SC4624 데이터 시트보기 (PDF) - Semtech Corporation

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SC4624

Low Input Voltage, High Efficiency, 4A Integrated FET Synchronous Step down DC/DC Regulator

Semtech Corporation 

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.

During start up or restart, A typical 4µA sourcing current

charges the capacitor and then the voltage of capacitor

ramp up the error amp reference slowly. The closed

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 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 VCC and SYNC/EN pin. The voltage on

the VCC pin determines the operation of the SC4624. As

VCC increases during start up, the UVLO block senses VCC

and keeps the high side and low side MOSFETs off and

the internal soft start voltage low until VCC reaches 2V. If

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

when VCC exceeds 2V. A typical 120mV hysteresis in the

UVLO comparator provides noise immunity during its start

up. (refer to Figure 1 to 2).

Shutdown

The SC4624 is disabled when VCC falls below 1.88V (typical)

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 VCC pin,

the internal soft start voltage is held low and the internal

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

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 VCC finally.

Pre-biased Output

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

adding external RC circuit, where R(10KΩ) is between VCC

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 VCC and EN is very important for pre-biased

output. VCC must lead EN. When VCC and EN voltage rise

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

is pull low before VCC reach to the voltage of UVLO. If this

isn’t desirable, RC(10KΩ and 0.1uF) must be added at EN

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.

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.

 2008 Semtech Corp.

11

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