SC202FEVB datasheet(12/17 Pages) SEMTECH

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SC202F

start time shorter.

Over-Voltage Protection

Over-voltage protection ensures the output voltage does

not rise to a level that could damage its load. When V

OUT

exceeds the regulation voltage by 15%, the PWM drive is

disabled. Switching does not resume until V

OUT

has fallen

below the regulation voltage by 2%.

Current Limit

The SC202F switching stage is protected by a current limit

function. If the output load exceeds the PMOS current

limit for 32 consecutive switching cycles, the device enters

fold-back current limit mode and the output current is

limited to approximately 150mA. Under these conditions,

the output voltage will be the product of I

FB-LIM

and the load

resistance. The load must fall below I

FB-LIM

for the device to

exit fold-back current limit mode. This function makes the

device capable of sustaining an indefinite short circuit on

its output under fault conditions.

Thermal Shutdown

The SC202F has a thermal shutdown feature to protect the

device if the junction temperature exceeds 160°C. During

thermal shutdown, the PMOS and NMOS switches are

both disabled, tri-stating the LX output. When the junc-

tion temperature drops by the hysteresis value (20°C), the

device goes through the soft-start process and resumes

normal operation.

Under-Voltage Lockout

UVLO (Under-Voltage Lockout) activates when the supply

voltage drops below the falling UVLO threshold. This pre-

vents the device from entering an ambiguous state in

which regulation cannot be maintained. Hysteresis of

approximately 200mV is included to prevent chattering

near the threshold.

OUT Selection

The internal voltage loop compensation in the SC202F

limits the minimum output capacitor value to 10μF. This

is due to its influence on the loop crossover frequency,

phase margin, and gain margin. Increasing the output

Applications Information (continued)

capacitor above this minimum value will reduce the cross-

over frequency and provide greater phase margin.

Capacitors with X7R or X5R ceramic dielectric are recom-

mended for their low ESR and superior temperature and

voltage characteristics. Y5V capacitors should not be used

as their temperature coefficients make them unsuitable

for this application.

In addition to ensuring stability, the output capacitor

serves other important functions. This capacitor deter-

mines the output voltage ripple — as capacitance

increases, ripple voltage decreases. It also supplies current

during a large load step for a few switching cycles until

the control loop responds (typically 3 switching cycles).

Once the loop responds, regulation is restored and the

desired output is reached. During the period prior to PWM

operation resuming, the relationship between output

voltage and output capacitance can be approximated

using the equation

DROOP

LOAD

OUT

This equation can be used to approximate the minimum

output capacitance needed to ensure voltage does not

droop below an acceptable level. For example, a load step

from 50mA to 400mA requiring droop less than 50mV

would require the minimum output capacitance to be

OUT

In this example, using a standard 10μF capacitor would be

adequate to keep voltage droop less than the desired

limit. Note that if the voltage droop limit were decreased

from 50mV to 25mV, the output capacitance would need

to be increased to at least 12μF (twice as much capaci-

tance for half the droop). Capacitance will decrease from

the nominal value when a ceramic capacitor is biased with

a DC current, so it is important to select a capacitor whose

value exceeds the necessary capacitance value at the pro-

grammed output voltage. Check the manufacturer’s

capacitance vs. DC voltage graphs when selecting an

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