LTC3611_15 Datasheet, PDF (16/26 Pages) Linear Technology – 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter

LTC3611
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. EXTV_CCconnectedtoanexternalsupply.Ahighefficiency
supply compatible with the MOSFET gate drive require-
ments (typically 5V) can improve overall efficiency.
3. EXTV_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 LTC3611
as well as a timer for soft-start and overcurrent latchoff.
Pulling the RUN/SS pin below 0.8V puts the LTC3611 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 LTC3611
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.
The 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.
3611fd
16