ltc3711 Linear Technology Corporation, ltc3711 Datasheet - Page 16

• Manufacturer Part Number: ltc3711
• Description: 5-bit Adjustable, Wide Operating Range, No Rsense? Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3711
EXTV
The EXTV
and control power from the output or another external
source during normal operation. Whenever the EXTV
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV
pin to INTV
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV
ing list summarizes the possible connections for EXTV
1. EXTV
internal 5V regulator.
2. EXTV
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV
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
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
current source to charge up the external timing capacitor
C
there is a delay before starting of about:
16
SS
Q
< 30 A). Releasing the pin allows an internal 1.2 A
. If RUN/SS has been pulled all the way to ground,
CC
CC
CC
CC
Connection
CC
CC
CC
grounded. INTV
connected to an output derived boost network.
connected to an external supply. A high effi-
pin and ensure that EXTV
pin can be used to provide MOSFET gate drive
. INTV
CC
voltage of about 6V.
CC
U
power is supplied from EXTV
U
CC
is always powered from the
W
CC
V
IN
. The follow-
U
CC
until
CC
CC
CC
:
When the voltage on RUN/SS reaches 1.5V, the LTC3711
begins operating with a clamp on I
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
is raised until its full 2.4V range is available. This takes an
additional 1.3s/ F, 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
slowly for the soft-start function.
After the controller has been started and given adequate
time to charge up the output capacitor, C
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 1.8 A cur-
rent then begins discharging C
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
antee that the output is in regulation by the time C
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:
Generally 0.1 F 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 5 A to the RUN/SS pin. The additional
current prevents the discharge of C
also shortens the soft-start period. Using a resistor to V
as shown in Figure 8a is simple, but slightly increases
shutdown current. Connecting a resistor to INTV
shown in Figure 8b eliminates the additional shutdown
C
t
DELAY
SS
> C
OUT
1 2
1 5
.
V
.
OUT
V
A
C
R
SS
SENSE
SS
be made large enough to guar-
1 3
(10
. /
s F C
– 4
SS
. If the fault condition
[F/V s])
SS
TH
SS
during a fault and
of approximately
SS
SS
to charge up
is used as a
SS
CC
has
3711f
TH
as
IN