LTC1539 LINER [Linear Technology], LTC1539 Datasheet - Page 17

• Manufacturer Part Number: LTC1539
• Description: Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
(DC) and the V
resistors as shown in Figure 5b. Controller 2 will force the
externally attenuated output voltage to 1.19V.
Power-On Reset Function (POR)
The power-on reset function monitors the output voltage
of the first 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 POR1 pin.
When power is first applied or when coming out of
shutdown, the POR1 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
down device turns off. The POR1 output is active when both
controllers are shut down as long as V
The POR1 output will go low whenever the output voltage
of the first 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 POR1 output is
pulled low even if the regulator’s output is held up by an
external source.
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
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
SS.
SENSE
When the voltage on RUN/SS1 (RUN/SS2)
OSENSE2
16
(V
U
(65536) clock cycles, the POR1 pull-
RUN/SS
IN
SENSE
by providing a gradual ramp-up of
pin is connected to the feedback
INFORMATION
U
(at V
2.7V). The output current
RUN/SS
W
IN
is powered.
= 1.3V) and ends
U
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
LTC1538-AUX/LTC1539 are put into 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
Functional Diagram. In a hard short (V
current will be reduced to approximately 25% of the
maximum output current. This technique may be used for
all applications with regulated output voltages of 1.8V or
greater.
Q
FB
< 200 A). These pins can be driven directly from logic
between the output and the I
OR 5V
3.3V
D1
Figure 6. RUN/SS Pin Interfacing
LTC1538-AUX/LTC1539
(RUN/SS2)
RUN/SS1
SS
C
to ramp up slowly providing the soft
SS
SS
can be deleted if soft start
TH
DS(ON)
pin as shown in the
(RUN/SS2)
OUT
RUN/SS1
MOSFETs are
= 0V) the
C
1538 F06
SS
17