8

 2002 Semtech Corp.

www.semtech.com

SC2453

POWER MANAGEMENT

PRELIMINARY

Applications Information

The SC2453 is designed to control and drive two N-Chan-

nel MOSFET synchronous rectified buck and two LDOs,

one positive and the other negative. The two bucks are

synchronized and out of phase operation for low input

ripple and noise. The switching frequency is program-

mable to optimize design. The SC2453 switching regula-

tor section features lossless current sensing while it pro-

vides a programmable cycle by cycle over current limit.

The SC2453 linear sections are low dropout regulators.

The voltage for the linear controllers LDO1 and LDO2 are

programmable.

SUPPLIES

Supplies VIN, PVCC and AVCC from the input source are

used to power the SC2353. An external PNP transistor

as a linear regulator supplies AVCC and PVCC. The VIN

supply provides the bias for the Internal Reference and

UVLO circuitry. The AVCC supply provides the bias for the

oscillator, the switchers, the LDO controllers, and the

Power Good circuitry. PVCC is used to drive the low side

MOSFET gate.

START UP SEQUENCE

A 5uA current source pulls up on the SS pin. When the

SS pin reaches 0.5V, the first converter will start. The

reference input of the error amplifier is ramped up with

the soft-start signal. When the SS pin reaches 2V the SS

pin is pulled down to approximately 0.7V and the second

converter will begin to soft-start in an identical fashion

to the first converter. When the SS pin reaches 2V for

the second time, the SS pin is pulled to approximately

0.7V again and the LDOs are started. The reference of

the positive LDO is ramped with the SS pin while

the negative LDO should be soft-started externally by

ramping the positive supply of the feedback resistors.

The SS pin will then be pulled up to supply, i.e. AVCC. The

SS time is controlled by the value of the SS cap. If the

SS pin is pulled below 0.5V, the SC2453 is disabled.

The power-okay circuitry monitors the FB inputs of the

converter error amplifiers. If the voltage on these inputs

goes above 0.55V or below 0.45V then the POK pin is

pulled low. The power-okay circuitry monitors the FB in-

puts of the converter error amplifiers. If the voltage on

these inputs goes above 0.55V or below 0.45V then the

POK pin is pulled low. The POK pin is held low until the

end of the start-up sequence.

OSCILLATOR

The switching frequency of the SC2453 is set by an ex-

ternal resistor using the following formula:

fs

4

.

8

p

10

1

R

freq

•

•

=

OVER CURRENT

SC2453 monitors any voltage drop in the lower MOSFETs

Rdson voltage due to an over current condition. This

method of current sensing minimizes any unnecessary

losses due to external sense resistance.

The SC2453 utilizes an internal current source and an

external resistor connected from the ILIM pins to the

AGND pin to program a current limit level. This limit is

programmable by choosing the resistor relative to the

level required. The value of the resistor can be selected

by the following formula:

)

Rdson

*

IIim

/(

2000

lim

Ri

=

An internal comparator with a reference from the level

set by the external resistor monitors the voltage drop

across the lower MOSFET. Once the Vdson of the MOSFET

exceeds this level, the low side gate is turned on and the

upper MOSFET is turned off.

GATE DRIVERS

The low side gate driver is supplied from PVCC and pro-

vides a peak source/sink current of 1A. The high side

gate drive is also capable of sourcing and sinking peak

currents of 1A. The high side MOSFET gate drive can be

provided by an external 12V supply that is connected

from BST to GND. The actual gate to source voltage of

the upper MOSFET will approximately equal 7V (12V-VCC).

If the external 12V supply is not available, a classical

bootstrap technique can be implemented from the PVCC

supply. A bootstrap capacitor is connected from BST to

Phase while PVCC is connected through a diode (Schottky

or other fast low VF diode) to the BST. This will provide a

gate to source voltage approximately equal to the VCC-

Vdiode drop.

Shoot through control circuitry provides a 30ns dead time

to ensure both the upper and lower MOSFET will not turn

on simultaneously and cause a shoot through condition.