LTC1438/LTC1439
17
14389fb
APPLICATIONS INFORMATION
WUU
U
the internal current limit.
Power supply sequencing
can
also be accomplished using this pin.
An internal 3µA current source charges up an external
capacitor C
SS.
When the voltage on RUN/SS1 (RUN/SS2)
reaches 1.3V the particular controller is permitted to start
operating. As the voltage on the pin continues to ramp
from 1.3V to 2.4V, the internal current limit is also ramped
at a proportional linear rate. The current limit begins at
approximately 50mV/R
SENSE
(at V
RUN/SS
= 1.3V) and ends
at 150mV/R
SENSE
(V
RUN/SS
≥ 2.7V). The output current
thus ramps up slowly, reducing the starting surge current
required from the input power supply. If RUN/SS has been
pulled all the way to ground there is a delay before starting
of approximately 500ms/µF, followed by a similar time to
reach full current on that controller.
By pulling both RUN/SS controller pins below 1.3V, the
LTC1438/LTC1439 are put into low current shutdown
(I
Q
< 25µA). These pins can be driven directly from logic as
shown in Figure 6. Diode D1 in Figure 6 reduces the start
delay but allows C
SS
to ramp up slowly providing the soft
start function; this diode and C
SS
can be deleted if soft start
is not needed. Each RUN/SS pin has an internal 6V Zener
clamp (See Functional Diagram).
Figure 5b. LTC1438/LTC1439 Adjustable Applications
LTC1438
LTC1439
V
PROG1
SENSE
–
1
SGND
GND: V
OUT
= 3.3V
INTV
CC
: V
OUT
= 5V
+
V
OUT
1438 F05a
C
OUT
Figure 5a. LTC1438/LTC1439 Fixed Output Applications
Power-On Reset Function (POR)
The power-on reset function (not available on the
LTC1438X) monitors the output voltage of the second
controller and turns on an open drain device when it is
below its properly regulated voltage. An external pull-up
resistor is required on the POR2 pin.
When power is first applied or when coming out of
shutdown, the POR2 output is held at ground. When the
output voltage rises above a level which is 5% below the
final regulated output value, an internal counter starts.
After this counter counts 2
16
(65536) clock cycles, the
POR2 pull-down device turns off.
The POR2 output will go low whenever the output voltage
of the second controller drops below 7.5% of its regulated
value for longer than approximately 30µs, signaling an
out-of-regulation condition. In shutdown, when RUN/SS1
and RUN/SS2 are both below 1.3V, the POR2 output is
pulled low even if the regulator’s output is held up by an
external source. The POR2 output is active during shut-
down if V
IN
is powered.
Run/ Soft Start Function
The RUN/SS1 and RUN/SS2 pins each serve two func-
tions. Each pin provides the soft start function and a
means to shut down each controller. Soft start reduces
surge currents from V
IN
by providing a gradual ramp-up of
D1
C
SS
3.3V
OR 5V
RUN/SS1
(RUN/SS2)
C
SS
1438 F06
RUN/SS1
(RUN/SS2)
Figure 6. RUN/SS Pin Interfacing
Foldback Current Limiting
As described in Power MOSFET and D1 Selection, the
worst-case dissipation for either MOSFET occurs with a
short-circuited output, when the synchronous MOSFET
conducts the current limit value almost continuously. In
most applications this will not cause excessive heating,
even for extended fault intervals. However, when heat
sinking is at a premium or higher R
DS(ON)
MOSFETs are
being used, foldback current limiting should be added to
reduce the current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding diode
D
FB
between the output and the I
TH
pin as shown in the
LTC1438
LTC1439
V
PROG2
*
V
OSENSE1,2
SGND
OPEN (DC)
1.19V ≤ V
OUT
≤ 9V
1438 F05b
100pF
R2
R1
R2
R1
V
OUT
= 1.19V 1 +
()
*LTC1439 ONLY
