lm25066apsqx National Semiconductor Corporation, lm25066apsqx Datasheet - Page 20

• Manufacturer Part Number: lm25066apsqx
• Description: Lm25066a System Power Management And Protection Ic With Pmbus
• Manufacturer: National Semiconductor Corporation

www.national.com
- R
at maximum load current (I
junction temperature above the manufacturer’s recommen-
dation.
- The gate-to-source voltage provided by the LM25066A can
be as high as 18.8V at turn-on when the output voltage is zero.
At turn-off the reverse gate-to-source voltage will be equal to
the output voltage at the instant the GATE pin is pulled low.
If the device chosen for Q
external zener diode must be added from its gate to source,
with the zener voltage less than the device maximum V
rating. The zener diode’s working voltage protects the MOS-
FET during turn-on, and its forward voltage protects the MOS-
FET during shutoff. The zener diode’s forward current rating
must be at least 190 mA to conduct the GATE pull-down cur-
rent when a circuit breaker condition is detected.
CURRENT LIMIT, (R
The LM25066A monitors the current in the external MOSFET
Q
connected from VIN to SENSE. The required resistor value is
calculated from:
where I
across R
POWER LIMIT THRESHOLD
The LM25066A determines the power dissipation in the ex-
ternal MOSFET (Q
current in R
resistor at the PWR pin (R
sipation for Q
where P
Q
rent Limit section. For example, if R
desired power limit threshold is 20W, R
kΩ. If Q
is modulated to regulate the load current, keeping Q
from exceeding the threshold. For proper operation of the
power limiting feature, R
power limiting circuit is active, the fault timer is active as de-
scribed in the Fault Timer & Restart section. Typically, power
1
1
, and R
DS(on)
by measuring the voltage across the sense resistor (R
LIM
1
R
MOSFET(LIM)
’s power dissipation reaches the threshold Q
should be sufficiently low that the power dissipation
S
PWR
S
reaches V
is the desired current limit threshold. If the voltage
is the current sense resistor described in the Cur-
S
), and the V
1
, and is calculated from the following equation:
= 1.71 x 10
1
is the desired power limit threshold for
CL
) by monitoring the drain current (the
S
)
, the current limit circuit modulates the
DS
5
PWR
1
PWR
L(max)
is not rated for these voltages, an
x R
of Q
) sets the maximum power dis-
must be
S
1
2
x P
(SENSE to OUT pins). The
x R
MOSFET(LIM)
DS(on)
S
PWR
≤
is 10 mΩ, and the
150 kΩ. While the
) does not raise its
FIGURE 7. Sense Resistor Connections
calculates to 34.2
1
’s power
1
’s gate
(1)
(2)
S
GS
),
20
gate of Q
limiting circuit is active, the fault timer is active as described
in the Fault Timer & Restart section. For proper operation,
R
V
software. This setting defaults to use of CL pin which when
grounded is 25mV, or high is 46mV. The value when powered
can
MFR_SPECIFIC_DEVICE_SETUP command, which de-
faults to the 25mV setting.
Once the desired setting is known, calculate the shunt based
on that input voltage and maximum current. While the maxi-
mum load current in normal operation can be used to deter-
mine the required power rating for resistor R
current limit value provides a more reliable design since the
circuit can operate near the current limit threshold continu-
ously. The resistor’s surge capability must also be considered
since the circuit breaker threshold is 1.8 or 3.6 times the cur-
rent limit threshold.
Connections from R
Kelvin techniques. In the suggested layout of
small pads at the lower corners of the sense resistor connect
only to the sense resistor terminals, and not to the traces car-
rying the high current. With this technique, only the voltage
across the sense resistor is applied to VIN and SENSE, elim-
inating the voltage drop across the high current solder con-
nections.
limit is reached during startup, or if the output voltage falls due
to a severe overload or short circuit. The programmed maxi-
mum power dissipation should have a reasonable margin
from the maximum power defined by the SOA chart if retry is
enabled since the MOSFET will be repeatedly stressed during
fault restart cycles. The MOSFET manufacturer should be
consulted for guidelines. If the application does not require
use of the power limit function the PWR pin can be left open.
The accuracy of the power limit function at turn-on may de-
grade if a very low value power dissipation limit is set. The
reason for this caution is that the voltage across the sense
resistor, which is monitored and regulated by the power limit
circuit, is lowest at turn-on when the regulated current is at a
minimum. The voltage across the sense resistor during power
limit can be expressed as follows:
CL
S
must be no larger than 200 mΩ.
can be set to either 25mV or 46mV via hardware and/or
be
1
to regulate the current at I
set
S
to the LM25066A should be made using
via
30146019
the
PMBus
LIM
. While the current
S
, basing it on the
Figure 7
with
the
the