ltc1629-6 Linear Technology Corporation, ltc1629-6 Datasheet - Page 18

• Manufacturer Part Number: ltc1629-6
• Description: Polyphase, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1629-6
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:
The time for the output current to ramp up is then:
By pulling the RUN/SS pin below 0.8V the LTC1629-6 is
put into low current shutdown (I
driven directly from logic as shown in Figure 6. Diode D1
in Figure 6 reduces the start delay but allows C
up slowly providing the soft-start function. The RUN/SS
pin has an internal 6V zener clamp (see Functional Dia-
gram).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to latch off the
controllers when an overcurrent condition is detected. The
RUN/SS capacitor, C
current of both controllers. After the controllers have been
started and been given adequate time to charge up the
output capacitors and provide full load current, the RUN/
SS capacitor is used for a short-circuit timer. If the output
voltage falls to less than 70% of its nominal value after C
reaches 4.1V, C
that the output is in an overcurrent condition. If the
condition lasts for a long enough period as determined by
the size of C
RUN/SS pin voltage is recycled. If the overload occurs
during start-up, the time can be approximated by:
If the overload occurs after start-up, the voltage on C
continue charging and will provide additional time before
latching off:
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, R
shown in Figure 6. This resistance shortens the soft-start
18
t
t
t
t
LO1
LO2
DELAY
RAMP
(C
(C
SS
SS
3
SS
1 2
1 5
V
.
1 2
• 0.6V)/(1.2 A) = 5 • 10
• 3V)/(1.2 A) = 2.5 • 10
, the controller will be shut down until the
.
.
SS
V
A
1 5
begins discharging on the assumption
.
A
U
C
SS
SS
V
, is used initially to limit the inrush
C
SS
U
1 25
.
1 25
SS
Q
s
.
< 40 A). RUN/SS can be
, to the RUN/SS pin as
/
s
W
F C
/
5
6
F C
(C
(C
SS
SS
SS
SS
)
)
SS
U
to ramp
SS
will
SS
period and prevents the discharge of the RUN/SS capaci-
tor during a severe overcurrent and/or short-circuit con-
dition. When deriving the 5 A current from V
figure, current latchoff is always defeated. Diode-
connecting this pull-up resistor to INTV
Figure 6, eliminates any extra supply current during shut-
down while eliminating the INTV
ing controller start-up.
Why should you defeat current latchoff? During the
prototyping stage of a design, there may be a problem with
noise pickup or poor layout causing the protection circuit
to latch off the controller. Defeating this feature allows
troubleshooting of the circuit and PC layout. The internal
short-circuit and foldback current limiting still remains
active, thereby protecting the power supply system from
failure. A decision can be made after the design is com-
plete whether to rely solely on foldback current limiting or
to enable the latchoff feature by removing the pull-up
resistor.
The value of the soft-start capacitor C
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
The minimum recommended soft-start capacitor of C
0.1 F will be sufficient for most applications.
Phase-Locked Loop and Frequency Synchronization
The LTC1629-6 has a phase-locked loop comprised of an
internal voltage controlled oscillator and phase detector.
This allows the top MOSFET turn-on to be locked to the
rising edge of an external source. The frequency range of
the voltage controlled oscillator is 50% around the
C
SS
3.3V OR 5V
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
> (C
OUT
D1
Figure 6. RUN/SS Pin Interfacing
)(V
V
IN
OUT
R
SS
*
RUN/SS
)(10
-4
C
SS
)(R
SENSE
CC
loading from prevent-
D1*
)
INTV
SS
may need to be
CC
R
SS
*
IN
RUN/SS
CC
as in the
, as in
1629 F06
C
SS
SS
16296f
=