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 2007 Semtech Corp.

www.semtech.com

SC4624

POWER MANAGEMENT

Application Information

Overview

The SC4624 is a programmable high switching frequency,

integrated 4A MOSFET, synchronous step down regulator.

This reduces external component count and makes it

effective for applications which are low in cost and sized

small. A non-overlap protection is provided for the gate

drive signals to prevent shoot through of the internal

MOSFET pair.

The SC4624 is capable of producing an output voltage as

low as 0.5V and Its operation frequency is programmable

up to 2MHz by an external resistor. It features lossless

current sensing of the voltage drop across the internal

drain to source resistance of the high side MOSFET during

its conduction period.

The quiescent supply current in shutdown mode is

typically lower than 1µA. An external soft start is provided

to prevent output voltage overshoot during start-up. Over

Temperature Protection, Power Good Indicator, External

Clock Synchronization are some of the internal added

features.

Enable

The SC4624 is enabled by applying a voltage greater than

2V (typical) to the V

the V

V

and keeps the high side and low side MOSFETs off and

the internal soft start voltage low until V

no faults are present, the SC4624 will initiate a soft start

when V

UVLO comparator provides noise immunity during its start

up. (refer to Figure 1 to 2).

Shutdown

The SC4624 is disabled when V

or shutdown mode operation is invoked by clamping the

SYNC/EN pin to a voltage below 0.8V. During the shutdown

mode, A typical 0.2µA current draw through the V

the internal soft start voltage is held low and the internal

MOSFETs are turned off. (refer to Figure 3 to 4).

Soft Start

The soft start function is required for step down controllers

to prevent excess in-rush current through the DC bus

during start up. An external capacitor is necessary for

the soft start function and is connected from SS pin to

AGND.

During start up or restart, A typical 6µA sourcing current

charges the capacitor and then the voltage of capacitor

loop creates narrow width driver pulses while the output

voltage is low and allows these pulses to increase to their

steady state duty cycle as the output voltage reaches

its regulated value. The duration of the soft start in the

SC4624 is controlled by an external capacitor.

The SC4624 starts up in asynchronous mode before SS

voltage reaches to 0.5V, and the bottom FET diode is used

for circulating current during the top FET off time. Ths SS

voltage level is clamped at V

Pre-biased Output

The SC4624 is able to start into pre-biased output by

CC

pin and EN pin, C(0.1uF) is between EN pin and AGND.

If there is a pre-biased load on the output of SC4624

during start-up, the internal low-side MOSFET of SC4624

is always disabled before SS reach to 0.5V, the output

voltage is maintained. The great feature avoids negative

voltage spikes or short circuit on the output, which could

cause damage to the down-stream IC during start-up.

Timing between V

output. V

at same time(tied together), the pre-biased output voltage

is pull low before V

to prevent this from happening.

Oscillator

The FS pin is used to set the PWM oscillator frequency

through an external resistor that is connected from the

FS pin to the AGND. The internal ramp is a triangle at the

PWM frequency with a peak voltage of 1.25V and a valley

voltage of 0.25V. The approximate operating frequency is

determined by the value of an external resistor as shown

in Figure 15.