Datasheet

ADM1186 Data Sheet
Rev. B | Page 16 of 28
SEQUENCE CONTROL USING A SUPPLY RAIL
The UP and
DOWN
inputs on the ADM1186-1 and the
UP/
DOWN
input on the ADM1186-2 are used to initiate
power-up and power-down sequences. These inputs are
designed for use with digital or analog signals, such as power
supply rails. Using a power supply rail to control the up and
down sequencing allows the ADM1186 to perform sequencing
and monitoring functions for five supply rails.
When using a supply rail to control an ADM1186-1 (with
the UP and
DOWN
pins connected) or an ADM1186-2,
some hysteresis is required. The hysteresis is added on the
joined UP and
DOWN
pins of the ADM1186-1 or on the
UP/
DOWN
pin of the ADM1186-2 to ensure that a slowly
ramping supply rail does not cause spurious rising or falling
edges that would otherwise cause state machine faults.
To provide the necessary hysteresis, a single additional resistor
(RH in Figure 27) is connected between the joined UP and
DOWN
pins of the ADM1186-1 and the OUT1 pin of the
device, or between the UP/
DOWN
pin of the ADM1186-2 and
the OUT1 pin of the device.
07153-038
VIN
3.3V
UP
OUT1
–
+
VP
DOWN
–
+
AD
M1186
-1
STATE
MACHINE
R1
RP
R2
RH
VCC
1.4V
Figure 27. Using a Supply Rail to Control Sequencing with Hysteresis
When OUT1 is low, the resistor RH sinks current from the
node at the midpoint of R1 and R2, slightly increasing the VIN
voltage needed to start a power-up sequence, referred to as V
H
.
When OUT1 is high, RH sources current into the midpoint of
R1 and R2, decreasing the VIN voltage necessary to start a
power-down sequence, referred to as V
L
.
The hysteresis at the VIN node is simply V
H
− V
L
. As the R1
and R2 resistors scale VIN down, the hysteresis on VIN is also
scaled down. The scaled hysteresis, V
SHYS
, at the inputs to the
UP and
DOWN
pins (ADM1186-1) or the UP/
DOWN
pin
(ADM1186-2) must be at least 75 mV. The value of RH is
selected to ensure that this is the case.












×
+
×+×=
RH R2
RH R2
R1V
H
14.1
















+
−
−×+=
RP RH
VP
R2
R1V
L
4.14.1
4.1
( )






+
×−=
R2 R1
R2
V V V
LH
SHYS
In the example application shown in Figure 27, the following
values could be used:
RP = 10 kΩ
VP = 5 V
VIN = 3.3 V
The values of the R1 and R2 resistors determine the midpoint
of the hysteresis, V
MID
, about which V
H
and V
L
set the levels at
which power-up and power-down sequences are initiated. For a
3.3 V supply, a threshold just below 3 V could be used, making
R1 = 11 kΩ and R2 = 10 kΩ and giving a midpoint of 2.94 V.
( )
( )
k10
k10k114.14.1 +×
=
+×
=
R2
R2R1
V
MID
V
MID
= 2.94 V
As a general rule, the value for RH is approximately 60 times
the value of R1 in parallel with R2. In this example, R1 in
parallel with R2 is 5.24 kΩ, so RH would be approximately
314 kΩ. Taking a value of 300 kΩ for RH and using this value
in the previous equations for V
H
, V
L
, and V
SHYS
, the following
values are obtained:












×
+
×+×=
k300k10
k300 k 10
k1114.1 V
H
V
H
= 2.991 V
















+
−
−×+=
k10k300
4.15
k10
4.1
k114.1 V
L
V
L
= 2.812 V
( )






+
×−=
k10k
11
k10
812.2991.2
SHYS
V
V
SHYS
= 0.085 V
Because the value of V
SHYS
is greater than the 75 mV of scaled
hysteresis required, the RH resistor value selected is sufficient.
If the value of V
SHYS
obtained is too small, the value of RH can
be reduced, increasing the scaled hysteresis provided.