LT1166 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1166

Power Output Stage Automatic Bias System

Linear Technology 

LT1166

APPLICATIONS INFORMATION

Overvoltage Protection

The supplies VTOP (Pin 1) and VBOTTOM (Pin 4) have clamp

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 VGS voltage. Current into Pin 1 or Pin 4 should be

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 VT (top) and VB

(bottom) of the LT1166 are set by the required “on”

voltage of the power devices. A reference current IREF sets

a constant VBE7 and VBE8. This voltage is across emitter

base of Q9 and Q10 which are 1/10 the emitter area of Q7

and Q8. The expression for this current multiplier is:

VBE7 + VBE8 = VBE9 + VBE10

or in terms of current:

(IC9)(IC10) = (IREF)2/100 = Constant

The product of IC9 and IC10 is constant. These currents are

mirrored and set the voltage on the (+) inputs of a pair of

VTOP

V+

RT

1k

1

M1

IREF

IREF

SHUNT

10

REGULATOR

Q7

Q9

× 10

×1

Q8

Q10

× 10

×1

8

VAB+

1Ω

1k

3

VO

1k

VAB–

1Ω

5

4

VBOTTOM

Figure 2. Constant Product Generator

M2

RB

1k

V–

1166 • F02

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 IREF

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 VT or VB to increase by an amount

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 VT and VB. When a

positive voltage VIN is applied to RIN, a small input current

flows into R2 and the emitter of Q2. This effect causes VO

to follow VIN within the gain error of the amplifier. The

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 RB and this causes VB to rise by the same amount

Q6

× 32

CEXT1

RIN

2

VIN

VTOP

1

V+

RT

1k

M1

Q5

×1

Q1

R1 Q11

R2 Q12

Q2

1Ω

3

VO

1Ω

Q4

× 32

CEXT2

Q3

×1

4

VBOTTOM

M2

RB

1k

V–

1166 • F03

Figure 3. Input Stage Driving Gates

5

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