SC2446 datasheet(9/38 Pages) SEMTECH

Part #	SC2446

Description	Dual-Phase Single or Two Output Synchronous Step-Down Controllers

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Manufacturer	SEMTECH [Semtech Corporation]

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 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2446

Operation

Overview

The SC2446 is a constant frequency 2–phase current-

mode step-down PWM switching controller driving all N-

channel MOSFET’s. The two channels of the controller

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. Also, with lower

inductor current and smaller inductor ripple current per

phase, overall I2R losses are reduced.

The SC2446 operates in synchronous continuous-

conduction mode. It 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 IN2- 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

synchronized to channel one.

Frequency Setting and Synchronization

The internal oscillator of the SC2446 runs at twice the

phase frequency. The free-running frequency of the

oscillator can be programmed with an external resistor

from the ROSC pin to the ground. The step-down controllers

are capable of operating up to 1 MHz. It is necessary to

consider the operating duty-ratio before deciding the

switching frequency. See Applications Information section

for more details.

When synchronized externally, the applied clock frequency

should be twice the desired phase frequency. The

synchronizing clock frequency should also be between 1-

1.33 times the set free-running frequency.

Control Loop

The SC2446 uses peak current-mode control for fast

transient response, ease of compensation and current

sharing in single output operation. The low-side MOSFET

of each channel is turned off at the falling-edge of the

phase timing clock. After a brief non-overlapping time

interval of 90ns, 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

conduction time of 90ns, 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, the high-side gate

drive voltage swing will be from approximately 2V

CC to the

ground. The power dissipated in the high-side gate driver

is not higher with higher voltage swing because the gate-

source voltage of the high-side MOSFET still swing from

zero to V

CC.The outputs of the low-side gate drivers swing

from V

C to the ground.

The SC2446 has internal ramp-compensation to prevent

sub-harmonic oscillation when operating above 50% duty

cycle. There is enough ramp internally for a sensed

voltage ripple between ¼ to 1/3 of the full-scale sensed

voltage limit of 75mV. The maximum sensed voltage limit

is unaffected by the compensation ramp.

Current-Sensing

There are two ways to sense the inductor current for

current-mode control with the SC2446. Since the peak

inductor current corresponds to 75mV of sensed voltage

(CS+ - CS-), resistor current sensing can be used at the

output without resulting in excessive power dissipation.

Although accurate and far easier to lay out than high-

side resistor sensing, a pair of precision sense resistors

adds cost to the converter. The SC2446 has provision to

reconstruct a differential voltage proportional to the

inductor current at the output of the converter (U.S. patent

6,441,597). The voltage to current ratio or the equivalent

sense resistance R

eq is a combination of high-side and low-

side MOSFET R

DS(ON) ’s and the inductor series resistance

(hence the name “Combi-Sense”). The SC2446 provides

the virtual phase voltages VPN1 and VPN2 (these are

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