Datasheet

LM5066
VIN _K
GND
OUT
Q1
GND
SENSE
C
L
R
S
R
L
V
IN
LM5066
VIN_K
GND
PGD
OUT
Q1
GND
SENSE
R
S
R
L
C
L
V
IN
LM5066
www.ti.com SNVS655G –JUNE 2011–REVISED FEBRUARY 2013
(4)
where C
L
is the load capacitance. For example, if V
IN
= 48V, C
L
= 200 µF, and I
LIM
= 5A, t
ON
calculates to 12 ms.
The maximum instantaneous power dissipated in the MOSFET is 12W. This calculation assumes the time from t
1
to t
2
in Figure 9(a) is small compared to t
ON
, the load does not draw any current until after the output voltage has
reached its final value, and PGD switches high (Figure 8A). The Fault Timeout Period must be set longer than
t
ON
to prevent a fault shutdown before the turn-on sequence is complete.
If the load draws current during the turn-on sequence (Figure 8B), the turn-on time is longer than the above
calculation, and is approximately equal to:
(5)
where R
L
is the load resistance. The Fault Timeout Period must be set longer than t
ON
to prevent a fault
shutdown before the turn-on sequence is complete.
A. No Load Current During Turn-On
B. Load Draws Current During Turn-On
Figure 8. Current During Turn-On
B) Turn-On with Power Limit and Current Limit: The maximum allowed power dissipation in Q
1
(P
MOSFET(LIM)
)
is defined by the resistor at the PWR pin, and the current sense resistor R
S
. See the POWER LIMIT
THRESHOLD section. If the current limit threshold (I
LIM
) is higher than the current defined by the power limit
threshold at maximum V
DS
(P
MOSFET(LIM)
/V
IN
) the circuit operates initially in the power limit mode when the V
DS
of
Q
1
is high, and then transitions to current limit mode as the current increases to I
LIM
and V
DS
decreases.
Assuming the load (R
L
) is not connected during turn-on, the time for the output voltage to reach its final value is
approximately equal to:
(6)
For example, if V
IN
= 48V, C
L
= 200 µF, I
LIM
= 1A, and P
MOSFET(LIM)
= 10W, t
ON
calculates to ≊24 ms, and the
initial current level (I
P
) is approximately 0.208A. The Fault Timeout Period must be set longer than t
ON
.
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 23
Product Folder Links: LM5066