3V to 12V Current-Limiting Hot Swap Controllers

with Autoretry, DualSpeed/BiLevel Fault Protection

22

______________________________________________________________________________________

For the MAX4273 in retry mode, the duty cycle can be

pulse duration to choose a maximum duty cycle, D.

Calculate the retry time:

Design Procedure (MAX4273)

GIVEN:

• Hot-swap 5V supply to a 1000µF card

Ω

• Operating current = 4A

• Overload current = 5A

• System current limit = 10A

• Retry enabled

PROCEDURE:

1) Select the current sense resistor:

Slow comparator threshold = 50mV

Overload current = 5A

Current-sense resistor value = 50mV/5A = 10m

Ω

The device will initiate a slow fault if the load current

is greater than 5A for longer than 20µs after startup.

2) Set fast comparator threshold (RTH).

The MOSFET pulsed drain current limit is 58A.

Another consideration for setting the current limit is

the system requirement. Systems may glitch if 58A

load transients are present. For this example, the

load transient will be limited to 10A:

System current limit = 10A

Current-sense resistor = 10m

Ω

Fast comparator threshold = 10m

Select RTH from Figure 7, RTH = 10k

Ω

3) Set Startup timer.

Startup current = System current limit = 10A

Give a factor of 2 guardband on the startup timer.

4) Select Retry Timeout.

100

10

1

0.1

0.01

0.001

0.0001

0.1

1

10

100

NOTES:

D = 0.50

0.20

0.10

0.05

0.02

0.01

SINGLE PULSE

(THERMAL RESPONSE)

Figure 17. Example Curves (from IRF7413A) for Maximum Effective Transient Thermal Impedance, Junction to Ambient