lm25066apsqx National Semiconductor Corporation, lm25066apsqx Datasheet - Page 21

• Manufacturer Part Number: lm25066apsqx
• Description: Lm25066a System Power Management And Protection Ic With Pmbus
• Manufacturer: National Semiconductor Corporation

where I
Q1. For example, if the power limit is set at 20W with R
mΩ, and V
13.3 mV, which is comfortably regulated by the LM25066A.
However, if the power limit is set lower (e.g., 2W), the sense
resistor voltage calculates to 1.33 mV. At this low level noise
and offsets within the LM25066A may degrade the power limit
accuracy. To maintain accuracy, the sense resistor voltage
should not be less than 5 mV.
TURN-ON TIME
The output turn-on time depends on whether the LM25066A
operates in current limit, or in both power limit and current
limit, during turn-on.
A) Turn-on with current limit only: The current limit thresh-
old (I
the current limit threshold is less than the current defined by
the power limit threshold at maximum V
at the current limit threshold only during turn-on. Referring to
Figure
voltage is controlled at V
the output voltage reaches its final value (V
current reduces to its normal operating value. The time for the
B) Turn-On with Power Limit and Current Limit: The max-
imum allowed power dissipation in Q
fined by the resistor at the PWR pin, and the current sense
resistor R
rent limit threshold (I
the power limit threshold at maximum V
V
when the V
mode as the current increases to I
suming the load (R
SYS
) the circuit operates initially in the power limit mode
LIM
8A, as the load current reaches I
L
) is determined by the current sense resistor (R
is the current in R
S
. See the Power Limit Threshold section. If the cur-
DS
DS
A. No Load Current During Turn-On
of Q
= 15V the sense resistor voltage calculates to
1
L
is high, and then transitions to current limit
) is not connected during turn-on, the time
LIM
) is higher than the current defined by
GSL
S
to maintain the current at I
, and V
LIM
DS
and V
1
LIM
DS
is the voltage across
(P
, the gate-to-source
the circuit operates
MOSFET(LIM)
DS
DS
DS
decreases. As-
≊
(P
0V) the drain
MOSFET(LIM)
) is de-
30146022
LIM
S
S
= 10
. As
). If
(3)
FIGURE 8.
/
21
OUT pin voltage to transition from zero volts to V
to:
Where C
C
maximum instantaneous power dissipated in the MOSFET is
12W. This calculation assumes the time from t
9(a) is small compared to t
current until after the output voltage has reached its final val-
ue, and PGD switches high
Period must be set longer than t
before the turn-on sequence is complete.
If the load draws current during the turn-on sequence
8B), the turn-on time is longer than the above calculation, and
is approximately equal to:
where R
must be set longer than t
the turn-on sequence is complete.
for the output voltage to reach its final value is approximately
equal to:
For example, if V
P
current level (I
Period must be set longer than t
MOSFET(LIM)
L
= 1000 µF, and I
B. Load Draws Current During Turn-On
L
L
is the load capacitance. For example, if V
is the load resistance. The Fault Timeout Period
= 10W, t
P
) is approximately 0.83A. The Fault Timeout
SYS
LIM
ON
= 12V, C
calculates to
ON
= 1A, t
ON
to prevent a fault shutdown before
, and the load does not draw any
(Figure
ON
ON
L
ON
= 1000 µF, I
to prevent a fault shutdown
.
calculates to 12 ms. The
8A). The Fault Timeout
≊
12.2 ms, and the initial
30146023
1
LIM
to t
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SYS
= 1A, and
SYS
2
in
is equal
(Figure
= 12V,
Figure
(4)
(5)
(6)