components.

The LTC1504 feedback loop includes a precision reference

feedback amplifier (FB) and an onboard PWM generator

(SAW and PWM). Two additional feedback comparators

(MIN and MAX) monitor the feedback voltage and override

the primary feedback amplifier when the regulated out falls

outside a

±3% window, improving transient response.

The internal sawtooth oscillator typically runs at 200kHz.

Q1 and Q2 are capable of carrying peak currents in excess

of 500mA, with the continuous output power level limited

primarily by the thermal dissipation of the SO-8 package.

With a 5V input and a 3.3V output, the LTC1504 can supply

500mA of continuous output current with an appropriate

layout. An on-chip current limit circuit, set with a single

external resistor, can be used to help limit power dissipa-

tion. See the Thermal Considerations section for more

information.

Theory of Operation

The LTC1504 primary feedback loop consists of the main

error amplifier FB, the PWM generator, the output drive

logic and the power switches. The loop is closed with the

external inductor and the output bypass capacitor. The

feedback amplifier senses the output voltage directly at the

SENSE pin for fixed output versions or through an external

resistor divider in the adjustable output version. This

feedback voltage is compared to the 1.265V internal

reference voltage by FB and an error signal is generated at

the COMP pin. COMP is a high impedance node that is

brought out to an external pin for optimizing the loop

compensation.

COMP is compared to a 200kHz sawtooth wave by com-

parator PWM. This raw pulse-width modulated signal is

logically combined with the outputs of the transient com-

parators MIN and MAX before reaching the output stage.

The output stage generates nonoverlapping drive for the

onboard P- and N-channel power MOSFETs, which drive

the SW pin with a low impedance image of the PWM

waveform. Typical open-loop output impedance at SW is

between 1

Ω and 3Ω, depending on supply voltage. This

high power pulse train is filtered by the external inductor

and capacitor, providing a steady DC value at the output

node. This node returns to FB or SENSE, closing the loop.

The MIN and MAX comparators in the feedback loop

provide high speed fault correction in situations where the

FB amplifier may not respond quickly enough. MIN com-

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

the internal reference. At this point, MIN overrides the FB

amplifier and forces the loop to full duty cycle. Similarly,

MAX 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.

The LTC1504 includes yet another feedback loop that

monitors the voltage at the SW pin while Q1 is on. It

peak current through Q1 rises, the voltage drop across it

controlled amount of current out of SS, the external soft

start pin. As SS falls, it pulls COMP down with it, limiting

the duty cycle and reducing the output voltage to control

the current. The speed at which the current limit circuit

reacts is set by the value of the external soft start capacitor.