MIC2584 datasheet(21/28 Pages) MICREL

21 / 28 page

March 2005

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 100

µA condition.

100 A

1.19V

100 A

11.9k

OV1(MIN)

≥

≥Ω

R7 is chosen as 13.0k

Ω ±1%

2) Thus, following the previous example and

substituting R6 and R7 for R12 and R13,

respectively, V

OV1(MIN) for VFB1(MAX), and 13.2V

overvoltage for 10.5V output “good,” the same

formula yields R6 of 131.2k

Ω. The nearest

standard 1% value is 130k

Ω.

Now, consider the 12.6V maximum input voltage

CC1 +5%), the higher tolerance for R7 and lower tolerance

for R6, 13.13k

Ω 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

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

0.033

µF

0.01

µF

SENSE1

VCC1

CPOR

OUT1

FB1

GATE1

GND

/POR

CFILTER

/FAULT

Long

Backplane

Connector

PCB Edge

Connector

Long

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

PCB Connection Sense

Si7892DP

(PowerPAK™ SO-8)

Ω

10.5k

Ω

20k

Ω

33k

Ω

*Q2

MIC2584

µF

0.01

µF

CLOAD1

1000

µF

33k

Ω

27.4k

Ω

VOUT1

5V@7A

VIN1

/FAULT

RSENSE1

0.005

Ω

Figure 9. PCB Connection Sense with ON/OFF Control

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