Si91845/6

Vishay Siliconix

Document Number: 71733

S-40591—Rev. B, 29-Mar-04

www.vishay.com

9

BLOCK DIAGRAM

Si91845/6

Reference

−

+

Thermal

Sensor

Shutdown

Control

GND

ERROR

Reverse Polarity

Protection

Current

Limit

SD

BP

Si91845

Only

DETAILED DESCRIPTION

The Si91845/6 is a low-noise, low drop-out and low quiescent

current linear voltage regulator, packaged in a small footprint

Thin SOT23-6 package. The Si91845/6 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.

An external bypass capacitor

connected to the BP pin reduces noise at the output. 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_, the device turns the p-channel pass

transistor off.

Reverse Battery Protection

The Si91845/6 has a battery reverse protection circuitry that

voltage. There is no current drawn in such an event. When the

reverse battery protection is not desired.

Noise Reduction

An external 10-nF bypass capacitor at BP is used to create a low

pass filter for noise reduction. The start-up time is fast, since a

power-on circuit pre-charges the bypass capacitor. After the

power-up sequence the pre-charge circuit is switched to standby

mode in order to save current. It is therefore not recommended

to use larger bypass capacitor values than 50 nF. When the

circuit is used without a capacitor, stable operation is guaranteed.

ERROR

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

(off) when SD pin is low.

Auto-Discharge/No-Discharge

SD pin is low. This applies only to the Si91845. The Si91846

does not have a discharge path when the SD pin is low.

Stability

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

6 nF/mA (= 1 mF @ 150 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 <0.4 W is necessary. Parasitic

inductance of about 10 nH can be tolerated.