Datasheet

52 PA
GATE
VCC
SENSE
Insertion time
2.2
mA Fault
Current Limit
Control
Gate
Control
VEE
OUT
VEE
System Gnd
Gate
Charge
V
SYS
/
Power Limit
110 mA
Initial Hold
-
down
UVLO/OVLO
C
IN
R
IN
Q1
R
S
Circuit Breaker/
LM5067
www.ti.com SNVS532C –OCTOBER 2007–REVISED MARCH 2013
Gate Control
The external N-channel MOSFET is turned on when the GATE pin sources 52 µA to enhance the gate. During
normal operation (t3 in Figure 6) Q1’s gate is held charged to approximately 13V above VEE, typically within 20
mV of the voltage at VCC. If the maximum V
GS
rating of Q1 is less than 13V, a lower voltage external zener
diode must be added between the GATE and SENSE pins. The external zener diode must have a forward
current rating of at least 110 mA.
When the system voltage is initially applied (before the operating voltage reaches the POR
IT
threshold), the
GATE pin is held low by a 110 mA pull-down current. The pull-down current helps prevent an inadvertent turn-on
of the MOSFET through its drain-gate capacitance as the applied system voltage increases.
During the insertion time (t1 in Figure 6) the GATE pin is held low by a 2.2 mA pull-down current. This maintains
Q1 in the off-state until the end of t1, regardless of the voltage at VCC and UVLO.
Following the insertion time, during t2 in Figure 6, the gate voltage of Q1 is modulated to keep the current or
Q1’s power dissipation level from exceeding the programmed levels. Current limiting and power limiting are
considered fault conditions, during which the voltage on the TIMER pin capacitor increases. If the current and
power limiting cease before the TIMER pin reaches 4V the TIMER pin capacitor is discharged, and the circuit
enters normal operation. See Fault Timer and Restart for details on the fault timer.
If the system input voltage falls below the UVLO threshold, or rises above the OVLO threshold, the GATE pin is
pulled low by the 2.2 mA pull-down current to switch off Q1.
Figure 7. Gate Control
Current Limit
The current limit threshold is reached when the voltage across the sense resistor R
S
(SENSE to VEE) reaches
50 mV. In the current limiting condition, the GATE voltage is controlled to limit the current in MOSFET Q1. While
the current limit circuit is active, the fault timer is active as described in the Fault Timer & Restart section. If the
load current reduces below the current limit threshold before the end of the Fault Timeout Period, the LM5067
resumes normal operation. For proper operation, the R
S
resistor value should be no larger than 100 mΩ.
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