SC197 datasheet(14/20 Pages) SEMTECH

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SC197

period and duty cycle while in PSAVE mode are solely

determined by V

IN and VOUT until PWM 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 SC197 exits PSAVE. Figure 2 illus-

trates the transitions from PWM mode to PSAVE mode and

back to PWM mode.

VOUT -2%

VOUT

VOUT +2.5%

Load

Demand

(

IOUT)

BURST

OFF

VLX

Time

PWM Mode at

Medium/High

Load

PSAVE Mode at

Light Load

PWM Mode at

Medium/High

Load

PSAVE

EXIT

Figure 2 — Transitions Between PWM and PSAVE Modes

Protection Features

The SC197 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 CTLx(A/B) code to a non-zero CTLx(A/B) code

enables the device. 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 ref-

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

•

Applications Information (continued)

start-up period of 260μs. If V

OUT reaches 90% of the target

within the first 2 steps, the device 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

OVP (Over-Voltage Protection) ensures the output voltage

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

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 SC197 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 SC197 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(A/B) output. When the

junction 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 UVLO threshold. This prevents

the device from entering an ambiguous state in which

regulation cannot be maintained. Hysteresis of approxi-

mately 200mV is included to prevent chattering near the

threshold.

Inductor Selection

The SC197 is designed to operate with a 1µH inductor

between the LX(A/B) pin and the OUT(A/B) pin. Other

values may lead to instability, malfunction, or out-of-

specification performance. The specified current levels

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