Datasheet

LM5066
www.ti.com SNVS655G –JUNE 2011–REVISED FEBRUARY 2013
Refer to Programming Guide section: After all hardware design is complete, refer to the programming guide
for a step by step procedure regarding software.
MOSFET SELECTION
It is recommended that the external MOSFET (Q
1
) selection is based on the following criteria:
- The BV
DSS
rating should be greater than the maximum system voltage (V
IN
), plus ringing and transients which
can occur at V
IN
when the circuit card, or adjacent cards, are inserted or removed.
- The maximum continuous current rating should be based on the current limit threshold (for example, 26 mV/R
S
for CL = VDD), 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
(48/96/193 mV/R
S
, depending on CL and CB configuration).
- 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 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
DS(on)
should be sufficiently low such that the power dissipation at maximum load current (I
LIM
2
x R
DS(on)
) does
not raise its junction temperature above the manufacturer’s recommendation.
- 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
1
must be able to tolerate this voltage for its V
GS
rating. An additional zener diode
can be added from GATE to OUT to lower this voltage and limit the peak V
GS
. 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
S
)
The LM5066 monitors the current in the external MOSFET Q
1
by measuring the voltage across the sense
resistor (R
S
), connected from VIN to SENSE. The required resistor value is calculated from:
(1)
where I
LIM
is the desired current limit threshold. If the voltage across R
S
reaches V
CL
, the current limit circuit
modulates the gate of Q
1
to regulate the current at I
LIM
. While the current limiting circuit is active, the fault timer is
active as described in the Fault Timer & Restart section. For proper operation, R
S
must be less than 200 mΩ.
V
CL
can be set to either 26 mV or 50 mV via hardware and/or 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 defaults to the 26 mV setting.
Once the desired setting is known, calculate the shunt based on that input voltage and maximum current. While
the maximum load current in normal operation can be used to determine the required power rating for resistor
R
S
, basing it on the current limit value provides a more reliable design since the circuit can operate near the
current limit threshold continuously. 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
S
to the LM5066 should be made using Kelvin techniques. In the suggested layout of
Figure 7 the small pads at the lower corners of the sense resistor connect only to the sense resistor terminals,
and not to the traces carrying 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.
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