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LT1185

1185ff

Block Diagram

A simplified block diagram of the LT1185 is shown in

Figure 1. A 2.37V bandgap reference is used to bias the

input of the error amplifier A1, and the reference amplifier

A2. A1 feeds a triple NPN pass transistor stage which has

the two driver collectors tied to ground so that the main

pass transistor can completely saturate. This topology

normally has a problem with unlimited current in Q1 and

Q2 when the input voltage is less than the minimum

required to create a regulated output. The standard “fix”

for this problem is to insert a resistor in series with Q1 and

Q2 collectors, but this resistor must be low enough in

value to supply full base current for Q3 under worst-case

Figure 1. Block Diagram

300mV

µA

350

Ω

200mV

–

VIN

0.055

Ω

–

VREF

2.37V

VOUT

GND

RLIM (EXTERNAL)

REF

LT1185 • BD

conditions, resulting in very high supply current when the

input voltage is low. To avoid this situation, the LT1185

uses an auxiliary emitter on Q3 to create a drive limiting

feedback loop which automatically adjusts the drive to Q1

so that the base drive to Q3 is just enough to saturate Q3,

but no more. Under saturation conditions, the auxiliary

emitter is acting like a collector to shunt away the output

current of A1. When the input voltage is high enough to

keep Q3 out of saturation, the auxiliary emitter current

drops to zero even when Q3 is conducting full load current.

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