lm5066pmhx National Semiconductor Corporation, lm5066pmhx Datasheet - Page 20

• Manufacturer Part Number: lm5066pmhx
• Description: High Voltage System Power Management And Protection Ic With Pmbus
• Manufacturer: National Semiconductor Corporation

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to a severe overload or short circuit. The programmed maxi-
mum power dissipation should have a reasonable margin
from the maximum power defined by the SOA chart, espe-
cially if retry is enabled, because the MOSFET will be repeat-
edly stressed during fault restart cycles. The MOSFET
manufacturer should be consulted for guidelines. If the appli-
cation does not require use of the power limit function the
PWR pin can be left open. The accuracy of the power limit
function at turn-on may degrade if a very low power dissipa-
tion limit is set. The reason for this caution is that the voltage
across the sense resistor, which is monitored and regulated
by the power limit circuit, is lowest at turn-on when the regu-
lated current is at a minimum. The voltage across the sense
resistor during power limit can be expressed as follows:
where I
Q
mΩ, and V
7.8 mV, which is comfortably regulated by the LM5066. How-
ever, if the power limit is set lower (e.g., 25W), the sense
resistor voltage calculates to 2.6 mV. At this low level noise
and offsets within the LM5066 may degrade the power limit
accuracy. To maintain accuracy, the sense resistor voltage
should not be less than 3 mV.
TURN-ON TIME
The output turn-on time depends on whether the LM5066 op-
erates 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
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
the circuit operates initially in the power limit mode when the
V
as the current increases to I
the load (R
DS
1
. For example, if the power limit is set at 75W with R
of Q
LIM
LIM
) is determined by the current sense resistor (R
1
S
. See the Power Limit Threshold section. If the cur-
is high, and then transitions to current limit mode
is the current in R
DS
L
) is not connected during turn-on, the time for the
A. No Load Current During Turn-On
= 48V the sense resistor voltage calculates to
LIM
) is higher than the current defined by
LIM
S
, and V
and V
DS
DS
1
decreases. Assuming
is the voltage across
(P
DS
MOSFET(LIM)
(P
MOSFET(LIM)
FIGURE 8. Current During Turn-On
) is de-
30115922
S
/V
S
= 5
). If
(3)
IN
)
20
the current limit threshold is less than the current defined by
the power limit threshold at maximum V
only at the current limit threshold 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
OUT pin voltage to transition from zero volts to V
to:
where C
C
imum 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.
output voltage to reach its final value is approximately equal
to:
For example, if V
(LIM)
level (I
must be set longer than t
L
= 200 µF, and I
= 10W, t
P
8A, as the load current reaches I
B. Load Draws Current During Turn-On
) is approximately 0.208A. The Fault Timeout Period
L
L
is the load capacitance. For example, if V
is the load resistance. The Fault Timeout Period
ON
calculates to
IN
LIM
= 48V, C
= 5A, t
ON
GSL
ON
to prevent a fault shutdown before
L
.
ON
to maintain the current at I
ON
= 200 µF, I
(Figure
≊
, the load does not draw any
calculates to 12 ms. The max-
ON
24 ms, and the initial current
to prevent a fault shutdown
8A). The Fault Timeout
LIM
DS
LIM
, the gate-to-source
the circuit operates
= 1A, and P
DS
≊
1
30115923
to t
0V) the drain
IN
2
IN
in
is equal
(Figure
MOSFET
LIM
= 48V,
Figure
. As
(4)
(5)
(6)