Datasheet

SENSE
RESISTOR
R
S
VIN_K
SENSE
FROM SYSTEM
INPUT VOLTAGE
TO DRAIN OF
MOSFET Q
1
HIGH CURRENT PATH
VIN
LM5066
SNVS655G –JUNE 2011–REVISED FEBRUARY 2013 www.ti.com
Figure 7. Sense Resistor Connections
POWER LIMIT THRESHOLD
The LM5066 determines the power dissipation in the external MOSFET (Q
1
) by monitoring the drain current (the
current in R
S
) and the V
DS
of Q
1
(SENSE to OUT pins). The resistor at the PWR pin (R
PWR
) sets the maximum
power dissipation for Q
1
, and is calculated from the following equation:
(2)
where P
MOSFET(LIM)
is the desired power limit threshold for Q
1
, and R
S
is the current sense resistor described in
the CURRENT LIMIT (R
S
) section. For example, if R
S
is 5 mΩ, V
IN
= 48V, and the desired power limit threshold
is 50W, R
PWR
calculates to 24.9 kΩ. If Q
1
’s power dissipation reaches the threshold Q
1
’s gate is modulated to
regulate the load current, keeping Q
1
’s power from exceeding the threshold. For proper operation of the power
limiting feature, R
PWR
must be ≤150 kΩ. While the power limiting circuit is active, the fault timer is active as
described in the Fault Timer & Restart section. Typically, power limit is reached during startup, or if the output
voltage falls due to a severe overload or short circuit. The programmed maximum power dissipation should have
a reasonable margin from the maximum power defined by the SOA chart, especially if retry is enabled, because
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 degrade if a very low 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:
(3)
where I
LIM
is the current in R
S
, and V
DS
is the voltage across Q
1
. For example, if the power limit is set at 75W
with R
S
= 5 mΩ, and V
DS
= 48V the sense resistor voltage calculates to 7.8 mV, which is comfortably regulated
by the LM5066. However, if the power limit is set lower (for example, 25W), the sense resistor voltage calculates
to 2.6 mV. At this low level noise and offsets within the LM5066 may degrade the power limit accuracy. To
maintain accuracy, the sense resistor voltage should not be less than 3 mV.
TURN-ON TIME
The output turn-on time depends on whether the LM5066 operates in current limit, or in both power limit and
current limit, during turn-on.
A) Turn-on with current limit only: The current limit threshold (I
LIM
) is determined by the current sense resistor
(R
S
). If the current limit threshold is less than the current defined by the power limit threshold at maximum V
DS
the circuit operates only at the current limit threshold during turn-on. Referring to Figure 8A, as the load current
reaches I
LIM
, the gate-to-source voltage is controlled at V
GSL
to maintain the current at I
LIM
. As the output voltage
reaches its final value (V
DS
≊ 0V) the drain current reduces to its normal operating value. The time for the OUT
pin voltage to transition from zero volts to V
IN
is equal to:
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