Micrel

MIC2589/MIC2595

December 2005

21

M9999-120505

(408) 955-1690

114

13

5

4

3

2

/PWRGD1

CFILTER

CNLD

VEE

/PWRGD3

PGTIMER

GATE

/PWRGD2

SENSE

VDD

-48V RTN

R2

11.8k

1%

2.2µF

0.068µF

*C2

0.1µF

0.01

5%

R4

10

M1

SUM110N10-09

C1

0.47µF

*D1

SMAT70A

-48V RTN

(Long Pin)

-48V RTN

(Long Pin)

-48V RTN

(Short Pin)

System

Reset

UV

MIC2589-2BM

DRAIN

OV

8

7

6

12

10

11

9

N/C

R1

689k

1%

*C3

0.1µF

*M2

*D2

ZMM5266B

C4

0.1µF

C5

100µF

R5

47k

R3

12.4k

1%

*Optional components (See Functional Description and Applications Information for more details)

M2 is an SOT-323, B8S138W or equivalent

D2 is a 68V, 500mW Zener diode

Figure 7. Optional Components for Added Performance/Protection

The circuit in Figure 7 consisting of M2, R5, and a

digital control signal, can be used to reset the

controller after the GATE (and output) turns off. Once

the output has been latched off, applying a low-high-

low pulse on the GATE of M2 via the System Enable

control can toggle the UV pin. System Enable is a

Sense Resistor Selection

The sense resistor is nominally valued at:

om)

HOT_SWAP(n

TRIP(typ)

SENSE(nom)

I

V

R

=

the nominal load current level necessary to trip the

internal circuit breaker.

To accommodate worse-case tolerances in the sense

resistor (for a ±1% initial tolerance, allow ±3%

tolerance for variations over time and temperature)

and circuit breaker threshold voltages, a slightly more

detailed calculation must be used to determine the

minimum and maximum hot swap load currents.

As the MIC2589’s minimum current limit threshold

voltage is 40mV, the minimum hot swap load current

is determined where the sense resistor is 3% high:

()

SENSE(nom)

SENSE(nom)

in)

HOT_SWAP(m

R

38.8mV

R

1.03

40mV

I

=

×

=

Keep in mind that the minimum hot swap load current

should be greater than the application circuit’s upper

steady-state load current boundary. Once the lower

practice to check the maximum hot swap load current

of tolerance build-up in the opposite direction. Here,

threshold of 60mV and a sense resistor 3% low in

value:

()

SENSE(nom)

SENSE(nom)

ax)

HOT_SWAP(m

R

61.9mV

R

0.97

60mV

I

=

×

=

In this case, the application circuit must be sturdy

enough to operate up to approximately 1.5x the

steady-state hot swap load current. For example, if an

MIC2589 circuit must pass a minimum hot swap load

set to:

10mΩ

4A

40mV

=

=

where the nearest 1% standard value is 10.0mΩ. At

circuit in question is then simply:

6.19A

10mΩ

61.9mV

I

ax)

HOT_SWAP(m

=

=

With a knowledge of the application circuit’s maximum

hot swap load current, the power dissipation rating of

the sense resistor can be determined using P = I

resistance

Thus, the sense resistor’s maximum power dissipation

is: