SC185 datasheet(14/18 Pages) SEMTECH

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SC185

Applications Information (continued)

Current Limit & OCP

The internal PMOS power device in the switching stage is

protected by a current limit feature. If the inductor current

is above the PMOS current limit for 16 consecutive cycles,

the part enters foldback current limit mode and the output

current is limited to the current limit holding current

CL_HOLD) which is approximately 900mA. Under this condi-

tion, the output voltage will be the product of I

CL_HOLD and

the load resistance. When the load presented falls below

the current limit holding level, the output will charge to

the upper PSAVE voltage threshold and return to normal

operation. The SC185 is capable of sustaining an indefi-

nite short circuit without damage. During the soft start, if

current limit has occurred before the SS voltage has

reached 400mV, the part enters foldback current limit

mode. Foldback current limit mode will be disabled during

soft-start after the SS voltage is higher than 400mV.

Over-Voltage Protection

In the event of a 15% over-voltage on the output, the

PWM drive is disabled with the LX pin floating. Switching

does not resume until the output voltage falls below the

nominal Vout regulation voltage.

Soft-Start

The soft-start mode is activated after AVIN reaches it’s

UVLO voltage threshold and EN is set high to enable the

part. A thermal shutdown event will also activate the soft

start sequence. The Soft-start mode controls the slew-rate

of the output voltage during startup thus limiting in-rush

current on the input supply. During start up, the reference

voltage for the error amplifier is clamped by the voltage

on SS pin. The output voltage slew rate during soft start is

determined by the value of the external capacitor con-

nected to the SS pin and the internal 5µA charging current.

The SC185 requires a minimum soft-start time from enable

to final regulation in the order of 200µs, including the

50µs enable delay. As a result the soft start capacitor, Css,

should be higher than 1.5nF. During start up, the chip

operates in forced PWM mode. The value of Css for the

desired soft-start time, tss, can be determined by Equation

(1)

The SC185 is capable of starting up into a pre-biased

output. When the output is pre-charged by another

supply rail, the SC185 will not discharge the output during

the soft start period.

Shut Down

When the EN pin is low, the SC185 will run in shutdown

mode, drawing less than 1μA (typ.) from the input power

supply. The internal switches and bandgap voltage will be

immediately turned off.

Thermal Shutdown

The device has a thermal shutdown feature to protect

the SC185 if the junction temperature exceeds 160°C.

During thermal shutdown, the on-chip power devices are

disabled, floating the LX output. When the temperature

drops by 10°C, it will initial a soft start cycle to resume

normal operation.

Under-Voltage Lockout

Under-Voltage Lockout (UVLO) is enabled when the

input voltage drops below the UVLO threshold. This

prevents the device from entering an ambiguous state

in which regulation cannot be maintained. Hysteresis of

approximately 300mV is included to prevent chattering

near the threshold. When the AVIN voltage reaches back

to the turn-on threshold and EN is high, the soft-start

mode is resumed.

Power Good

The power good (PGOOD) is an open-drain output. When

the output voltage drops below 10% of nominal voltage,

the PGOOD pin is pulled low after a 20μs delay. During

start-up, PGOOD will be asserted 1.8ms (typ.) after the

output voltage reaches 90% of the final regulation

voltage. The faults of over voltage, fold-back current limit

mode and thermal shutdown will force PGOOD low after

a 20µs delay. When recovering from a fault, PGOOD will be

asserted 1.8ms (typ.) after Vout reaches 90% of the final

regulation voltage.

Enable

The EN input is used to enable or disable the device when

the device is not in UVLO. When EN is low (grounded), the

device enters shutdown mode and consumes less than

1μA of current. In shutdown mode, the device tri-states

the LX pin and pulls down the SS pin. The EN pin has a

500kΩ internal pull-down resistor. This resistor is switched

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