SC2441ITSTRT datasheet(11/35 Pages) SEMTECH

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 2005 Semtech Corp.

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POWER MANAGEMENT

SC2441

Overview

The SC2441 is a constant frequency triple-output

switching regulator especially designed for operating from

input voltages as low as 1.8V. It consists of two current-

mode step-down switch-mode PWM controllers driving

all N-channel MOSFET’s and a 1.7A step-up current-mode

controller with integrated 1.7A power switch. A low

voltage input (3.3V, 2.5V or 1.8V) can be stepped up to

5V-10V locally using the boost regulator to provide

sufficient gate drives for the step-down converters. The

boost converter can also be used to generate a third

output.

The two step-down channels of the SC2441 operate at

180 degrees out of phase from each other. Since input

currents are interleaved in a two-phase converter, input

ripple current is lower and smaller input capacitor can be

used for filtering.

The step-down controllers of the SC2441 operate in

synchronous continuous-conduction mode. They can be

configured either as two independent step-down

controllers producing two separate outputs or as a dual-

phase single-output controller by tying the FB2 pin to V

CC.

In single output operation, the channel-one error amplifier

controls both channels and the channel-two error

amplifier is disabled. Soft-start and overload hiccup of

both channels is also synchronized to channel one.

Frequency Setting and Synchronization

The step-up regulator in the SC2441 runs at twice the

frequency of step-down controllers. Each step-down

controller runs at one-half of the oscillator frequency and

is 180 degrees out of phase from the other step-down

controller. The switching frequency of the step-up

regulator is the oscillator frequency and can be set with

an external resistor from the R

OSC pin to the ground. The

boost regulator and the step-down controllers are capable

of operating up to 2 MHz and 1 MHz respectively. It is

necessary to consider the operating duty-ratio range

before deciding the switching frequency. See Applications

Information section for more details.

When synchronized externally, the applied clock frequency

(hence switching frequency of the step-up converter)

should be twice the individual phase frequency of the

step-down controllers. The synchronizing clock frequency

should also be between 1-1.33 times the set free-running

frequency.

If not synchronized, the SYNC/SHDN pin should be tied

to the input. Pulling the SYNC/SHDN pin below 0.5V shuts

off the SC2441 after 85

µs time delay.

Control Loop

The step-down controllers and the boost regulator in the

SC2441 use peak current-mode control for fast transient

response, ease of compensation and current sharing in

single output operation. The low-side MOSFET of each

step-down channel is turned off at the falling-edge of

the phase clock. After a brief non-overlapping conduction

interval of 74ns, the high-side MOSFET is turned on. The

phase inductor current ramps up. When the sensed

inductor current reaches the threshold determined by

the error amplifier output and ramp compensation, the

high-side MOSFET is turned off. After a non-overlapping

delay of 62ns, the low-side MOSFET is turned on.

The supply voltages for the high-side gate drivers are

obtained from two diode-capacitor bootstrap circuits. If

the bootstrap capacitor is charged from V

CC, then the

high-side gate drive voltage will swing from approximately

CC to the ground. The outputs of the low-side gate

drivers swing from V

CC to the ground.

All three converters in the SC2441 have internal ramp-

compensation to prevent sub-harmonic oscillation when

operating above 50% duty cycle. The internal

compensating ramp is designed for an inductor ripple-

current of between ¼ to 1/3 of the maximum inductor

current and the peak-to-peak current-sense voltage (CSP-

CSN of the step-down controllers) of between ¼ to 1/3

of the current-limit threshold (50mV). The current-limits

of all three converters are unaffected by the

compensation ramps.

Current-Sensing

Since the inductor current ramp is used as the modulating

ramp in current-mode control, the inductor current needs

to be sensed. There are two current sensing methods for

the step-down controllers. Since the maximum current-

sense voltage (CSP-CSN) is only 50mV, a precision sense

resistor in series with the inductor can be used at the

output without resulting in excessive power dissipation.

Operation

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