5
LT1431
PI FU CTIO S
U
UU
COLL (Pin 1): Open collector of the output transistor. The
maximum pin voltage is 26V. The saturation voltage at
100mA is approximately 1V.
COMP (Pin 2): Base of the driver for the output transistor.
This pin allows additional compensation for complex
feedback systems and shutdown of the regulator. It must
be left open if unused.
V
+
(Pin 3): Bias voltage for the entire shunt regulator. The
maximum input voltage is 36V and the minimum to
operate is equal to V
REF
(2.5V). The quiescent current is
typically 0.6mA.
R
TOP
(Pin 4): Top of the on-chip 5k-5k resistive divider
that guarantees 1% accuracy of operation as a 5V shunt
regulator with no external trim. The pin is tied to COLL for
self-contained 5V operation. It may be left open if unused.
See note on parasitic diodes below.
GND-S (Pin 5): Ground reference for the on-chip resistive
divider and shunt regulator circuitry except for the output
transistor. This pin allows external current limit of the
output transistor with one resistor between GND-F (force)
and GND-S (sense).
GND-F (Pin 6): Emitter of the output transistor and sub-
strate connection for the die.
R
MID
(Pin 7): Middle of the on-chip resistive divider string
between R
TOP
and GND-S. The pin is tied to REF for self-
contained 5V operation. It may be left open if unused.
REF (Pin 8): Control pin of the shunt regulator with a 2.5V
threshold. If V
+
> 3V, input bias current cancellation
reduces I
B
to 0.2µA typical.
COMP, R
TOP
, R
MID
, and REF have static discharge protec-
tion circuits that must not be activated on a continuous
basis. Therefore, the absolute maximum DC voltage on
these pins is 6V, well beyond the normal operating condi-
tions.
As with all bipolar ICs, the LT1431 contains parasitic
diodes which must not be forward biased or else anoma-
lous behavior will result. Pin conditions to be avoided are
R
TOP
below R
MID
in voltage and any pin below GND-F in
voltage (except for GND-S).
The following pin definitions apply to the Z package.
CATHODE (Pin 1): Corresponds to COLL and V
+
tied
together.
ANODE (Pin 2): Corresponds to GND-S and GND-F tied
together.
REF (Pin 3): Corresponds to REF.
W
IDAGRA
B
L
O
C
K
LT1431 • BD01
COMPV
+
GND-FORCE
GND-SENSE
R
TOP
REF
R
MID
2.5V
+
–
gm =
4mA/V
5k
COLLECTOR
8
7
5
4321
6
5k
