17
M9999-122012
Application Information
External ON/OFF Control
The MIC2586/MIC2586R has an ON pin input that is
used to enable the controller to commence a start-up
sequence upon card insertion or to disable controller
operation upon card removal. In addition, the ON pin
can be used to reset the MIC2586/MIC2586R’s internal
electronic circuit breaker in the event of a load current
fault. To reset the electronic circuit breaker, the ON pin
is toggled LOW then HIGH. The ON pin is internally
connected to an analog comparator with 80mV of
hysteresis. When the ON pin voltage falls below its
internal V
ONL
threshold, the GATE pin is immediately
pulled low. The GATE pin will be held low until the ON
pin voltage is above its internal V
ONH
threshold. The
external circuit's ON threshold voltage level is
programmed using a resistor divider (R1 and R2) as
shown in the typical application circuit. The equations to
set the trip points are shown below. For the following
example, the external circuit's ON threshold is set to
V
ONH(EX)
= +37V, a value commonly used in +48V Central
Office power distribution applications.
+
×=
R2
R2R1
VV
ONHONH(EX)
Eq. 13
Given V
ONH
and R2, a value for R1 can be determined.
A suggested value for R2 is that which will provide
approximately 100µA of current through the voltage
divider chain at V
CC
= V
ONH
. This yields the following as
a starting point:
13.13kΩ
100µA
1.313V
100µA
V
R2
ONH(TYP)
===
Eq. 14
The closest standard 1% value for R2 is 13kΩ. Now,
solving for R1 yields:
353.3kΩ
1
1.313V
37V
13kΩ1
V
V
R2R1
ONH(TYP)
ONH(EX)
=
−×=−×=
Eq.15
The closest standard 1% value for R1 is 357kΩ.
Using standard 1% resistor values, the external circuit's
nominal ON and OFF thresholds are V
ON(EX)
= +36V and
V
OFF(EX)
= +34V. In solving for V
OFF(EX)
, replace V
ONH
with
V
ONL
in Equation 13.
Output Voltage PWRGD Detection
The MIC2586/86R includes an analog comparator used
to monitor the output voltage of the controller through an
external resistor divider as shown in the typical
application circuit. The FB input pin is connected to the
non-inverting input and is compared against an internal
reference voltage. The analog comparator exhibits a
hysteresis of 80mV.
Setting the PWRGD threshold for the circuit follows a
similar approach as setting the circuit's ON/OFF input
voltage. The equations to set the trip points are shown
below. For the following +48V telecom application,
power-is-good output signal PWRGD1 (or /PWRGD1) is
to be de-asserted when the output supply voltage is
lower than +48V-10% (+43.2V):
×
R6
R6+R5
V=V
FBL
GOOD) OUT(NOT
Eq.16
Given V
FBL
and R6, a value for R5 can be determined. A
suggested value for R6 is that which will provide
approximately 100µA of current through the voltage
divider chain at V
OUT(NOT GOOD)
= V
FBL
. This yields the
following equation as a starting point:
12.33kΩ
100µA
1.233V
100µA
V
R6
FBL(TYP)
===
Eq. 17
The closest standard 1% value for R6 is 12.4kΩ. Now,
solving for R5 yields:
422k
Ω
1
1.233V
43.2V
12.4kΩ1
V
V
R6R5
FBL(TYP)
GOOD) OUT(NOT
=
−×=−×=
Eq. 18
The closest standard 1% value for R5 is 422kΩ.
Using standard 1% resistor values, the external circuit's
nominal "power-is-good" and "power-is-not-good" output
voltages are V
OUT(GOOD)
= +46V and V
OUT(NOT GOOD)
=
+43.2V
