Datasheet

R
S
=
55 mV
I
LIM
Shutdown
Control
VIN
V
SYS
UVLO
OVLO
GND
R1
R2
R3
LM5069
LM5069
SNVS452D –SEPTEMBER 2006–REVISED MAY 2013 www.ti.com
Over-Voltage Lock-Out (OVLO)
The series pass MOSFET (Q1) is enabled when the input supply voltage (V
SYS
) is within the operating range
defined by the programmable under-voltage lockout (UVLO) and over-voltage lock-out (OVLO) levels. If V
SYS
raises the OVLO pin voltage above 2.5V Q1 is switched off by the 2 mA pull-down current at the GATE pin,
denying power to the load. When the OVLO pin is above 2.5V, the internal 21 µA current source at OVLO is
switched on, raising the voltage at OVLO to provide threshold hysteresis. When V
SYS
is reduced below the OVLO
level Q1 is enabled. See the Applications Section for a procedure to calculate the threshold setting resistor
values.
Shutdown Control
The load current can be remotely switched off by taking the UVLO pin below its 2.5V threshold with an open
collector or open drain device, as shown in Figure 28. Upon releasing the UVLO pin the LM5069 switches on the
load current with in-rush current and power limiting.
Figure 28. Shutdown Control
Power Good Pin
The Power Good indicator pin (PGD) is connected to the drain of an internal N-channel MOSFET capable of
sustaining 80V in the off-state, and transients up to 100V. An external pull-up resistor is required at PGD to an
appropriate voltage to indicate the status to downstream circuitry. The off-state voltage at the PGD pin can be
higher or lower than the voltages at VIN and OUT. PGD is switched high when the voltage from SENSE to OUT
(the external MOSFET’s V
DS
) decreases below 1.25V. PGD switches low when the MOSFET’s V
DS
is increased
past 2.5V. If the UVLO pin is taken below 2.5V, or the OVLO pin taken above 2.5V, to disable the LM5069, PGD
switches low within 10 µs without waiting for the voltage at OUT to fall 2.5V below the voltage at SENSE. The
PGD output pin is high when the voltage at VIN is less than 5V.
APPLICATION INFORMATION
(REFER TO Figure 22)
CURRENT LIMIT, R
S
The LM5069 monitors the current in the external MOSFET (Q1) 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 55 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 twice the current limit threshold. Connections from R
S
to the LM5069 should be made using
Kelvin techniques. In the suggested layout of Figure 29 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 and SENSE, eliminating the voltage drop across the
high current solder connections.
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