SC2453_05 datasheet(9/22 Pages) SEMTECH

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 2005 Semtech Corp.

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SC2453

POWER MANAGEMENT

Applications Information

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

nel MOSFET PWM synchronous buck switchers and two

positive linear regulators. The two PWM switchers are

synchronized 180° out of phase for low input ripple and

noise. The switching frequency is programmable to opti-

mize design. The SC2453 PWM switchers feature lossless

current sensing and programmable over current limit. The

two positive linear regulators output voltages are adjust-

able.

Power Supplies

Supplies VIN, PVCC and AVCC from the input source are

used to power the SC2453. An external PNP transistor

linear regulator supplies AVCC and PVCC. The AVCC sup-

ply provides the bias for the oscillator, the switchers, the

linear regulator controllers and the POK circuitry. PVCC is

used to drive the low side MOSFET gate. In low shut-

down current mode, the PNP transistor is turned off, dis-

abling AVCC and PVCC.

Soft-start, Sequencing and Disabling

A 10A current source pulls up on the SS/SHDN pin. When

the SS/SHDN pin reaches 0.5V, the first switcher is acti-

vated and the reference input of the error amplifier is

ramped up with the soft-start voltage. When the SS/

SHDN pin reaches 2V, the SS/SHDN pin is pulled down

to approximately 0.7V and the second switcher begins

to soft-start in an identical fashion to the first switcher.

When the SS/SHDN pin reaches 2V for the second time,

the SS/SHDN pin is pulled down to approximately 0.7V

again, and then the positive linear regulators ramp up

with the SS/SHDN pin voltage. The SS/SHDN pin is even-

tually pulled up to the supply AVCC. The soft-start time is

controlled by the value of the capacitor connected to

the SS/SHDN pin.

If the SS/SHDN pin is pulled down below 0.5V, the

SC2453 is disabled. If the SS/SHDN pin is pulled down

below 0.34V, the bias PNP transistor for SC2453 is dis-

abled and the supply current is only 100uA.

The power-ok circuitry monitors the FB inputs of the

error amplifiers of the switchers. 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.

Frequency Setting and Synchronization

The internal oscillator free-running frequency of the

SC2453 is set by an external resistor using the following

formula:

freq

−

When it is synchronized externally, the applied clock fre-

quency should be equal or greater than the free-running

frequency.

Setting Current Limit

SC2453 monitors the voltage drop in the lower MOSFETs

Rdson voltage to sense 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

Rilim should be between 10K and 100K.

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 in the next switching cycle.

Gate Drives

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-

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