LTC1439 Linear Technology, LTC1439 Datasheet - Page 17

• Manufacturer Part Number: LTC1439
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

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
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 30 s, 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
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
Figure 5a. LTC1438/LTC1439 Fixed Output Applications
Figure 5b. LTC1438/LTC1439 Adjustable Applications
IN
is powered.
*LTC1439 ONLY
LTC1438
LTC1439
V
LTC1438
LTC1439
OSENSE1,2
SENSE
V
PROG2
V
SGND
PROG1
SGND
U
–
*
IN
1
by providing a gradual ramp-up of
INFORMATION
OPEN (DC)
U
V
GND: V
INTV
OUT
16
CC
= 1.19V 1 +
(65536) clock cycles, the
OUT
: V
100pF
OUT
1.19V
= 3.3V
W
= 5V
+
V
R2
R1
OUT
R2
R1
C
1438 F05b
OUT
1438 F05a
V
9V
OUT
U
the internal current limit. Power supply sequencing can
also be accomplished using this pin.
An internal 3 A current source charges up an external
capacitor C
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
at 150mV/R
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/ F, 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
as shown in Figure 6. Diode D1 in Figure 6 reduces the start
delay but allows C
start function; this diode and C
is not needed. Each RUN/SS pin has an internal 6V Zener
clamp (See Functional Diagram).
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
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
Q
FB
< 25 A). These pins can be driven directly from logic
between the output and the I
OR 5V
3.3V
SS.
SENSE
D1
Figure 6. RUN/SS Pin Interfacing
When the voltage on RUN/SS1 (RUN/SS2)
(RUN/SS2)
RUN/SS1
SS
(V
C
RUN/SS
to ramp up slowly providing the soft
SS
SENSE
LTC1438/LTC1439
(at V
SS
2.7V). The output current
can be deleted if soft start
RUN/SS
TH
DS(ON)
pin as shown in the
= 1.3V) and ends
(RUN/SS2)
RUN/SS1
MOSFETs are
C
1438 F06
SS
17