Where:
This may require iteration to get to the final junction
temperature. Figures 13, 14, and 17 show the
PI2121 internal MOSFET final junction temperature
curves versus conducted current at given ambient
temperatures and air flow.
: UV threshold voltage at Vin.
: OV threshold voltage at Vin.
: UV voltage set
V(UVTH )
V(OVTH )
V (UV )
IRa
:
current.
Ra
OV/UV resistor selection:
Alternatively,
a
two-resistor voltage divider
The UV and OV comparator inputs are used to
monitor the input voltage and will indicate a fault
condition when this voltage is out of range. The UV
& OV pins can be configured in two different ways,
either with a divider on each pin, or with a three-
resistor divider to the same node, enabling the
elimination of one resistor. Under-voltage is
monitored by the UV pin input and over-voltage is
monitored with the OV pin input.
configuration can be used and is shown in (Figure
20).
The UV resistor voltage divider can be obtained from
the following equations:
V (UVTH )
R1UV
=
IRUV
Set R1UV value based on system allowable current
IRUV ≥100μA
The Fault pin (
) will indicate a fault (active low)
FT
when the UV pin is below the threshold or when the
OV pin is above the threshold. The threshold is
0.50V typical with 25mV hysteresis and the input
current is less than ±1µA. It is important to consider
the maximum current that will flow in the resistor
divider and maximum error due to UV and OV input
current. Set the resistor current to 100µA or higher
to maintain better than 1% accuracy for UV and OV
due to the bias current.
⎛
⎞
V (UV )
⎜
⎜
⎟
⎟
R2UV = R1UV
−1
V (UVTH )
⎝
⎠
Where:
V (UVTH )
: UV threshold voltage
:
current
R1UV
IRUV
V (UVTH )
R1UV
=
IRUV
The three-resistor voltage divider configuration for
both UV and OV to monitor the same voltage node
is shown in Figure 19:
Set R1OV value based on system allowable current
IRUV ≥100μA
⎛
⎞
V (OV )
V (OVTH
⎜
⎜
⎟
⎟
R2OV = R1OV
−1
)
⎝
⎠
Where:
: OV threshold voltage
)
V (OVTH
:
current
R1OV
IROV
Figure 19: UV & OV three-resistor divider
configuration
V (OVTH )
Ra =
IRa
Set Ra value based on system allowable current
IRa
⎛
⎞
V (OV )
V (UV )
Figure 20: UV & OV two-resistor divider
⎜
⎜
⎟
⎟
Rb = Ra
−1
configuration
⎝
⎠
⎛
⎜
⎞
V (UV )
⎟
⎟
Rc =
(
Ra + Rb
)
−1
⎜
V (UVTH )
⎝
⎠
Picor Corporation • picorpower.com
PI2121
Rev. 1.0 Page 15 of 21
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