MIC2589_11 Datasheet, PDF (19/29 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controller/Sequencer

Micrel
following as a starting point:
R3 = ^VOVH(typ) = ^1.223V = 12.23kΩ
100µA 100µA
The closest standard 1% value for R3 = 12.4kΩ.
Solving for R2 and R1 yields:
R2
=
R3 ×
⎢⎢⎣⎡⎜⎜⎝⎛
V_OV
V_UV
⎟⎟⎠⎞
⎤
− 1_⎥
⎥⎦
R2
=
12.4kΩ
×
⎢⎡⎜⎛
⎣⎝
72V
37V
⎟⎞
⎠
−
⎤
1_⎥
⎦
=
11.73kΩ
The closest standard 1% values for R2 = 11.8kΩ.
Lastly, the value for R1 is calculated:
R1
=
R3
×
⎡
⎢
⎣
(V_OV − 1.223V
1.223V
)⎤
⎥
⎦
−
R2
R1
=
12.4kΩ
×
⎡
⎢⎣
(72V − 1.223V
1.223V
)⎤
⎥⎦
−
11.8kΩ
R1 = 705.81kΩ
The closest standard 1% value for R1 = 698kΩ.
Using standard 1% resistor values, the circuit’s
nominal UV and OV thresholds are:
V_UV = 36.5V
V_OV = 71.2V
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.
Programmable UVLO Hysteresis (MIC2595 and
MIC2595R)
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 V_ONH 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 V_OFFL
threshold value, or the part turns off due to an external
MIC2589/MIC2595
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 V_ONH
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 V_ON = 40V and the circuit’s OFF threshold is set to
V_OFF = 35V.
V_ON
=
V_ONH (typ) ×
(R1+ R2 + R3)
R3
V_OFF
=
V_OFFL (typ) ×
(R1+ R2 + R3)
R2 + R3
Given V_OFF, V_ON, 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 V_DD = V_OFF. This
yields the following as a starting point:
R3 = ^{VOFFL (typ)} = ^1.223V = 12.23kΩ
100µA 100µA
The closest standard 1% value for R3 = 12.4kΩ.
Solving for R2 and R1 yields:
R2
=
R3
×
⎢⎢⎣⎡⎜⎜⎝⎛
V_ON
V_OFF
− 1_{⎟⎟⎠⎞⎥⎥⎦⎤}
R2
=
12.4kΩ
×
⎢⎡⎜⎛
⎣⎝
40V
35V
− 1⎟⎞⎥⎤
⎠⎦
= 1.77kΩ
The closest standard 1% value for R2 = 1.78kΩ.
Lastly, the value for R1 is calculated:
R1 = R3 × ^{(VON − 1.223V )} − R2
1.223V
R1 = 12.4kΩ × ^{40V − 1.223V} − 1.78kΩ
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:
V_ON = 40.1V
V_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.
December 2005
19
M9999-120505
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