Datasheet
t
CBT
=
4V x C
T
4V x 450nF
240PA
= 7.5ms
=
240PA
I
SHORT-CIRCUIT(MAX)
=
120mV
40m:
= 3A
100PF x 100V x 40m: x 240PA
4V x 80mV
C
T
= = 300nF
R
S
=
40mV
1A
= 40m:
C
L
x V
IN(MAX)
x R
S
x 240PA
4V x 80mV
C
T
=
C
L
x V
IN(MAX)
I
INRUSH(MIN)
t
CL CHARGE
=
I
SHORT-CIRCUIT(MAX)
=
120mV
R
S
80mV
I
INRUSH(MIN)
=
R
S
R
S
=
V
CB(MIN)
I
L(MAX)
40mV
=
I
L(MAX)
LM5068
SNVS254C –JANUARY 2004–REVISED MARCH 2013
www.ti.com
Sense Resistor (R
s
), Timer Capacitor (C
T
) and N-Channel Mosfet (Q1) Selection
To select the proper MOSFET, the following safe operating area (SOA) parameters are needed: maximum input
voltage, maximum current and the maximum current conduction time.
First, R
S
is calculated for the maximum operating load current (I
L(MAX)
) and the minimum circuit breaker trip point
(V
CB(MIN)
):
(8)
During the initial charging process, the LM5068 may operate the MOSFET in current limit, forcing V
AC(MIN)
(80mV)
to V
AC(MAX)
(120mV) across R
S
.
The minimum in-rush current and maximum short-circuit limit are calculated from:
(9)
(10)
The value of TIMER capacitor (C
T
) is calculated in order to prevent C
T
from timing out before the load capacitor
is fully charged using the slowest expected charging rate of the load capacitor. Assuming there is no initial
resistive loading, the time necessary to charge the load capacitor C
L
is calculated from:
(11)
Applying Equation 9 and Equation 11 to Equation 6 gives the TIMER capacitor value of:
(12)
Finally, the SOA curves of a prospective MOSFET are checked using V
IN (MAX)
, and I
SHORT-CIRCUIT (MAX)
calculated
from equation Equation 10 and time of the current flow from Equation 6.
Example: For: I
L
=1A, V
DD
= 48V, V
DD (MAX)
= 100V and C
L
=100µF,
(13)
(14)
To account for tolerances of R
S
, C
L
, TIMER current and TIMER threshold voltage, the computed C
T
value should
be increased, for this example 50% was selected, therefore:
C
T
= 300nF • 1.5 = 450nF
The maximum active current limiting value and duration are:
(15)
(16)
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