LT1185 LINER [Linear Technology], LT1185 Datasheet - Page 5

• Manufacturer Part Number: LT1185
• Description: Low Dropout Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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
U U
V
2.37V
REF
–
+
V
IN
A2
300mV
W
–
A5
D2
+
I1
2 A
U
Figure 1. Block Diagram
Q4
REF
R
D1
LIM
+
(EXTERNAL)
R1
350
A4
D4
–
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.
–
A1
+
+
200mV
A3
D3
–
Q1
LT1185 • BD
Q2
Q3
R2
0.055
GND
FB
V
OUT
LT1185
5