Micrel
MIC2589/MIC2595
following as a starting point:
fault condition. Should either event occur, the GATE
pin is immediately pulled low and will remain low until
the ON pin voltage once again rises above its VONH
threshold. The circuit’s turn-on and turn-off voltage
levels are set using the resistor divider R1, R2, and
R3 similar to the “Typical Application” circuit and the
equations to set the trip points are shown below. For
the following example, the circuit’s ON threshold is set
to VON = 40V and the circuit’s OFF threshold is set to
VOVH(typ)
100µA
1.223V
100µA
R3 =
=
= 12.23kꢀ
The closest standard 1% value for R3 = 12.4kꢀ.
Solving for R2 and R1 yields:
⎡
⎢
⎤
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
V
OV
R2 = R3 ×
− 1
⎥
V
⎢
⎣
⎥
⎦
UV
V
OFF = 35V.
⎡
⎛
⎤
72V
37V
⎞
⎟
R2 = 12.4kꢀ ×
− 1 = 11.73kꢀ
⎜
⎢
R1+ R2 + R3
)
⎥
VON = VONH(typ)×
VOFF = VOFFL (typ)×
⎝
⎣
⎠
⎦
R3
R1+ R2 + R3
R2 + R3
The closest standard 1% values for R2 = 11.8kꢀ.
Lastly, the value for R1 is calculated:
(
)
(
VOV − 1.223V
)
⎡
⎤
Given VOFF, VON, and any one of the resistor values,
the remaining two resistor values can be readily
determined. A suggested value for R3 is selected to
provide approximately 100µA (or more) of current
through the voltage divider chain at VDD = VOFF. This
yields the following as a starting point:
R1 = R3 ×
− R2
⎢
⎣
⎥
1.223V
⎦
(
72V − 1.223V
)
⎡
⎤
R1 = 12.4kꢀ ×
− 11.8kꢀ
⎢
⎥
1.223V
⎣
⎦
R1 = 705.81kꢀ
The closest standard 1% value for R1 = 698kꢀ.
VOFFL (typ)
100µA
1.223V
100µA
R3 =
=
= 12.23kꢀ
Using standard 1% resistor values, the circuit’s
nominal UV and OV thresholds are:
The closest standard 1% value for R3 = 12.4kꢀ.
V
V
UV = 36.5V
OV = 71.2V
Solving for R2 and R1 yields:
⎡
⎢
⎤
⎥
⎛
⎜
⎜
⎝
⎞
VON
⎟
− 1
R2 = R3 ×
Good general engineering design practices must
consider the tolerances associated with these
parameters, including but not limited to, power supply
tolerance, undervoltage and overvoltage threshold
tolerances, and the tolerances of the external passive
components.
⎟
VOFF
⎢
⎣
⎥
⎦
⎠
⎡
⎤
40V
35V
⎛
⎞
R2 = 12.4kꢀ ×
− 1 = 1.77kꢀ
⎜
⎟
⎢
⎥
⎝
⎣
⎠
⎦
The closest standard 1% value for R2 = 1.78kꢀ.
Lastly, the value for R1 is calculated:
Programmable UVLO Hysteresis (MIC2595 and
MIC2595R)
VON − 1.223V
)
− R2
R1 = R3 ×
1.223V
The MIC2595 and the MIC2595R devices have user-
programmable hysteresis by means of the ON and
OFF pins (Pins 4 and 3, respectively). This allows
setting the MIC2595/MIC2595R 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
example, a battery supply). The MIC2595/MIC2595R
holds the output off until the voltage at the ON pin is
above its VONH threshold value given in the “Electrical
Characteristics” table. Once the output has been
enabled by the ON pin, it will remain on until the
voltage at the OFF pin falls below its respective VOFFL
threshold value, or the part turns off due to an external
40V − 1.223V
R1 = 12.4kꢀ ×
− 1.78kꢀ
1.223V
R1 = 391.38kꢀ
The closest standard 1% value for R1 = 392kꢀ.
Using standard 1% resistor values, the circuit’s
nominal ON and OFF thresholds are:
V
V
ON = 40.1V
OFF = 35V
Good general engineering design practices must
consider the tolerances associated with these
parameters, including but not limited to, power supply
tolerance, undervoltage and overvoltage threshold
tolerances, and the tolerances of the external passive
components.
19
M9999-120505
(408) 955-1690
December 2005
MICREL [ MICREL SEMICONDUCTOR ]
