SC1548 datasheet(13/14 Pages) SEMTECH

13 / 14 page

ã 2001 Semtech Corp.

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SC1548

POWER MANAGEMENT

Please see Table 1 below for recommended resistor

values for some standard output voltages. All resistors

are 1%, 1/10W.

)

(

W)(

Table 1: Recommended Resistor Values For SC1548

The maximum output voltage that can be obtained from

the adjustable option is determined by the input supply

voltage and the R

DS(ON) and gate threshold voltage of the

external MOSFET. Assuming that the MOSFET gate

threshold voltage is sufficiently low for the output

voltage chosen and a worst-case drive voltage of 9V,

OUT(MAX) is given by:

)

MAX

)(

(

)

MAX

(

OUT

)

MIN

(

PWR

)

MAX

(

OUT

•

−

Short Circuit Protection

The short circuit protection feature of the SC1548 is

implemented by using the R

DS(ON) of the MOSFET. As the

output current increases, the regulation loop maintains

the output voltage by turning the FET on more and more.

Eventually, as the R

DS(ON) limit is reached, the MOSFET

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

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

approximately 50% of nominal, the LDO controller is

latched off, setting output voltage to 0V. Power must be

cycled to reset 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 at a preset time (nominally 5ms)

after both IN and EN rise above their lockout points. If

EN is left floating (using the internal resistor pullup), then

PWR should come up before VIN, or the device will latch

off. If the enable function is not being used, EN should

be tied to V

PWR.

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 200mW 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 (V

PWR) will help to hold up the power supply

during fast load changes, thus improving overall transient

response. The 12V supply should be bypassed with a

0.1µF ceramic capacitor.

Layout Guidelines

One of the advantages of using the SC1548 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 supply pin, and connected directly to the ground plane.

The ground pin of the device should also be connected

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 possible.

Power dissipation within the device is practically

negligible, requiring no special consideration during

layout.

Applications Infomation (Cont.)

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