Datasheet

R
S
=
50 mV
I
LIM
Shutdown
Control
VIN
UVLO/EN
OVLO
GND
R1
R2
R3
LM25069
V
SYS
LM25069
www.ti.com SNVS607E –FEBRUARY 2011–REVISED MARCH 2013
Shutdown Control
The load current can be remotely switched off by taking the UVLO pin below its threshold with an open collector
or open drain device, as shown in Figure 26. Upon releasing the UVLO pin the LM25069 switches on the load
current with in-rush current and power limiting.
Figure 26. 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 17V in the off-state, and transients up to 20V. 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 1.3V. PGD switches low when the MOSFET’s V
DS
is increased
past 1.9V. If the UVLO pin is taken below its threshold or the OVLO pin taken above its threshold, to disable the
LM25069, PGD switches low within 10 µs without waiting for the voltage at OUT to fall. The PGD output pin is
high when the voltage at VIN is less than 1.6V.
Application Information (Refer to Figure 21)
CURRENT LIMIT, R
S
The LM25069 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:
where
• I
LIM
is the desired current limit threshold (1)
If 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 200 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 LM25069 should
be made using Kelvin techniques. In the suggested layout of Figure 27 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.
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LM25069