SC195 datasheet(13/18 Pages) SEMTECH

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SC195

mode resumes. This can result in the switching frequency

being much lower than the PWM mode frequency.

If the output load current increases enough to cause V

OUT

to decrease below the PSAVE exit threshold (V

OUT

-2%), the

device automatically exits PSAVE and operates in continu-

ous PWM mode. Note that the PSAVE high and low

threshold levels are both set at or above V

OUT

to minimize

undershoot when the SC195 exits PSAVE. Figure 1 illus-

trates the transitions from PWM mode to PSAVE mode and

back to PWM mode.

VOUT

VOUT +2.5%

VOUT -2%

Load

Demand

OUT)

BURST

OFF

VLX

Time

PWM Mode at

Medium/High

Load

PSAVE Mode at

Light Load

PWM Mode at

Medium/High

Load

PSAVE

EXIT

Figure 1 — Transitions Between PWM and PSAVE Modes

Protection Features

The SC195 provides the following protection features:

Soft-Start Operation

Over-Voltage Protection

Current Limit

Thermal Shutdown

Under-Voltage Lockout

Soft-Start

The soft-start sequence is activated after a transition from

an all zeros CTL code to a non-zero CTL code enables the

part. Soft-start is also activated by any overload event

that causes the part to shutdown until the overload goes

away. At start-up, the PMOS current limit is stepped

•

through four levels: 25%, 40%, 60%, and 100%. Each step

is maintained for 60μs following an internal reference

start up of 20μs, resulting in a total nominal start-up

period of 260μs. If V

OUT

reaches 90% of the target within

the first 2 steps, the chip continues in PSAVE mode at the

end of soft-start; otherwise, it goes into PWM mode. Note

the V

OUT

ripple in PSAVE mode can be larger than the

ripple in PWM mode.

Over-Voltage Protection

Over-voltage protection is provided to ensure 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 SC195 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 func-

tion makes the device capable of sustaining an indefinite

short circuit on its output under fault conditions.

Thermal Shutdown

The SC195 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

Under-Voltage Lockout (UVLO) is enabled when the

supply voltage drops below the UVLO threshold. This

prevents 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.

Applications Information (continued)

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