Datasheet

SENSE
RESISTOR
R
S
FROM
SYSTEM
INPUT
VOLTAGE
TO MOSFET' S
DRAIN
HIGH CURRENT PATH
VIN
3
4
5
SENSE
9
8
7
6
10
LM25061
R
S
=
50 mV
I
LIM
LM25061
www.ti.com SNVS611E –FEBRUARY 2011–REVISED MARCH 2013
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. The PGD pin switches high when the voltage at the FB pin
exceeds its threshold. Typically the output voltage threshold is set with a resistor divider (R3-R4) as shown in
Figure 21, although the monitored voltage need not be the output voltage. Any other voltage can be monitored by
connecting R3 to that voltage as long as the voltage at the FB pin does not exceed its maximum rating. Referring
to the Block Diagram, when the voltage at the FB pin is below its threshold, the internal 22 µA current source at
FB is disabled. As the output voltage increases, taking FB above its threshold, the current source is enabled,
sourcing current out of the pin, raising the voltage at FB to provide threshold hysteresis.
The PGD output is low when the UVLO pin is below its threshold. The PGD output is high when the voltage at
VIN is less than 1.6V.
APPLICATION INFORMATION
(Refer to Figure 21)
CURRENT LIMIT, R
S
The LM25061 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 LM25061 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.
Figure 27. Sense Resistor Connections
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