LT1166 Linear Technology, LT1166 Datasheet - Page 5

• Manufacturer Part Number: LT1166
• Description: Power Output Stage Automatic Bias System
• Manufacturer: Linear Technology

APPLICATIONS
Overvoltage Protection
The supplies V
diodes that turn on when they exceed 12V. These diodes
act as ESD protection and serve to protect the LT1166
when used with large power MOS devices that produce
high V
limited to 75mA maximum.
Multiplier Operation
Figure 2 shows the current multiplier circuit internal to the
LT1166 and how it works in conjunction with power
output transistors. The supply voltages V
(bottom) of the LT1166 are set by the required “on”
voltage of the power devices. A reference current I
a constant V
base of Q9 and Q10 which are 1/10 the emitter area of Q7
and Q8. The expression for this current multiplier is:
or in terms of current:
The product of I
mirrored and set the voltage on the (+) inputs of a pair of
REGULATOR
SHUNT
V
(I
BE7
C9
)(I
GS
+ V
C10
voltage. Current into Pin 1 or Pin 4 should be
Q7
Q8
10
10
BE8
) = (I
Figure 2. Constant Product Generator
BE7
V
TOP
I
BOTTOM
= V
REF
V
C9
TOP
and V
REF
and I
(Pin 1) and V
BE9
U
)
2
I
/100 = Constant
REF
10
Q9
Q10
BE8
+ V
C10
1
1
INFORMATION
. This voltage is across emitter
BE10
U
is constant. These currents are
BOTTOM
–
+
+
–
W
(Pin 4) have clamp
V
V
1
4
AB
AB
+
–
T
(top) and V
1k
1k
U
8
3
5
R
1k
R
1k
REF
T
B
1166 • F02
sets
M
M
1
1
2
1
V
V
V
O
+
–
B
internal op amps. The feedback of the op amps force the
same voltage on the (–) inputs and these voltages then
appear on the sense resistors in series with the power
devices. The product of the two currents in the power
devices is constant, as one increases the other decreases.
The excellent logging nature of Q9 and Q10 allows this
relation to hold over many decades in current.
The total current in Q7 and Q8 is actually the sum of I
and a small error current from the shunt regulator. During
high output current conditions the error current from the
regulator decreases. Current conducted by the regulator
also decreases allowing V
needed to drive the power devices.
Driving the Input Stage
Figure 3 shows the input transconductance stage of the
LT1166 that provides a way to drive V
positive voltage V
flows into R2 and the emitter of Q2. This effect causes V
to follow V
input current is then mirrored by Q3/Q4 and current
supplied to Q4’s collector is sourced by power device M1.
The signal current in Q4’s emitter is absorbed by external
resistor R
R
V
C
IN
IN
EXT1
B
IN
and this causes V
Q6
Q4
32
2
32
Figure 3. Input Stage Driving Gates
within the gain error of the amplifier. The
C
EXT2
IN
is applied to R
Q1
Q2
Q5
Q3
R1
R2
T
1
1
or V
B
Q11
Q12
to rise by the same amount
B
to increase by an amount
IN
V
BOTTOM
V
, a small input current
TOP
T
1
4
and V
3
R
1k
R
1k
LT1166
T
B
B
. When a
1166 • F03
M
M
1
1
2
1
V
V
V
+
O
–
5
REF
O