Datasheet

C
T
=
t
FAULT
x 85 PA
4V
= t
FAULT
x 2.13 x 10
-5
C
T
=
t1 x 5.5 PA
4V
= t1 x 1.38 x 10
-6
LM5069
www.ti.com SNVS452D –SEPTEMBER 2006–REVISED MAY 2013
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 (55 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 (105 mV/R
S
).
- The SOA (Safe Operating Area) chart of the device, and the thermal properties, should be used to determine
the maximum power dissipation threshold set by the R
PWR
resistor. The programmed maximum power dissipation
should have a reasonable margin from the maximum power defined by the FET's SOA chart if the LM5069-2 is
used since the FET will be repeatedly stressed during fault restart cycles. The FET manufacturer should be
consulted for guidelines.
- R
DS(on)
should be sufficiently low that the power dissipation at maximum load current (I
L(max)
2
x R
DS(on)
) does
not raise its junction temperature above the manufacturer’s recommendation.
If the device chosen for Q1 has a maximum V
GS
rating less than 12V, an external zener diode must be added
from its gate to source, with the zener voltage less than the maximum V
GS
rating. The zener diode’s forward
current rating must be at least 250 mA to conduct the GATE pull-down current during startup and in the circuit
breaker mode.
TIMER CAPACITOR, C
T
The TIMER pin capacitor (C
T
) sets the timing for the insertion time delay, fault timeout period, and restart timing
of the LM5069-2.
A) Insertion Delay - Upon applying the system voltage (V
SYS
) to the circuit, the external MOSFET (Q1) is held
off during the insertion time (t1 in Figure 24) to allow ringing and transients at V
SYS
to settle. Since each
backplane’s response to a circuit card plug-in is unique, the worst case settling time must be determined for each
application. The insertion time starts when VIN reaches the POR
IT
threshold, at which time the internal 5.5 µA
current source charges C
T
from 0V to 4.0V. The required capacitor value is calculated from:
For example, if the desired insertion delay is 250 ms, C
T
calculates to 0.345 µF. At the end of the insertion delay,
C
T
is quickly discharged by a 1.5 mA current sink.
B) Fault Timeout Period - During in-rush current limiting or upon detection of a fault condition where the current
limit and/or power limit circuits regulate the current through Q1, the fault timer current source (85 µA) is switched
on to charge C
T
. The Fault Timeout Period is the time required for the TIMER pin voltage to reach 4.0V, at which
time Q1 is switched off. The required capacitor value for the desired Fault Timeout Period t
FAULT
is calculated
from:
(3)
For example, if the desired Fault Timeout Period is 16 ms, C
T
calculates to 0.34 µF. After the Fault Timeout
Period, the LM5069-1 latches the GATE pin low until a power up sequence is initiated by external circuitry. C
T
is
discharged by the 2.5 µA current sink at the end of the Fault Timeout Period. See the Fault Timer & Restart
section and Figure 26. When the Fault Timeout Period of the LM5069-2 expires, a restart sequence starts as
described below (Restart Timing). Since the LM5069 normally operates in power limit and/or current limit during
a power up sequence, the Fault Timeout Period must be longer than the time required for the output voltage to
reach its final value. See the Turn-on Time section.
C) Restart Timing For the LM5069-2, after the Fault Timeout Period described above, C
T
is discharged by the
2.5 µA current sink to 1.25V. The TIMER pin then cycles through seven additional charge/discharge cycles
between 1.25V and 4.0V as shown in Figure 27. The restart time ends when the TIMER pin voltage reaches
0.3V during the final high-to-low ramp. The restart time, after the Fault Timeout Period, is equal to:
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