March 2005 21 MIC2584/2585
MIC2584/2585 Micrel
Input Overvoltage Protection
A similar design approach as the previous Undervoltage
Detection example is recommended for the overvoltage
protection circuitry, resistors R6 and R7 for OV1, in Figure 1.
For input overvoltage protection, the first consideration is to
establish the input voltage level that indicates an overvoltage
triggering a system (output voltage) shut down. For our
example, the input value for which the Channel 1 12V supply
will signal an “output shutdown” is 13.2V (+10%). Similarly,
from the previous example:
1) Choose R7 to satisfy 100µA condition.
R7
V
100 A
1.19V
100 A
11.9k
OV1(MIN)
≥
µ
≥
µ
≥Ω
R7 is chosen as 13.0kΩ ±1%
2) Thus, following the previous example and
substituting R6 and R7 for R12 and R13,
respectively, V
OV1(MIN)
for V
FB1(MAX)
, and 13.2V
overvoltage for 10.5V output “good,” the same
formula yields R6 of 131.2kΩ. The nearest
standard 1% value is 130kΩ.
Now, consider the 12.6V maximum input voltage
(V
CC1
+5%), the higher tolerance for R7 and lower tolerance
for R6, 13.13kΩ and 128.7kΩ, respectively. With 12.6V input,
the voltage sensed at the OV1 pin is below V
OV1(MIN)
, and the
MIC2584/85 will not indicate an overvoltage condition until
V
CC1
exceeds approximately 13.2V considering the given
tolerances. A similar approach should be used for Channel 2.
PCB Connection Sense
There are several configuration options for the MIC2584/85’s
ON pin to detect if the PCB has been fully seated in the
backplane before initiating a start-up cycle. In Figure 1, the
MIC2584/85 is mounted on the PCB with a resistive divider
network connected to the ON pin. R4 is connected to a short
pin on the PCB edge connector. Until the connectors mate,
the ON pin is held low which keeps the GATE output charge
pump off. Once the connectors mate, the resistor network is
pulled up to the input supply, 12V in this example, and the ON
pin voltage exceeds its threshold (V
ON
) of 1.235V and the
MIC2584/85 initiates a start-up cycle. In Figure 9, the connec-
tion sense consisting of a discrete logic-level MOSFET
and a few resistors allows for interrupt control from the
processor or other signal controller to shut off the output of the
MIC2584/85. R4 pulls the GATE of Q2 to V
IN
and the ON pin
is held low until the connectors are fully mated. Once the
connectors fully mate, a logic LOW at the /ON_OFF signal
turns Q2 off and allows the ON pin to pull up above its
threshold and initiate a start-up cycle. Applying a logic HIGH
at the /ON_OFF signal will turn Q2 on and short the ON pin
of the MIC2584/85 to ground which turns off the GATE output
charge pump.
GND
ON_OFF
C3
0.033µF
C4
0.01µF
SENSE1VCC1
ON
CPOR
OUT1
FB1
GATE1
GND
/POR
CFILTER
/FAULT
Long
Pin
Backplane
Connector
PCB Edge
Connector
Long
Pin
Medium or
Short Pin
Undervoltage (Output) = 4.45V
/POR Delay = 16.5ms
START-UP Delay = 4ms
Circuit Breaker Response Time = 5ms
*Q2 is TN0201T (SOT-23)
Channel 2 and additional pins omitted for clarity.
Downstream
Signal
Short
Pin
PCB Connection Sense
Q1
Si7892DP
(PowerPAK™ SO-8)
R5
10Ω
R7
10.5kΩ
1%
R4
20kΩ
R1
33kΩ
R3
33Ω
*Q2
MIC2584
C1
1µF
C2
0.01µF
C
LOAD1
1000µF
R2
33kΩ
R6
27.4kΩ
1%
V
OUT1
5V@7A
V
IN1
5V
/FAULT
11
97
8
16 15
12
13
6
10
14
R
SENSE1
0.005Ω
5%
12
34
Figure 9. PCB Connection Sense with ON/OFF Control
