MAX4272 MAXIM [Maxim Integrated Products], MAX4272 Datasheet - Page 22

• Manufacturer Part Number: MAX4272
• Description: 3V to 12V Current-Limiting Hot-Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
• Manufacturer: MAXIM [Maxim Integrated Products]

3V to 12V Current-Limiting Hot-Swap Controllers
with Autoretry, DualSpeed/BiLevel Fault Protection
For the MAX4273 in retry mode, the duty cycle can be
adjusted. Use the MOSFET Z
pulse duration to choose a maximum duty cycle, D.
Calculate the retry time:
Use Figure 8 to determine CTIM given t
GIVEN:
• Hot-swap 5V supply to a 1000µF card
• MOSFET IRF7413A: R
• Operating current = 4A
• Overload current = 5A
• System current limit = 10A
• Retry enabled
PROCEDURE:
1) Select the current sense resistor:
2) Set fast comparator threshold (RTH).
22
Figure 17. Example Curves (from IRF7413A) for Maximum Effective Transient Thermal Impedance, Junction to Ambient
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.
100
0.1
______________________________________________________________________________________
10
0.0001
1
D = 0.50
0.10
0.05
0.20
0.02
0.01
Design Procedure (MAX4273)
t
RETRY
(THERMAL RESPONSE)
DS(on)
= t
SINGLE PULSE
START
0.001
θJA
= 0.0135Ω
curve and the t
/ D
RETRY
.
0.01
START
t
1
, RECTANGULAR PULSE DURATION (s)
3) Set Startup timer.
4) Select Retry Timeout.
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
Select RTH from Figure 7, RTH = 10kΩ
Startup current = System current limit = 10A
V
C
t
Give a factor of 2 guardband on the startup timer.
t
From Figure 8, C
V
I
Peak junction temperature, T
Peak ambient temperature, T
ON
START
MAX
0.1
IN
IN
BOARD
= 5V
= 5V.
= 1000µF
= 10A.
= 2
= 1000µF
✕
t
ON
✕
5V / 10A = 500µs
= 1.0ms
TON
1
= 3000pF.
J
A
P
NOTES:
1. DUTY FACTOR D = t
2. PEAK T
= 150°C
DM
= 85°C
10
J
✕
= P
t
1
DM
10A = 100mV
A
J
t
X Z
2
θJA
1
/ t
+ T
2
A
100