SIP4282 datasheet(8/9 Pages) VISHAY

8 / 9 page

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Document Number: 73851

S-70069–Rev. C, 22-Jan-07

Vishay Siliconix

SiP4282

New Product

Protection Against Reverse Voltage Condition

The P-channel MOSFET pass transistor has an intrinsic

diode that is reversed biased when the input voltage is

greater than the output voltage. Should VOUT exceed VIN,

this intrinsic diode will become forward biased and allow

excessive current to flow into the IC thru the VOUT pin and

potentially damage the IC device. Therefore extreme care

should be taken to prevent VOUT from exceeding VIN.

In conditions where VOUT exceeds VIN a Schottky diode in

parallel with the internal intrinsic diode is recommended to

protect the SiP4282.

Thermal Considerations

The SiP4282 is designed to maintain a constant output load

current. Due to physical limitations of the layout and assem-

bly of the device the maximum switch current is 1.2 A, as

stated in the ABS MAX table. However, another limiting char-

acteristic for the safe operating load current is the thermal

power dissipation of the package. To obtain the highest

power dissipation (and a thermal resistance of 90 °C/W) the

power pad of the device should be connected to a heat sink

on the printed circuit board.

The maximum power dissipation in any application is depen-

dant on the maximum junction temperature, TJ(MAX) =

125 °C, the junction-to-ambient thermal resistance for the

SC-75 PPAK package,

J-A = 90 °C/W, and the ambient tem-

perature, TA, which may be formulaically expressed as:

It then follows that, assuming an ambient temperature of

70 °C, the maximum power dissipation will be limited to about

610 mW.

So long as the load current is below the 1.2 A limit, the max-

imum continuous switch current becomes a function two

things: the package power dissipation and the RDS(ON) at the

ambient temperature.

As an example let us calculate the worst case maximum load

current at TA = 70 °C. The worst case RDS(ON) at 25 °C

occurs at an input voltage of 1.8 V and is equal to 480 m

Ω.

The RDS(ON) at 70 °C can be extrapolated from this data

using the following formula

RDS(ON) (at 70 °C) = RDS(ON) (at 25 °C) x (1 + TC x ΔT)

Where TC is 3300 ppm/°C. Continuing with the calculation

we have

RDS(ON) (at 70 °C) = 480 mΩ x (1 + 0.0033 x (70 °C - 25 °C))

= 551 m

The maximum current limit is then determined by

which in case is 1.05 A. Under the stated input voltage con-

dition, if the 1.05 A current limit is exceeded the internal die

temperature will rise and eventually, possibly damage the

device.

Vishay Siliconix maintains worldwide manufacturing capability. Products may be manufactured at one of several qualified locations. Reliability data for Silicon Tech-

nology and Package Reliability represent a composite of all qualified locations. For related documents such as package/tape drawings, part marking, and reliability

data, see http://www.vishay.com/ppg?73851.

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