MIC2588_05 Datasheet, PDF (13/21 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controllers

MIC2588/MIC2594
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 nominal ON threshold is set to
V_ON = 40V and the circuit’s nominal 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 a minimum of 100µA of cur-
rent through the voltage divider chain at V_DD = V_OFF. This
yields the following as a starting point:
R3 = ^VOFFL(typ)  12.23k
100A
The closest standard 1% value for R3 = 12.4kΩ.
solving for R2 and R1 yields:
R2
=
R3



V_ON
V_OFF


–

1

Micrel
R2
=
12.4k


40V
35V

–
1^{}
R2 = 1.771k
The closest standard 1% value for R2 = 1.78kΩ.
  R1= R3  VON –1.223V – R2
1.223V
R1= 12.4k  ^{40V – 1.223V} – 1.78kΩ
1.223V
R1= 391.380k
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, undervoltgae
and overvoltage tolerances, and the tolerances of the
external passive components.
M9999-083005
13
September 2005