Datasheet

UVLO
LM25119
+
-
1.25V
R
UV2
R
UV1
20 PA
+
-
0.4V
SHUTDOWN
STANDBY
C
FT
V
IN
VIN - 1.25
1.25V x R
UV2
R
UV1
=
20 PA
V
HYS
R
UV2
=
V
OUT
COMP
FB
LM25119
+
-
R
FB2
R
FB1
R
COMP
C
COMP
C
HF
0.8V
LM25119/25119Q
www.ti.com SNVS680G –AUGUST 2010–REVISED JANUARY 2014
2.21 kΩ was chosen for R
FB1
in this design which results in a R
FB2
value of 6.98 kΩ for V
OUT1
of 3.3 V. A
reasonable guide is to select the value of R
FB1
in the range between 500 Ω and 10 kΩ. The value of R
FB1
should
be large enough to keep the total divider power dissipation small.
Figure 8. Feedback Configuration
UVLO DIVIDER
The UVLO threshold is internally set to 1.25 V at the UVLO pin. The LM25119 is enabled when the system input
voltage VIN causes the UVLO pin to exceed the threshold voltage of 1.25 V. When the UVLO pin voltage is
below the threshold, the internal 20 μA current source is disabled. When the UVLO pin voltage exceeds the 1.25
V threshold, the 20 μA current source is enabled causing the UVLO pin voltage to increase, providing hysteresis.
The values of R
UV1
and R
UV2
can be determined from the following equation:
(34)
(35)
V
HYS
is the desired UVLO hysteresis at VIN, and VIN in the second equation is the desired UVLO release (turn-
on) voltage. For example, if it is desired for the LM25119 to be enabled when VIN reaches 5.6 V, and the desired
hysteresis is 1.05 V, then R
UV2
should be set to 52.5 kΩ and R
UV1
should be set to 15.1 kΩ. For this application
R
UV2
was selected to be 52.3 kΩ, R
UV1
was selected to be 15 kΩ. The LM25119 can be remotely shutdown by
taking the UVLO pin below 0.4 V with an external open collector or open drain device. The outputs and the VCC
regulator are disabled in shutdown mode. Capacitor C
FT
provides filtering for the divider. A value of 100 pF was
chosen for C
FT
. The voltage at the UVLO pin should never exceed 15 V when using the external set-point
divider. It may be necessary to clamp the UVLO pin at high input voltages.
Figure 9. UVLO Configuration
MOSFET SELECTION
Selection of the power MOSFETs is governed by the same tradeoffs as switching frequency. Breaking down the
losses in the high-side and low-side MOSFETs is one way to compare the relative efficiencies of different
devices. When using discrete SO-8 MOSFETs, generally the output current capability range is 2 A to 10 A.
Losses in the power MOSFETs can be broken down into conduction loss, gate charging loss, and switching loss.
Conduction loss P
DC
is approximately:
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