regulate with shunt currents as low as 1
mA (at 1.24 V) and as high as 8 A (a 78-
dB dynamic range).

The circuits can be used anywhere a

power- zener diode function would be
utilized. Some typical applications are:

1) A high-current (up to 8 A) preci-

sion shunt-regulator (“power zener”).

2) A precise +3.3-V/8-A power supply

derived from regulated +5 V.

1

3) A precision, high-current voltage

clamp for overvoltage protection of dc
power supplies.

IDEAS FOR DESIGN

132

ELECTRONIC DESIGN • February 7, 2000

Robert N. Buono

Buono Consulting, 31 Upper Lakeview Ave., Ringwood, NJ 07456;
[phone]; fax [phone]; e-mail: [e-mail]

High-Current, Low-Voltage
Shunt Regulator

CIRCLE 521

T

his design idea describes a high-
current (up to 8 A) shunt-regulator
built around the TLV431 low-volt-

age, adjustable, precision shunt-regula-
tor IC. Special attention was paid to
implementing this design as a “two-ter-
minal” circuit block, greatly increasing
the versatility of the design. The circuit
block can be conceptualized and imple-
mented in any application where a

power-zener diode would be used.
However, this circuit has much greater
precision, accuracy, and range of opera-
tional currents than such a diode.

Two circuit varieties are described,

based on the voltage range desired. For
shunt voltages ranging from 1.75 V to 6
V, the circuit in Figure 1 can be used; for
voltages between 1.24 V and 1.75 V, the
circuit of Figure 2 must be used.

Both circuits exploit

the best features of the
TLV431 shunt-regula-
tor—very low opera-
tional bias current levels
(80

µ

A max.), and a pre-

cision reference voltage
(1.24 V). These two fea-
tures allow the circuits to
regulate accurately over a
wide range of currents.
The circuit shown in Fig-
ure 1 will regulate pre-
cisely with shunt cur-
rents as low as 200

µ

A

and as high as 8 A. That’s
a 91.8-dB dynamic range.
The Figure 2 circuit will

1.

This high-current shunt-regulator is designed for applications

requiring shunt voltages ranging from 1.75 V to 6 V.

RA

1%

VREF

Q1

ZTX788B

RB

10k

1%

Q2

D44VH10

R1

1k

+

–

R3

1k

U1

TLV431C

TLV431AC

R2

100

All resistors are 1/8 W, 5%

except for RA and RB

V

SHUNT

R

B

(1%)

R

A

(1%)

1.24 V

OPEN

SHORT

1.50 V

10.0k

2.10k

1.70 V

10.0k

3.74k

TABLE 2: SUGGESTED

FIGURE 2 RESISTOR VALUES

V

SHUNT

R

B

(1%)

R

A

(1%)

R5 (1%)

1.75 V

10.0k

4.12k

100

2.00 V

10.0k

6.19k

100

2.50 V

10.0k

10.1k

100

5.00 V

10.0k

30.1k

100

TABLE 1: SUGGESTED

FIGURE 1 RESISTOR VALUES

The reason for the two different circuit

implementations (Figs. 1 and 2) is the
compliance voltage range of the cathode
terminal of the TLV431. The cathode ter-
minal can reach as high as 6 V, but can
only go approximately 200 mV below
the reference voltage (1.24 V) while
sinking current. For shunt voltages of
1.75 V up to 6 V, implemented using the
circuit of Figure 1, the cathode voltage
need only go as low as V

SHUNT

−

V

BE

. So

for V

SHUNT

= 1.75V, and V

BE

= 0.6V,

V

CATHODE

will be 1.15 V. This is within

the cathode’s operational voltage range.

Operation of the circuit in Figure 1 is

as follows: For voltages below the
shunt voltage, only the bias current of
the TLV431 will be drawn. The voltage
developed across R1, due to this bias
current, won’t be sufficient to turn on
Q1. Thus, Q1 and Q2 will be off. As
soon as the shunt voltage increases to
the point where V

REF

reaches 1.24 V

(defined by the resistive divider con-
sisting of R

A

and R

B

), the TLV431 will

begin to sink current at its cathode ter-
minal. This current will turn on Q1,
which in turn drives Q2. As a result, the
collector currents of both these transis-
tors are controlled by U1. The total
shunt current (I

SHUNT

)

∪

I

CATHODE

+

I

cQ1

+ I

cQ2

. Also, I

SHUNT

=

I

CATHODE

(

β

Q1)(

β

Q2). The high Beta (

β

= 400

min.) of Q1 makes a Darlington tran-
sistor unnecessary. It also allows lower
voltage operation, since the extra V

BE

drop of a Darlington is avoided.

For shunt voltages less than 1.75 V,

the circuit of Figure 2 should be used.
This circuit allows shunt voltages as low
as the reference voltage (1.24 V) to be

tightly regulated, with shunt currents
ranging from 1 mA up to 8 A. The main
difference between the two circuits is
that the current-mirror consisting of Q1
and Q2 allows Q3 and Q4 to be fully
driven with just a 150-mV range of cath-
ode voltage. Thus, the cathode voltage
of the TLV431 drops just 150 mV below
the reference voltage when regulating a
shunt voltage of 1.24 V.

Operation is as follows: For shunt

voltages below the set point, a bias cur-
rent will flow through Q1, R1, R2, and
R3. The bias current flowing through R2
creates an offset voltage that keeps Q2
off. This, in turn, keeps Q3 and Q4 off.
When the shunt voltage is high enough
so that V

REF

reaches 1.24 V (defined by

resistive divider R

A

and R

B

), U1 will start

to sink cathode current.

This cathode current will create a volt-

age drop across R1, causing Q1’s emitter
voltage to drop. The voltages on the
base and collector of Q1 will follow the
emitter. When the voltage at the collec-
tor of Q1 drops low enough, it will be-
gin to turn on Q2. Then Q3 and Q4 will
be driven just as Q1 and Q2 of Figure 1
were driven. (Note: Q3 and Q4 of Fig-
ure 2 are complementary devices to Q1
and Q2 of Figure 1). A drop across R1 of
150 mV maximum is sufficient to cause
8 A of shunt current to flow. Q2 of Fig-
ure 1 and Q4 of Figure 2 must have a
heat sink suitable for their level of
power dissipation, given by P

D

=

(I

SHUNT

)(V

SHUNT

).

Reference:
1. “Single IC in TO-220 Case Converts

5-V Rail to 3.3 V,”

ELECTRONIC DESIGN

, De-

cember 17, 1992, p. 37.

IDEAS FOR DESIGN

134

ELECTRONIC DESIGN • February 7, 2000

RA

1%

VREF

RB

10k

1%

R1

36

R2

68

R3

1k

R5

1k

+

–

U1
TLV431C
TLV431AC

All resistors are 1/8 W, 5%

except for RA and RB

R4

43

R6

1k

Q4

D45VH10

Q3

ZTX690B

Q1

2N3906

Q2

2N3906

1.70 V

3.3 V

1 mA min,

8 A max

+3.3 V @ 8 A

derived from +5.0 V

+

+

–

–

1 mA min

8 A max

VSHUNT

Precision shunt-regulator

or voltage clamp

5.0 V

+

–

VSHUNT

+

+

–

–

2.

When shunt voltages range from 1.24 V to 1.75 V, this circuit becomes a must.

3.

Shown are block-diagram representations of some possible design applications.