MAX4271/MAX4272/MAX4273
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
adjusted. Use the MOSFET Z
θJA
curve and the t
START
pulse duration to choose a maximum duty cycle, D.
Calculate the retry time:
t
RETRY
= t
START
/ D
Use Figure 8 to determine CTIM given t
RETRY
.
Design Procedure (MAX4273)
GIVEN:
• Hot-swap 5V supply to a 1000µF card
• MOSFET IRF7413A: R
DS(on)
= 0.0135Ω
• 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 = 10mΩ
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 = 10mΩ
Fast comparator threshold = 10mΩ
✕
10A = 100mV
Select RTH from Figure 7, RTH = 10kΩ
3) Set Startup timer.
Startup current = System current limit = 10A
V
IN
= 5V
C
BOARD
= 1000µF
t
ON
= 1000µF
✕
5V / 10A = 500µs
Give a factor of 2 guardband on the startup timer.
t
START
= 2
✕
t
ON
= 1.0ms
From Figure 8, C
TON
= 3000pF.
4) Select Retry Timeout.
V
IN
= 5V.
I
MAX
= 10A.
Peak junction temperature, T
J
= 150°C
J
Peak ambient temperature, T
A
= 85°C
A
