Si91872

Vishay Siliconix

Document Number: 72013

S-51147—Rev. F, 20-Jun-05

www.vishay.com

9

DETAILED DESCRIPTION

The Si91872 is a low-noise, low drop-out and low quiescent

current linear voltage regulator, packaged in a small footprint

MLP33-5 package. The Si91872 can supply loads up to

300 mA. As shown in the block diagram, the circuit consists of

a bandgap reference, error amplifier, p-channel pass transistor

and feedback resistor string. Additional blocks, not shown in

the block diagram, include a precise current limiter, reverse

battery and current protection, and thermal sensor.

Thermal Overload Protection

The thermal overload protection limits the total power

dissipation and protects the device from being damaged.

When the junction temperature exceeds 150_C, the device

turns the p-channel pass transistor off.

Reverse Battery Protection

The Si91872 has a battery reverse protection circuitry that

GND voltage. There is no current drawn in such an event.

pin is allowed when reverse battery protection is not desired.

ERROR

less than 4% of its normal value. To obtain a logic level output,

impedance (off) when SD pin is low.

Auto-Discharge

when SD pin is low for the Si91872.

Stability

The circuit is stable with only a small output capacitor equal to

6 nF/mA (= 2 mF @ 300 mA). Since the bandwidth of the error

amplifier is around 1−3 MHz and the dominant pole is at the

output node, the capacitor should be capacitive in this range,

i.e., for 150-mA load current, an ESR <0.2 W is necessary.

Parasitic inductance of about 10 nH can be tolerated.

Safe Operating Area

The ability of the Si91872 to supply current is ultimately

dependent on the junction temperature of the pass device.

Junction temperature is in turn dependent on power

dissipation in the pass device, the thermal resistance of the

package and the circuit board, and the ambient temperature.

The power dissipation is defined as

Junction temperature is defined as

To calculate the limits of performance, these equations must

be rewritten.

Allowable power dissipation is calculated using the equation

While allowable output current is calculated using the equation

Ratings of the Si91872 that must be observed are

approximately 46 _C/W.

Figure 1 shows the performance limits graphically for the

Si91872 mounted on the circuit board used for thermal

characterization.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0

123

456

Figure 1. Safe Operating Area

Vishay Siliconix maintains worldwide manufacturing capability. Products may be manufactured at one of several qualified locations. Reliability data for Silicon Technology 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?72013.