SC202F datasheet(11/17 Pages) SEMTECH

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SC202F

V OUT

SC202F

CTL3

CTL2

CTL1

CTL0

SN S

OU T

GN D

V IN

C OUT

C IN

E nable

C FF

R FB1

R FB2

Figure 1 – Application Circuit with External Resistors

To simplify the design, it is recommended to program the

output setting to 1.0V, use resistor values smaller than

10kΩ, and include a feed-forward capacitance calculated

with the previous equation. If the output voltage is set to

1.0V, the previous equation reduces to

OUT

Example:

An output voltage of 1.3V is desired, but this is not a pro-

grammable option. What external component values for

Figure 1 are needed?

Solution: To keep the circuit simple, set R

FB2

to 10kΩ so

current into the SNS pin can be neglected and set the

CTL3-0 pins to 0010 (1.0V setting). The necessary compo-

nent values for this situation are

SET

OUT

PWM Operation

In order to start up from pre-charged output voltage,

SC202F does not allow inductor current to go negative

in soft start stage, refer to the soft start waveforms in the

Typical Characteristics page. When the output voltage

exceeds 90% of the set point, determined by the CTLx

combinations, the device will enter fixed 3.5MHz PWM

operation. An internal synchronous NMOS rectifier turns

on complementary to the top PMOSFET.

100% Duty Cycle Operation

The duty cycle (percentage of time PMOS is active)

increases as VIN decreases to maintain output voltage

regulation. As the input voltage approaches the pro-

grammed output voltage, the duty cycle approaches

100% (PMOS always on) and the device enters a pass

through mode until the input voltage increases or the

load decreases enough to allow PWM switching to

resume.

Protection Features

The SC202F provides the following protection features:

Soft-Start Operation

Over-Voltage Protection

Current Limit

Thermal Shutdown

Under-Voltage Lockout

Soft Start with Current Limit

The soft-start sequence is activated after a CTL code tran-

sition from an all zeros code to a non-zero code enables

the start up process. From the beginning of a start-up

process, the internal reference start-up takes typically

50μs, then PMOS current limit is stepped through four

levels: 25%, 40%, 60%, and 100%. Each step is maintained

for typically 75μs. A complete 4 steps start-up period

could take 350μs. Before VOUT reaches 90%, the inductor

current is not allowed to go negative. The inductor

current is confined between different current limit levels,

25%, 40%, 60%, or 100% to zero.

As VOUT reaches 90% of the target value, the device will

transition into a fixed 3.5MHz operation allowing induc-

tor current in both directions.

Due to current limit operation, if the load is heavy or the

output capacitor is large, the soft start process may expe-

rience all 4 current limit steps before reaching 90% target

voltage. This will make the soft start time long. If the load

is light and output capacitor is small, the device may only

need the first current limit step, reaching 90% of target,

and transition into PWM operation. This makes the soft

•

Applications Information (continued)

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