SC120 datasheet(13/21 Pages) SEMTECH

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PRELIMINARY

SC120

Detailed Description

The SC120 is a synchronous step-up 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 4.0V from

an input voltage as low as 0.7V, with a (output unloaded)

start up input voltage of 0.85V.

The device operates in two modes: PWM and automatic

PSAVE mode. In PWM operation, the devices uses pulse

width modulation control to regulate the output under

moderate to heavy load conditions. It switches to PSAVE

mode when lightly loaded. Quiescent current consump-

tion is as little as 50μA, into the OUT pin, when in PSAVE

mode.

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, PWM operation, and

PSAVE operation. An external inductor must be connected

between the IN pin and the LX pin. When the n-channel

FET is turned on, the LX pin is internally grounded, con-

necting the inductor between IN and GND. This is called

the on-state. During the on-state, inductor current flows

to ground and is increasing. When the n-channel FET is

turned off and the p-channel FET is turned on (known as

the off-state), the inductor is then connected between IN

and OUT. The (now decreasing) inductor current flows

from the input to the output, boosting the output voltage

above the input voltage.

Output Voltage Selection

The SC120 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 4.0V can be programmed with

a resistor voltage divider between OUT and the FB pin as

shown in Figure 1.

OUT

CFB

(optional)

Figure 1 — Output Voltage Feedback Circuit

The values of the resistors in the voltage divider network

are chosen to satisfy the equation:

OUT

The value of R

should be 590kΩ or larger for stability.

Otherwise, the values of R

and R

can be as large as

desired to achieve low quiescent current.

PWM Operation

The PWM cycle runs at a fixed frequency (f

osc

= 1.2MHz),

with a variable duty cycle (D). PWM operation continu-

ally draws current from the input supply (except for dis-

continuous mode, described below). During the on-state,

of the PWM cycle, the n-channel FET is turned on, ground-

ing the inductor at the LX pin. This causes the current

flowing from the input supply through the inductor to

ground to ramp up. During the off-state, the n-channel

FET is turned off and the p-channel FET (synchronous

rectifier) is turned on. This causes the inductor current to

flow from the input supply through the inductor into the

output capacitor and load, boosting the output voltage

above the input voltage. The cycle then repeats to re-

energize the inductor.

Ideally, the steady state (constant load) duty cycle is

determined by D = 1 – (V

OUT

), but must be greater in

practice to overcome dissipative losses. The SC120 PWM

controller constrains the value of D such that 0.15 < D < 0.9,

(approximately).

The average inductor current during the off-state multi-

plied by (1-D) is equal to the average load current. The

inductor current is alternately ramping up (on-state) and

down (off-state) at a rate and amplitude determined by

Applications Information

SC120 Datasheet(PDF) 13 Page - Semtech Corporation

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