Typical Application Example 1:
Rds(on) is 2.0mΩ maximum at 25°C and will
increase as the Junction temperature increases.
From Figure 11, at 118°C Rds(on) will increase by
~35%, then
Requirement:
Redundant Bus Voltage = 3.3V
Load Current = 20A (assume through each
redundant path)
Maximum Ambient Temperature = 60°C, no air flow
Auxiliary Voltage = 12V (10V to 14V)
maximum at 118°C
Rds(on) = 2.0mΩ∗1.35= 2.7mΩ
Maximum power dissipation is:
Pdmax = Iin2 ∗Rds(on) = (20A)2 ∗2.7mΩ =1.08W
Solution:
A single PI2121 for each redundant 3.3V power
source should be used, configured as shown in the
circuit schematic in Figure 22.
Recalculate TJ:
54°C
⎛
⎞
⎠
T
= 60°C +
∗(20A)2 ∗2.7mΩ =118.3°C
⎜
⎟
J max
W
⎝
Vaux:
Since the Vaux voltage does not exceed the VC pin
clamp voltage, connect the Vaux directly to the VC
pin
SP and SN pins:
Connect each SP pin to the S pins and each SN pin
to the D pins
BK pin:
Connect the BK pin to ground to achieve the
minimum reverse current response time.
SL pin: Not required, so leave floating
pin:
FT
Connect to the logic input and to the logic power
supply via a 10KΩ resistor.
Program UV and OV to monitor input voltage:
Program UV at 3.0V and OV at 3.6V
Use the three-resistor divider configuration:
Figure 21: Example 1 final junction temperature at
20A/60°C TA
IRa = 200μA
Reverse Current Threshold:
500mV
The following procedure demonstrates how to
calculate the minimum required reverse current in
the internal MOSFET to generate a reverse fault
condition and turn off the internal MOSFET.
or 2.49kΩ 1%
Ra =
= 2.5kΩ
3.6V
200μA
⎛
⎜
⎞
⎠
or 499Ω 1%
Rb = 2.49kΩ
−1 = 498Ω
⎟
3.0V
⎝
At room temperature (25°C) typical Rds(on):
3.0V
⎛
⎝
⎞
⎠
Rc =
(
2.49kΩ + 499Ω
)
−1 =14.95kΩ
⎜
⎟
500mV
Vth.reverse − 6mV
Is.reverse =
=
= −4A
or 15kΩ 1%
Rds(on)
1.5mΩ
Power Dissipation and Junction Temperature:
First use Figure 13 (Junction Temperature vs. Input
Current) to find the final junction temperature for 20A
load current at 60°C ambient temperature. In Figure
13 (illustrated in Figure 21) draw a vertical line from
20A to intersect the 60°C ambient temperature line.
At the intersection draw a horizontal line towards the
Y-axis (Junction Temperature). The Junction
Temperature at full load current (20A) and 60°C
ambient is 118°C.
At maximum junction temperature (118°C) and
maximum Rds(on):
Vth.reverse − 6mV
Is.reverse =
=
= −2.2A
Rds(on)
2.7mΩ
Picor Corporation • picorpower.com
PI2121
Rev. 1.0 Page 16 of 21
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