Datasheet
0
0
Drain Current
0
t1
0
0
t2
t3
a) Current Limit Only
V
SYS
V
DS
I
LIM
I
P
V
GATE
V
GSL
V
TH
t
ON
Source Voltage-toGate-
b) Power Limit and Current Limit
V
DS
Drain Current
t
ON
0
V
SYS
I
LIM
V
GATE
V
GSL
V
TH
Source Voltage-toGate-
t
ON
=
C
L
x V
SYS
2
2 x P
FET(LIM)
C
L
x P
FET(LIM)
2 x I
LIM
2
+
VCC
OUT
GND
GATESENSEVEE
VEE
LM5067
C
IN
R
IN
R
S
V
SYS
R
L
C
L
Q1
LM5067
SNVS532C –OCTOBER 2007–REVISED MARCH 2013
www.ti.com
• R
L
is the load resistance and V
SYS
is the absolute value of the system input voltage (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 12. Load Draws Current During Turn-On
B) Turn-on with power limit and current limit: The power dissipation limit in Q1 (P
FET(LIM)
) is defined by the
resistor at the PWR pin, and the current sense resistor R
S
. See POWER LIMIT THRESHOLD. If the current limit
threshold (I
LIM
) is higher than the current defined by the power limit threshold at maximum V
DS
(P
FET(LIM)
/V
SYS
)
the circuit operates initially in power limit mode when the V
DS
of Q1 is high, and then transitions to current limit
mode as the current increases to I
LIM
as V
DS
decreases. See Figure 13b. 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
SYS
= -48V, C
L
= 1000 µF, I
LIM
= 1A, and P
FET(LIM)
= 20W, t
ON
calculates to ≊68 ms, and the
initial current level (I
P
) is approximately 0.42A. The Fault Timeout Period must be set longer than t
ON
.
Figure 13. MOSFET Power Up Waveforms
MOSFET SELECTION
It is recommended that the external MOSFET (Q1) selection be based on the following criteria:
• The BV
DSS
rating should be greater than the maximum system voltage (V
SYS
), plus ringing and transients
which can occur at V
SYS
when the circuit card, or adjacent cards, are inserted or removed.
• The maximum continuous current rating should be based on the current limit threshold (50 mV/R
S
), not the
maximum load current, since the circuit can operate near the current limit threshold continuously.
• The Pulsed Drain Current spec (I
DM
) must be greater than the current threshold for the circuit breaker function
(100 mV/R
S
).
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