LTC3728L/LTC3728LX
13
3728lxff
operaTion
Constant-Frequency Operation
When the FCB pin is tied to INTV
CC
, Burst Mode opera-
tion is disabled and the forced minimum output current
requirement is removed. This provides constant-frequency,
discontinuous current (preventing reverse inductor cur-
rent) operation over the widest possible output current
range. This constant-frequency operation is not as efficient
as Burst Mode operation, but does provide a lower noise,
constant-frequency operating mode down to approximately
1% of the designed maximum output current.
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least efficient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When sinking current
while in forced continuous operation, current will be forced
back into the main power supply potentially boosting the
input supply to dangerous voltage levels—BEWARE!
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
CC
pin. When
the EXTV
CC
pin is left open, an internal 5V low dropout
linear regulator supplies INTV
CC
power. If EXTV
CC
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 al-
lows the INTV
CC
power to be derived from a high efficiency
external source such as the output of the regulator itself
or a secondary winding, as described in the Applications
Information section.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious condi-
tions that may overvoltage the output. In this case, the top
MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
Power Good (PGOOD) Pin
The PGOOD pin is connected to an open drain of an internal
MOSFET. The MOSFET turns on and pulls the pin low when
either output is not within ±7.5% of the nominal output
level as determined by the resistive feedback divider. When
both outputs meet the ±7.5% requirement, the MOSFET is
turned off within 10µs and the pin is allowed to be pulled
up by an external resistor to a source of up to 7V.
Foldback Current, Short-Circuit Detection
and Short-Circuit Latchoff
The RUN/SS capacitors are used initially to limit the inrush
current of each switching regulator. After the controller
has been started and been given adequate time to charge
up the output capacitors and provide full load current, the
RUN/SS capacitor is used in a short-circuit time-out circuit.
If the output voltage falls to less than 70% of its nominal
output voltage, the RUN/SS capacitor begins discharging
on the assumption that the output is in an 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(s) voltage(s) are recycled. This built-in latchoff can
be overridden by providing a >5µA pull-up at a compliance
of 5V to the RUN/SS pin(s). This current shortens the soft
start period but also prevents net discharge of the RUN/
SS capacitor(s) during an overcurrent and/or short-circuit
condition. Foldback current limiting is also activated when
the output voltage falls below 70% of its nominal level
whether or not the short-circuit latchoff circuit is enabled.
Even if a short is present and the short-circuit latchoff is
not enabled, a safe, low output current is provided due to
internal current foldback and actual power wasted is low
due to the efficient nature of the current mode switching
regulator.
(Refer to Functional Diagram)
