Datasheet

72026)(7¶6
SOURCE
HIGH CURRENT PATH
1
2
3
4
10
9
8
6
VEE
SENSE
SENSE
RESISTOR
FROM
SYSTEM
INPUT
VOLTAGE
LM5067
S
R
R
S
=
50 mV
I
LIM
LM5067
SNVS532C –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
APPLICATION INFORMATION (REFER TO FIGURE 4)
R
IN
, C
IN
The LM5067 operating voltage is determined by an internal 13V shunt regulator which receives its current from
the system voltage via R
IN
. When the system voltage exceeds 13V, the LM5067 operating voltage (VCC – VEE)
is between VEE and VEE+13V. The remainder of the system voltage is dropped across the input resistor R
IN
,
which must be selected to pass at least 2 mA into the LM5067 at the minimum system voltage. The resistor’s
power rating must be selected based on the power dissipation at maximum system voltage, calculated from:
P
RIN
= (V
SYS(max)
– 13V)
2
/R
IN
(1)
CURRENT LIMIT, R
S
The LM5067 monitors the current in the external MOSFET (Q1) by measuring the voltage across the sense
resistor (R
S
), connected from SENSE to VEE. The required resistor value is calculated from:
where
• I
LIM
is the desired current limit threshold (2)
When the voltage across R
S
reaches 50 mV, the current limit circuit modulates the gate of Q1 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 no larger than 100 mΩ.
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 approximately twice the current limit threshold. Connections from R
S
to the LM5067 should
be made using Kelvin techniques. In the suggested layout of Figure 10 the small pads at the upper 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 VEE and SENSE, eliminating the voltage
drop across the high current solder connections.
Figure 10. Sense Resistor Connections
POWER LIMIT THRESHOLD
The LM5067 determines the power dissipation in the external MOSFET (Q1) by monitoring the drain current (the
current in R
S
), and the V
DS
of Q1 (OUT to SENSE pins). The resistor at the PWR pin (R
PWR
) sets the maximum
power dissipation for Q1, and is calculated from the following equation:
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