Datasheet

VIN
GND
PGD
OUT
Q1
GND
LM25069
V
SYS
C
L
R
L
R
S
VIN
GND
PGD
OUT
Q1
GND
V
SYS
C
L
R
L
R
S
LM25069
t
ON
= -(R
L
x C
L
) x In
(I
LIM
x R
L
) - V
SYS
(I
LIM
x R
L
)
t
ON
=
V
SYS
x C
L
I
LIM
LM25069
www.ti.com SNVS607E –FEBRUARY 2011–REVISED MARCH 2013
TURN-ON TIME
The output turn-on time depends on whether the LM25069 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 threshold (I
LIM
) is determined by the current sense resistor
(R
S
). If the current limit threshold is less than the current defined by the power limit threshold at maximum V
DS
the circuit operates at the current limit threshold only during turn-on. Referring to Figure 30a, as the load current
reaches I
LIM
, the gate-to-source voltage is controlled at V
GSL
to maintain the current at I
LIM
. As the output voltage
reaches its final value, (V
DS
≊ 0V) the drain current reduces to its normal operating value. The time for the OUT
pin voltage to transition from zero volts to V
SYS
is equal to:
where
• C
L
is the load capacitance (4)
For example, if V
SYS
= 12V, C
L
= 1000 µF, and I
LIM
= 1A, t
ON
calculates to 12 ms. The maximum instantaneous
power dissipated in the MOSFET is 12W. This calculation assumes the time from t1 to t2 in Figure 30a is small
compared to t
ON
, and the load does not draw any current until after the output voltage has reached its final value,
and PGD switches high (Figure 28). If the load draws current during the turn-on sequence (Figure 29), the turn-
on time is longer than the above calculation, and is approximately equal to:
where
• R
L
is the load resistance (5)
The Fault Timeout Period must be set longer than t
ON
to prevent a fault shutdown before the turn-on sequence is
complete.
Figure 28. No Load Current During Turn-On
Figure 29. Load Draws Current During Turn-On
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