LTC3608
16
3608fc
applications inForMation
The EXTV
CC
pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTV
CC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV
CC
pin to INTV
CC
. INTV
CC
power is supplied from EXTV
CC
until this pin drops below 4.5V. Do not apply more than
7V to the EXTV
CC
pin and ensure that EXTV
CC
≤ V
IN
. The
following list summarizes the possible connections for
EXTV
CC
:
1. EXTV
CC
grounded. INTV
CC
is always powered from the
internal 5V regulator.
2. E
XTV
CC
connected to an external supply. A high efficiency
supply compatible with the MOSFET gate drive require-
ments (typically 5V) can improve overall efficiency.
3.
EXT
V
CC
connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The
system will start-up using the internal linear regulator
until the boosted output supply is available.
Soft-Start
and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3608 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC3608 into a low quiescent current shutdown (I
Q
<
30µA). Releasing the pin allows an internal 1.2µA current
source to charge up the external timing capacitor C
SS
. If
RUN/SS has been pulled all the way to ground, there is a
delay before starting of about:
t
DELAY
=
1.5V
1.2µA
C
SS
= 1.3s/µF
( )
C
SS
When the voltage on RUN/SS reaches 1.5V, the LTC3608
begins operating with a clamp on I
TH
of approximately
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
TH
is raised until its full 2.4V range is available. This takes an
additional 1.3s/µF, during which the load current is folded
back until the output reaches 75% of its final value.
After the controller has been started and given adequate
time to charge up the output capacitor, C
SS
is used as a
short-circuit timer. After the RUN/SS pin charges above 4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8µA current then
begins discharging C
SS
. If the fault condition persists until
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
T
h
e overcurrent protection timer requires that the soft-start
timing capacitor, C
SS
, be made large enough to guarantee
that the output is in regulation by the time C
SS
has reached
the 4V threshold. In general, this will depend upon the
size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor
can be estimated from:
C
SS
> C
OUT
V
OUT
R
SENSE
(10
–4
[F/V s])
Generally 0.1µF is more than sufficient.
Overcurrent latchoff operation is not always needed or
desired. Load current is already limited during a short
circuit by the current foldback circuitry and latchoff op-
eration can prove annoying during troubleshooting. The
feature can be overridden by adding a pull-up current
greater than 5µA to the RUN/SS pin. The additional cur-
rent prevents the discharge of C
SS
during a fault and also
shortens the soft-start period. Using a resistor to V
IN
as
shown in Figure 5a is simple, but slightly increases shut-
down current. Connecting a resistor to INTV
CC
as shown
in Figure 5b eliminates the additional shutdown current,
but requires a diode to isolate C
SS
. Any pull-up network
must be able to pull RUN/SS above the 4.2V maximum
threshold of the latchoff circuit and overcome the 4µA
maximum discharge current.
