LTC3711 Datasheet, PDF (16/24 Pages) Linear Technology – 5-Bit Adjustable, Wide Operating Range, No RSENSE

LTC3711
APPLICATIO S I FOR ATIO
EXTV_CC Connection
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 follow-
ing list summarizes the possible connections for EXTV_CC:
1. EXTV_CC grounded. INTV_CC is always powered from the
internal 5V regulator.
2. EXTV_CC connected to an external supply. A high effi-
ciency supply compatible with the MOSFET gate drive
requirements (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.
External Gate Drive Buffers
The LTC3711 drivers are adequate for driving up to about
30nC into MOSFET switches with RMS currents of 50mA.
Applications with larger MOSFET switches or operating at
frequencies requiring greater RMS currents will benefit
from using external gate drive buffers such as the LTC1693.
Alternately, the external buffer circuit shown in Figure 7
can be used. Note that the bipolar devices reduce the
signal swing at the MOSFET gate and benefit from an
increased EXTV_CC voltage of about 6V.
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3711 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC3711 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:
16
( ) ^tDELAY
=
1.5V
1.2µA
C_SS
=
1.3s/µF C_SS
When the voltage on RUN/SS reaches 1.5V, the LTC3711
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. The pin
can be driven from logic as shown in Figure 8. Diode D1
reduces the start delay while allowing C_SS to charge up
slowly for the soft-start function.
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 cur-
rent then begins discharging C_SS. If the fault condition
persists until the RUN/SS pin drops to 3.5V, then the con-
troller 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 guar-
antee 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
operation 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
current 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 8a is simple, but slightly increases
shutdown current. Connecting a resistor to INTV_CC as
shown in Figure 8b eliminates the additional shutdown
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