18
LTC3729
sn3729 3729fas
The time for the output current to ramp up is then:
t
V
V
A
C
s
F C
RAMP
SS
SS
=
−
µ
=
µ
(
)
3
1 5
1 2
1 25
.
.
.
/
By pulling the RUN/SS pin below 0.8V the LTC3729 is put
into low current shutdown (I
Q
< 40
µ
A). RUN/SS 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. 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
SS
, is used initially to limit the inrush
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
SS
reaches 4.1V, C
SS
begins discharging on the assumption
that the output is in an overcurrent condition. If the
condition lasts for a long enough period as determined by
the size of C
SS
, the controller will be shut down until the
RUN/SS pin voltage is recycled. If the overload occurs
during start-up, the time can be approximated by:
t
LO1
≈
(C
SS
• 0.6V)/(1.2
µ
A) = 5 • 10
5
(C
SS
)
If the overload occurs after start-up, the voltage on C
SS
will
continue charging and will provide additional time before
latching off:
t
LO2
≈
(C
SS
• 3V)/(1.2
µ
A) = 2.5 • 10
6
(C
SS
)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, R
SS
, to the RUN/SS pin as
shown in Figure 6. This resistance shortens the soft-start
period and prevents the discharge of the RUN/SS capaci-
tor during a severe overcurrent and/or short-circuit
condition. When deriving the 5
µ
A current from V
IN
as in
the figure, current latchoff is always defeated. Diode-
connecting this pull-up resistor to INTV
CC
, as in Figure␣ 6,
eliminates any extra supply current during shutdown
while eliminating the INTV
CC
loading from preventing
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
SS
may need to be
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
C
SS
> (C
OUT
)(V
OUT
)(10
-4
)(R
SENSE
)
The minimum recommended soft-start capacitor of C
SS
=
0.1
µ
F will be sufficient for most applications.
Phase-Locked Loop and Frequency Synchronization
The LTC3729 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
center frequency f
O
. A voltage applied to the PLLFLTR pin
of 1.2V corresponds to a frequency of approximately
400kHz. The nominal operating frequency range of the
LTC3729 is 250kHz to 550kHz.
APPLICATIO S I FOR ATIO
W
U
U
U
Figure 6. RUN/SS Pin Interfacing
3.3V OR 5V
RUN/SS
V
IN
INTV
CC
RUN/SS
D1
D1*
C
SS
R
SS
*
C
SS
R
SS
*
3729
F06
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
