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 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC190

Applications Information

The output load in switcher mode has to be reduced to a

load that the LDO regulator can support before switching

between the two modes to minimize output voltage

deviation. The transition from LDO regulator mode to

switcher mode requires the load to be kept at levels the

LDO regulator can support for a given time period after

the MODE pin has been pulled low (see Timing Diagram

on page 7).

Protection Features

The SC190 provides the following protection features:

•

Thermal Shutdown

•

Current Limit

•

Overvoltage Protection

•

Soft-Start

Thermal Shutdown

The device has a thermal shutdown feature to protect the

device if the junction temperature exceeds 150°C. In

thermal shutdown the PWM drive is disabled tri-stating

the LX output and the front-end LDO is disabled. The

device will not be enabled again until the temperature

reduces by 10°C. If during this time the output falls by

greater than 60% of its regulation voltage a soft start will

be invoked. Thermal shutdown is not active during LDO

mode in order to minimize supply current.

Current Limit

The part has a number of current limit functions. The Front-

end LDO regulator has a current limit set at approximately

500mA, which will protect the FELDO regulator in the

event of a pulsed short circuit.

The PMOS and NMOS power devices of the buck switcher

stage are protected by current limit functions.

In the case of a short to ground on the output, the part

enters frequency foldback mode which causes the

switching frequency to divide by a factor determined by

the output voltage, stopping the inductor current “stair

stepping.”

The back-end LDO regulator has a current limit feature

which will limit the maximum output current during LDO

regulator mode to 50mA.

Overvoltage Protection

Overvoltage protection is provided on the SC190. In the

event of an overvoltage on the output in switcher mode,

the drive to the PWM stage is disabled and the part will

not resume switching until the output voltage has fallen to

below 2% of the regulation voltage.

Soft-Start

The soft-start mode is enabled after every shutdown cycle

to limit in-rush current. In conjunction with the frequency

foldback this controls the maximum current during the

start-up. The switcher’s PMOS current limit is stepped

from 25%, to 50%, to 75%, and then 100% of its typical

value by the internal oscillator. The oscillator frequency

1

/

1

/

output voltage thresholds causing modulation of the soft-

start timer (see Current Limit description in the Protection

Features section). As soon as the part reaches regulation,

soft-start mode is disabled.

100 Percent Duty Cycle Operation

The SC190C has a 100% duty cycle mode of operation

to allow the switcher to regulate the output at low input

voltage to high output voltage conditions. As the input

supply drops towards the output voltage, the PMOS on-

time increases linearly until a point where the PMOS

FET is on for 100% of the time. Once the PMOS is on DC

the output voltage will track the input voltage minus the

voltage drop across the PMOS power device according to

the following relationship:

V

V

V

V

in

= Input Voltage

I

out

= Output current

R

R

ind

= Series resistance of the inductor