LTC1435A Linear Technology, LTC1435A Datasheet - Page 12

• Manufacturer Part Number: LTC1435A
• Description: High Efficiency Low Noise Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

LTC1435A
APPLICATIONS
The external resistive divider is connected to the output as
shown in Figure 5 allowing remote voltage sensing.
Run/ Soft Start Function
The RUN/SS pin is a dual purpose pin that provides the soft
start function and a means to shut down the LTC1435A. Soft
start reduces surge currents from V
ing 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
the LTC1435A begins operating. As the voltage on RUN/SS
continues to ramp from 1.3V to 2.4V, the internal current
limit is also ramped at a proportional linear rate. The cur-
rent limit begins at approximately 50mV/R
SS
output current thus ramps up slowly, charging the output
capacitor. If RUN/SS has been pulled all the way to ground
there is a delay before starting of approximately 500ms/ F,
followed by an additional 500ms/ F to reach full current.
Pulling the RUN/SS pin below 1.3V puts the LTC1435A into
a low quiescent current shutdown (I
be driven directly from logic as shown in Figure 6. Diode
D1 in Figure 6 reduces the start delay but allows C
up slowly for the soft start function; this diode and C
be deleted if soft start is not needed. The RUN/SS pin has
an internal 6V Zener clamp (See Functional Diagram).
12
= 1.3V) and ends at 150mV/R
t
DELAY
Figure 5. Setting the LTC1435A Output Voltage
3.3V OR 5V
= 5(10
SS.
Figure 6. RUN/SS Pin Interfacing
When the voltage on RUN/SS reaches 1.3V
5
D1
LTC1435A
)C
U
V
SS
OSENSE
RUN/SS
SGND
Seconds
C
INFORMATION
SS
U
SENSE
100pF
1.19V
1435A F05
IN
Q
W
(V
by gradually increas-
< 25 A). This pin can
V
OUT
R2
R1
RUN/SS
9V
SENSE
RUN/SS
> 2.7V). The
1435 F06
SS
U
C
SS
(at V
to ramp
SS
RUN/
can
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 appli-
cations this will not cause excessive heating, even for
extended fault intervals. However, when heat sinking is at
a premium or higher R
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
rent will be reduced to approximately 25% of the maximum
output current. This technique may be used for all applica-
tions with regulated output voltages of 1.8V or greater.
SFB Pin Operation
When the SFB pin drops below its ground referenced 1.19V
threshold, continuous mode operation is forced. In continu-
ous mode, the large N-channel main and synchronous
switches are used regardless of the load on the main output.
In addition to providing a logic input to force continuous
synchronous operation, the SFB pin provides a means to
regulate a flyback winding output. Continuous synchronous
operation allows power to be drawn from the auxiliary
windings without regard to the primary output load. The SFB
pin provides a way to force continuous synchronous op-
eration as needed by the flyback winding.
The secondary output voltage is set by the turns ratio of the
transformer in conjunction with a pair of external resistors
returned to the SFB pin as shown in Figure 4a. The second-
ary regulated voltage, V
where N is the turns ratio of the transformer and V
the main output voltage sensed by V
Minimum On-Time Considerations
Minimum on-time, t
that the LTC1435A is capable of turning the top MOSFET
FB
V
SEC
between the output and the I
N
1
V
OUT
ON(MIN)
DS(ON)
SEC
1 19 1
, is the smallest amount of time
.
, in Figure 4a is given by:
MOSFETs are being used,
TH
R
R
pin as shown in the
6
5
OSENSE
OUT
= 0V) the cur-
.
OUT
is