SC1112STRT datasheet(18/23 Pages) SEMTECH

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POWER MANAGEMENT

SC1112

Short Circuit Protection

The VTT short circuit protection feature of the SC1112

is implemented by using the R

DS(ON) of the MOSFET. As

the output current increases, the regulation loop main-

tains the output voltage by turning the FET on more and

more. Eventually, as the R

DS(ON) limit is reached, the MOS-

FET will be unable to turn on any further, and the output

voltage will start to fall. When the VTT output voltage falls

to approximately 700mV, the LDO controller is latched off,

setting output voltage to 0V. Power must be cycled to re-

set the latch.

To prevent false latching due to capacitor inrush currents

or low supply rails, the current limit latch is initially

disabled. It is enabled once the short circuit delay time

has elapsed. Timing diagram on pages 4 to 5 will show a

detailed operation of the Short Circuit protection circuitry.

To be most effective, the MOSFET R

DS(ON) should not be

selected artificially low. The MOSFET should be

chosen so that at maximum required current, it is almost

fully turned on. If, for example, a supply of 1.5V at 4A is

required from a 3.3V ± 5% rail, the maximum allowable

DS(ON) would be:

()

Ω

≈

•

−

•

400

025

)

MAX

)(

(

To allow for temperature effects 200m

Ω would be a

suitable room temperature maximum, allowing a peak

short circuit current of approximately 15A for a short time

before shutdown.

Capacitor Selection

Output Capacitors: Low ESR aluminum electrolytic or tan-

talum capacitors are recommended for bulk

capacitance, with ceramic bypass capacitors for decoupling

high frequency transients.

Input Capacitors: Placement of low ESR aluminum

electrolytic or tantalum capacitors at the input to the

MOSFET (VTTIN) will help to hold up the power supply

during fast load changes, thus improving overall transient

response. The +5VSTBY supply should be bypassed with a

10µF ceramic capacitor.

Layout Guidelines

One of the advantages of using the SC1112 to drive an

external MOSFET is that the bandgap reference and

control circuitry do not need to be located right next to the

power device, thus a very accurate output voltage can be

obtained since heating effects will be minimal.

The 0.1µF bypass capacitor should be located close to the

+5VSTBY supply pin, and connected directly to the ground

plane. The ground pin of the device should also be con-

nected directly to the ground plane. The sense or adjust

pin does not need to be close to the output voltage plane,

but should be routed to avoid noisy traces if at all pos-

sible.

Power dissipation within the device is practically

negligible, requiring no special consideration during

layout.

Applications Infomation (Cont.)

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