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

disconnects the internal circuitry when V

IN

drops below the

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

When the SD pin is hardwired to V

IN

, the user must connect

the SD pin to V

IN

via a 100-k resistor if reverse battery

protection is desired. Hardwiring the SD pin directly to the V

IN

pin is allowed when reverse battery protection is not desired.

ERROR

ERROR is an open drain output that goes low when V

OUT

is

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

connect a pull-up resister from ERROR to V

OUT

or any other

voltage equal to or less than V

IN

. ERROR pin is high

impedance (off) when SD pin is low.

Auto-Discharge

V

OUT

has an internal 100- (typ.) discharge path to ground

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 F @ 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 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

P

D

= (V

IN

– V

OUT

) * I

OUT

.

Junction temperature is defined as

T

J

= T

A

+ ((P

D

* (R

θ

JC

+ R

θ

CA

)).

To calculate the limits of performance, these equations must

be rewritten.

Allowable power dissipation is calculated using the equation

P

D

= (T

J

T

A

)/ (R

θ

JC

+ R

θ

CA

)

While allowable output current is calculated using the equation

I

OUT

= (T

J

T

A

)/ (R

θ

JC

+ R

θ

CA

) * (V

IN

– V

OUT

).

Ratings of the Si91872 that must be observed are

T

Jmax

= 125 C, T

Amax

= 85 C, (V

IN

– V

OUT

)

max

= 5.3 V,

R

θ

JC

= 8 C/W.

The value of R

θ

CA

is dependent on the PC board used. The

value of R

θ

CA

for the board used in device characterization is

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

1

2

3

4

5

6

Figure 1.

Safe Operating Area

(

I

O

V

IN

V

OUT

(V)

(V

IN

V

OUT

)

MAX

= 5.3 V

T

A

= 50 C

T

A

= 85 C

T

A

= 70 C

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