11
LTC3729
sn3729 3729fas
OPERATIO
U
(Refer to Functional Diagram)
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
of the IC circuitry is derived from INTV
CC
. When the
EXTV
CC
pin is left open, an internal 5V low dropout
regulator supplies INTV
CC
power. If the EXTV
CC
pin is
taken above 4.7V, the 5V regulator is turned off and an
internal switch is turned on connecting EXTV
CC
to INTV
CC
.
This allows the INTV
CC
power to be derived from a high
efficiency external source such as the output of the regu-
lator itself or a secondary winding, as described in the
Applications Information section. An external Schottky
diode can be used to minimize the voltage drop from
EXTV
CC
to INTV
CC
in applications requiring greater than
the specified INTV
CC
current. Voltages up to 7V can be
applied to EXTV
CC
for additional gate drive capability.
Differential Amplifier
This amplifier provides true differential output voltage
sensing. Sensing both V
OUT
+
and V
OUT
–
benefits regula-
tion in high current applications and/or applications hav-
ing electrical interconnection losses.
Power Good (PGOOD)
The PGOOD pin is connected to the drain of an internal
MOSFET. The MOSFET turns on when the output is not
within
±
7.5% of its nominal output level as determined by
the feedback divider. When the output is within
±
7.5% of
its nominal value, the MOSFET is turned off within 10
µ
s
and the PGOOD pin should be pulled up by an external
resistor to a source of up to 7V.
Short-Circuit Detection
The RUN/SS capacitor is used initially to limit the inrush
current from the input power source. Once the controllers
have been given time, as determined by the capacitor on
the RUN/SS pin, to charge up the output capacitors and
provide full load current, the RUN/SS capacitor is then
used as a short-circuit timeout circuit. If the output voltage
falls to less than 70% of its nominal output voltage the
RUN/SS capacitor begins discharging assuming that the
output is in a severe overcurrent and/or short-circuit
condition. If the condition lasts for a long enough period
as determined by the size of the RUN/SS capacitor, the
controller will be shut down until the RUN/SS pin voltage
is recycled. This built-in latchoff can be overidden by
providing a >5
µ
A pull-up current at a compliance of 5V to
the RUN/SS pin. This current shortens the soft-start
period but also prevents net discharge of the RUN/SS
capacitor during a severe overcurrent and/or short-circuit
condition. Foldback current limiting is activated when the
output voltage falls below 70% of its nominal level whether
or not the short-circuit latchoff circuit is enabled.
APPLICATIO S I FOR ATIO
W
U
U
U
The basic LTC3729 application circuit is shown in Figure␣ 1
on the first page. External component selection is driven
by the load requirement, and begins with the selection of
R
SENSE1, 2
. Once R
SENSE1, 2
are known, L1 and L2 can be
chosen. Next, the power MOSFETs and D1 and D2 are
selected. The operating frequency and the inductor are
chosen based mainly on the amount of ripple current.
Finally, C
IN
is selected for its ability to handle the input
ripple current (that PolyPhase operation minimizes) and
C
OUT
is chosen with low enough ESR to meet the output
ripple voltage and load step specifications (also minimized
with PolyPhase). The circuit shown in Figure␣ 1 can be
configured for operation up to an input voltage of 28V
(limited by the external MOSFETs).
R
SENSE
Selection For Output Current
R
SENSE1, 2
are chosen based on the required output
current. The LTC3729 current comparator has a maxi-
mum threshold of 75mV/R
SENSE
and an input common
mode range of SGND to 1.1( INTV
CC
). The current com-
parator threshold sets the peak inductor current, yielding
a maximum average output current I
MAX
equal to the peak
value less half the peak-to-peak ripple current,
∆
I
L
.
Allowing a margin for variations in the LTC3729 and
external component values yields:
R
SENSE
= (50mV/I
MAX
)N
where N = number of stages.
