lm5066pmhx National Semiconductor Corporation, lm5066pmhx Datasheet - Page 19

• Manufacturer Part Number: lm5066pmhx
• Description: High Voltage System Power Management And Protection Ic With Pmbus
• Manufacturer: National Semiconductor Corporation

- The maximum continuous current rating should be based on
the current limit threshold (e.g., 26 mV/R
the maximum load current, since the circuit can operate near
the current limit threshold continuously.
- The Pulsed Drain Current spec (I
the current threshold for the circuit breaker function
(48/96/193 mV/R
- The SOA (Safe Operating Area) chart of the device, and the
thermal properties, should be used to determine the maxi-
mum power dissipation threshold set by the R
The programmed maximum power dissipation should have a
reasonable margin from the maximum power defined by the
MOSFET’s SOA curve. If the device is set to infinitely retry,
the MOSFET will be repeatedly stressed during fault restart
cycles. The MOSFET manufacturer should be consulted for
guidelines.
- R
pation at maximum load current (I
its junction temperature above the manufacturer’s recom-
mendation.
- The gate-to-source voltage provided by the LM5066 can be
as high as 16.5V, limited by the internal zener diode from
GATE to OUT. Q
V
GATE to OUT to lower this voltage and limit the peak V
The zener diode’s forward current rating must be at least 110
mA to conduct the GATE pull-down current when a circuit
breaker condition is detected.
CURRENT LIMIT (R
The LM5066 monitors the current in the external MOSFET
Q
connected from VIN to SENSE. The required resistor value is
calculated from:
POWER LIMIT THRESHOLD
The LM5066 determines the power dissipation in the external
MOSFET (Q
R
the PWR pin (R
Q
GS
S
1
1
) and the V
, and is calculated from the following equation:
DS(on)
by measuring the voltage across the sense resistor (R
rating. An additional zener diode can be added from
should be sufficiently low such that the power dissi-
1
) by monitoring the drain current (the current in
DS
PWR
of Q
S
1
, depending on CL and CB configuration).
must be able to tolerate this voltage for its
) sets the maximum power dissipation for
S
1
)
(SENSE to OUT pins). The resistor at
LIM
DM
2
x R
) must be greater than
DS(on)
S
for CL = VDD), not
FIGURE 7. Sense Resistor Connections
) does not raise
PWR
resistor.
GS
(2)
S
),
.
19
where I
across R
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
connected to VDD is 26 mV, or grounded is 50 mV. The value,
when powered, can be set via the PMBus with the
MFR_SPECIFIC_DEVICE_SETUP command, which de-
faults to the 26 mV 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 1.94 or 3.87 times the
current 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_K and SENSE,
eliminating the voltage drop across the high current solder
connections.
where P
Q
rent Limit section. For example, if R
the desired power limit threshold is 50W, R
24.9 kΩ. If Q
gate is modulated to regulate the load current, keeping Q
power 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
limit is reached during startup, or if the output voltage falls due
CL
S
1
, and R
must be less than 200 mΩ.
can be set to either 26 mV or 50 mV via hardware and/or
LIM
MOSFET(LIM)
S
1
S
reaches V
is the desired current limit threshold. If the voltage
to regulate the current at I
is the current sense resistor described in the Cur-
1
’s power dissipation reaches the threshold Q
is the desired power limit threshold for
S
CL
to the LM5066 should be made using
, the current limit circuit modulates the
30115919
PWR
must be
S
is 5 mΩ, V
LIM
≤
. While the current
150 kΩ. While the
S
PWR
, basing it on the
IN
Figure 7
calculates to
www.national.com
= 48V, and
the
(1)
1
1
’s
’s