MIC2589/2595
Micrel
R1+R2 +R3
(
(typ)×
ONH
)
V
V
= V
OV
ON
R2 =R3 ×
–1
R3
V
UV
R1+R2 +R3
(
(typ)×
OFFL
)
V
= V
OFF
72V
37V
R2+R3
R2 = 12.4kΩ ×
–1 = 11.73kΩ
Given V
, V , and any one resistor value, the remaining
OFF ON
two resistor values can be readily determined. A suggested
value for R3 is that which will provide approximately 100µA of
The closest standard 1% values for R2 = 11.8kΩ. Lastly, the
value for R1 is calculated:
current through the voltage divider chain at V = V
. This
DD
OFF
yields the following as a starting point:
V
–1.223V
(
)
–R2
OV
R1=R3 ×
V
(typ)
1.223V
OFFL
1.223V
R3 =
=
= 12.23kΩ
100µA
100µA
The closest standard 1% value for R3 = 12.4kΩ. Solving for
R2 and R1 yields:
72V –1.223V
(
)
–R2
R1=12.4kΩ ×
R1= 705.81kΩ
1.223V
V
ON
R2 =R3 ×
–1
V
OFF
The closest standard 1% value for R1 = 698kΩ.
Using standard 1% resistor values, the circuit’s nominal
UV and OV thresholds are:
40V
R2 =12.4kΩ ×
–1 = 1.77kΩ
35V
V
V
= 36.5V
= 71.2V
UV
OV
The closest standard 1% value for R2 = 1.78 kΩ. Lastly, the
Programmable UVLO Hysteresis (MIC2595 and
MIC2595R)
value for R1 is calculated:
V
–1.223V
(
)
–R2
The MIC2595 and the MIC2595R devices have user-pro-
grammable hysteresis by means of the ON and OFF pins
(Pins 4 and 3, respectively). This allows setting the
MIC2595/95R to turn on at a voltage V1, and not turn off until
a second voltage V2, where V2 < V1. This can significantly
simplify dealing with source impedances in the supply buss
while at the same time increasing the amount of available
operating time from a loosely regulated power rail (for ex-
ample, a battery supply). The MIC2595/95R holds the output
ON
R1=R3 ×
1.223V
40V –1.223V
(
)
–R2
R1=12.4kΩ ×
1.223V
R1 = 391.38kΩ
The closest standard 1% value for R1 = 392kΩ.
Using standard 1% resistor values, the circuit’s nominal
ON and OFF thresholds are:
off until the voltage at the ON pin is above its V
threshold
ONH
value given in the “Electrical Characteristics” table. Once the
output has been enabled by the ON pin, it will remain on until
V
V
= 40.1V
= 35V
ON
OFF
the voltage at the OFF pin falls below its respective V
OFFL
threshold value, or the part turns off due to an external fault
condition. Should either event occur, the GATE pin is imme-
diately pulled low and will remain low until the ON pin voltage
once again above its V
threshold. The circuit’s turn-on
ONH
and turn-off voltage levels are set using the resistor divider
R1, R2, and R3 as shown in the “Typical Application” where
the equations to set the trip points are shown below. For the
following example, the circuit’s ON threshold is set to V
=
ON
40V and the circuit’s OFF threshold is set to
= 35V.
V
OFF
M9999-031504
14
March 2004
MICREL [ MICREL SEMICONDUCTOR ]
