Tlv431 – Diodes TLV431 User Manual
Page 8
TLV431
Document number: DS32088 Rev. 6 - 2
8 of 14
October 2012
© Diodes Incorporated
TLV431
A Product Line of
Diodes Incorporated
Application Notes
In a conventional shunt regulator application (Figure 1), an external series resistor (R
3
) is connected between the supply voltage, V
IN
, and the
TLV431.
R
3
determines the current that flows through the load (I
L
) and the TLV431 (I
K
). The TLV431 will adjust how much current it sinks or “shunts” to
maintain a voltage equal to V
REF
across its feedback pin. Since load current and supply voltage may vary, R
3
should be small enough to supply
at least the minimum acceptable I
KMIN
to the TLV431 even when the supply voltage is at its minimum and the load current is at its maximum
value. When the supply voltage is at its maximum and I
L
is at its minimum, R
3
should be large enough so that the current flowing through the
TLV431 is less than 15mA.
R
3
is determined by the supply voltage, (V
IN
), the load and operating current, (I
L
and I
K
), and the TLV431’s reverse breakdown voltage, V
KA
.
Figure 1
K
L
KA
IN
3
I
I
V
V
R
+
−
=
where
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
×
=
2
1
REF
KA
R
R
1
V
V
and V
KA
= V
OUT
The values of R1 and R2 should be large enough so that the current flowing through them is much smaller than the current through R3 yet not
too large that the voltage drop across them caused I
REF
affects the reference accuracy.
The most frequent application of the TLV431 is in isolated low output voltage power supplies where the regulated output is galvanically isolated
from the controller. As shown in Figure 2 the TLV431 drives current, I
F
, through the opto-coupler’s LED which in turn drives the isolated transistor
which is connected to the controller on the primary side of the power supply.
This completes the feedback path through the isolation barrier and ensures that a stable isolated supply is maintained.
Assuming a forward drop of 1.4V across the opto-coupler diode allows output voltages as low as 2.7V to be regulated.
Regulated Supply
Regulated Supply
To controller
To controller
R3
I
F
R1
R2
TLV431
GND
Optocoupler
⎟
⎠
⎞
⎜
⎝
⎛ +
=
2
R
1
R
1
V
V
REF
OUT
mA
15
7
.
2
V
3
R
I
7
.
2
V
(max)
OUT
(min)
F
OUT
−
≥
>
−
Figure 2. Using the TLV431 as the Regulating Element in an Isolated PSU