MIC2588 Datasheet, PDF (12/14 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controllers

MIC2588/MIC2594
R1, R2, and R3 as shown in the “Typical Application.” The
equations to set the trip points are shown below. For the
following example, the circuit’s UV threshold is set to V_UV = 37V
and the OV threshold is placed at V_OV = 72V, values commonly
used in Central Office power distribution applications.
V_UV
=
V_UVL(typ) ×
(R1+ R2 + R3)
(R2 +R3)
V_OV
=
V_OVH(typ) ×
(R1+ R2 + R3)
R3
Given V_UV, V_OV, and any one resistor value, the remaining
two resistor values can be found. A suggested value for R3
is that which will provide approximately 100µA of current
through the voltage divider chain at V_DD = V_UV. This yields the
following as a starting point:
R3 = ^VOVH(typ) = 12.23kΩ
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^{}
R2 = 11.729kΩ
The closest standard 1% value for R2 = 11.8kΩ. Next, the
value for R1 is calculated:
R1 =
R3
×


V_OV – 1.223V
1.223V


–
R2
R1 =
12.4kΩ
×

72V – 1.223V
1.223V

–
R2
R1= 705.808kΩ
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
Programmable UVLO Hysteresis—MIC2594
The MIC2594 has user-programmable hysteresis by means of
the ON and OFF pins. This allows setting the part 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 bus while at the same time increasing
the amount of available operating time from a loosely regulated
power supply (for example, a battery supply). Similarly to the
MIC2588, each of these pins is internally connected to an
Micrel
analog comparator with 20mV of hysteresis. The MIC2594
holds the output off until the voltage at the ON pin exceeds 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 V_OFFL
threshold value, or the part turns off due to a fault. Should either
event occur, the GATE pin is immediately pulled low and will
remain low until the ON pin once again exceeds its V_ONH
threshold. The circuit’s turn-on and turn-off points are set using
the resistor divider R1, R2, and R3 as shown in the “Typical
Application.” The equations to establish the trip points are
shown below. In the following example, the circuit’s ON thresh-
old is set to V_ON = 40V and the circuit’s OFF threshold is 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 resistor value, the remaining
two resistor values can be readily found. A suggested value
for R3 is that which will provide approximately 100µA of
current through the voltage divider chain at V_DD = V_OFF. This
yields the following as a starting point:
R3 = ^VOFFL(typ) = 12.23kΩ
100µA
The closest standard 1% value for R3 = 12.4kΩ.
Then, solving for R2 and R1 yields:
R2
=
R3
×


V_ON
V_OFF


–

1

R2
=
12.4kΩ
×

40V
35V

–
1^{}
R2 = 1.771kΩ
The closest standard 1% value for R2 = 1.78kΩ.
( ) R1= R3 × VON – 1.223V – R2
1.223V
R1= 12.4kΩ × ^{(40V – 1.223V)} – R2
1.223V
R1= 391.380kΩ
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
M9999-122303
12
December 2003