LTC1735
U
W
U U
APPLICATIO S I FOR ATIO
V
OUT
VOUT resistive divider. The maximum current flowing out
of the sense pins is:
R2
V
ISENSE+ + ISENSE– = (2.4V – VOUT)/24k
OSENSE
47pF
R1
LTC1735
SGND
SinceVOSENSE isservoedtothe0.8Vreferencevoltage, we
can choose R1 in Figure 4 to have a maximum value to
absorb this current:
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Figure 4. Setting the LTC1735 Output Voltage
0.8V
R1
= 24k
(MAX)
2.4V – V
OUT
Topside MOSFET Driver Supply (CB, DB)
Regulating an output voltage of 1.8V, the maximum value
of R1 should be 32k. Note that at output voltages above
2.4V no maximum value of R1 is necessary to absorb the
sense pin currents; however, R1 is still bounded by the
VOSENSE feedback current.
An external bootstrap capacitor CB connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET.CapacitorCBintheFunctionalDiagramischarged
though external diode DB from INTVCC when the SW pin
is low. Note that the voltage across CB is about a diode
drop below INTVCC. When the topside MOSFET is to be
turned on, the driver places the CB voltage across the
gate-source of the MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage
SW rises to VIN and the BOOST pin rises to VIN + INTVCC.
The value of the boost capacitor CB needs to be 100 times
greater than the total input capacitance of the topside
MOSFET. In most applications 0.1µF to 0.33µF is ad-
equate. The reverse breakdown on DB must be greater
Soft-Start/Run Function
The RUN/SS pin is a multipurpose pin that provides a soft-
start function and a means to shut down the LTC1735.
Soft-start reduces surge currents from VIN by gradually
increasing the controller’s current limit ITH(MAX). This pin
can also be used for power supply sequencing.
Pulling the RUN/SS pin below 1.5V puts the LTC1735 into
shutdown. This pin can be driven directly from logic as
shown in Figure 5. The VIN quiescent current is a function
of RUN/SS voltage (refer to Typical Performance Charac-
teristics graphs on page 6). Releasing the RUN/SS pin
allows an internal 1.2µA current source to charge up the
external soft-start capacitor CSS. If RUN/SS has been
pulled all the way to ground there is a delay before starting
of approximately:
than VIN(MAX)
.
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If you make a change
and the input current decreases, then you improved the
efficiency. If there is no change in input current, then there
is no change in efficiency.
SENSE+/SENSE– Pins
1.5V
1.2µA
tDELAY
=
CSS = 1.25s/µF C
SS
(
)
The common mode input range of the current comparator
is from 0V to 1.1(INTVCC). Continuous linear operation in
step-down applications is guaranteed throughout this
range allowing output voltages anywhere from 0.8V to 7V.
A differential NPN input stage is used and is biased with
internal resistors from an internal 2.4V source as shown
in the Functional Diagram. This causes current to either be
sourced or sunk by the sense pins depending on the
output voltage. If the output voltage is below 2.4V current
will flow out of both sense pins to the main output. This
forces a minimum load current that can be fulfilled by the
When the voltage on RUN/SS reaches 1.5V the LTC1735
begins operating with a current limit at approximately
25mV/RSENSE.AsthevoltageontheRUN/SSpinincreases
from 1.5V to 3.0V, the internal current limit is increased
from 25mV/RSENSE to 75mV/RSENSE. The output current
limit ramps up slowly, taking an additional 1.25s/µF to
reach full current. The output current thus ramps up
slowly, reducing the starting surge current required from
the input power supply.
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17
Linear [ Linear ]
