SC121 datasheet(16/23 Pages) SEMTECH

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SC121

Detailed Description

The SC121 is a synchronous step-up fixed frequency Pulse

Width Modulated (PWM) DC-DC converter utilizing a

1.2MHz fixed frequency current mode architecture. It is

designed to provide output voltages in the range 1.8V to

5.0V from an input voltage as low as 0.7V, with a (output

unloaded) start up input voltage of 0.85V. Quiescent

current consumption is typically 3.5mA, entirely into the

OUT pin during boost regulation. (See footnote 1 of the

Electrical Characteristics table.)

The regulator control circuitry is shown in the Block

Diagram. It is comprised of a programmable feedback

controller, an internal 1.2MHz oscillator, an n-

channel Field Effect Transistor (FET) between the LX and

GND pins, and a p-channel FET between the LX and OUT

pins. The current flowing through both FETs is monitored

and limited as required for startup and PWM operation.

An external inductor must be connected between the IN

pin and the LX pin.

Output Voltage Selection

The SC121 output voltage can be programmed to an

internally preset value or it can be programmed with

external resistors. The output is internally programmed to

3.3V when the FB pin is connected to GND. Any output

voltage in the range 1.8V to 5.0V can be programmed with

a resistor voltage divider between OUT and the FB pin as

shown in Figure 1.

The values of the resistors in the voltage divider network

are chosen to satisfy the equation

191

OUT

A large value of R

, ideally 590kΩ or larger, is preferred for

stability for V

within approximately 400mV of V

OUT

. For

lower V

, lower resistor values can be used. The values of

and R

can be as large as desired to achieve low quies-

cent current. C

= 22pF is recommended to improve

transient response.

The Enable Pin

The EN pin is a high impedance logical input that can be

used to enable or disable the SC121 under processor

control. V

< 0.2V will disable regulation, set the LX pin

in a high-impedance state (turn off both FET switches),

and turn on an active discharge device to discharge the

output capacitor via the OUT pin. Synchronous rectifier

(p-channel FET) bulk switching prevents pass-through

conduction from LX to OUT while disabled. V

> 0.85V

will enable the output. The startup sequence from the

EN pin is identical to the startup sequence from the appli-

cation of input power.

Applications Information

CIN

VOUT

OUT

SC121

COUT

GND

CFB

Figure 1 — Output Voltage Feedback Circuit

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