SC46208

PIN CONFIGURATION

SC46208

5

4

1

2

3

EN

GND

SW

PIN DESCRIPTION

Pin no.

Pin name

I/O

Descriptions

1

EN

I

Enable input port, this pin should not float.

2

GND

--

Ground.

3

SW

I/O

Power switch which connect to inductor and output voltage.

4

--

Supply input pin.

5

I

Output voltage feedback pin. In the adjustable version, this pin used for

connecting external resistive divider.

FUNCTION DESCRIPTIONS

SC46208 is a 1.25MHz constant frequency, current mode PWM step-down converter. The device integrates

PMOS switch and a NMOS synchronous rectifier. It also includes a voltage reference, error amplifier, oscillator,

PWM comparator, current comparator, slope compensation, current sense, logic driver, etc...

In normal PWM mode, in one cycle, when the oscillator sets RS trigger, internal PMOS conduct, while PWM

comparator resets the RS trigger, PMOS is cut off, the peak current of inductor is controlled by error amplifier

output. When PMOS is off, the synchronous NMOS will be turned on and stay on until the inductor current

reverses (controlled by current comparator ICOMP) or the next clock cycle begins.

SC46208 detects the load current automatically, when the load current decreases to a threshold(relate to the

input/output voltage and inductance), the system starts power saving mode. In this mode, the dissipation

current is only 20

μA, thus to improve the conversion efficiency under light loads. Pay attention to the ripple of

output voltage, you can add a phase-lead capacitor (pF grade) between the VOUT and FB to reduce the ripple

in power saving mode.

Dropout Operation

When the input voltage decreases toward the value of output voltage, the duty increases, and the input voltage

decrease more, PMOS switch on in one cycle and the duty reaches 100%. The output voltage then is the input

voltage minus the voltage drop across the PMOS switch and the inductor.

Short Circuit Protection

When the output is short to ground, the frequency of internal oscillator decreases to 600KHz in order to remain

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

REV:1.1

2008.09.10

Http://www.silan.com.cn

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