LTC3611 Просмотр технического описания (PDF) - Linear Technology
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LTC3611 Datasheet PDF : 24 Pages
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LTC3611
APPLICATIONS INFORMATION
The basic LTC3611 application circuit is shown on the front
page of this data sheet. External component selection is
primarily determined by the maximum load current. The
LTC3611 uses the on-resistance of the synchronous power
MOSFET for determining the inductor current. The desired
amount of ripple current and operating frequency also
determines the inductor value. Finally, CIN is selected for its
ability to handle the large RMS current into the converter
and COUT is chosen with low enough ESR to meet the
output voltage ripple and transient specification.
VON and PGOOD
The LTC3611 has an open-drain PGOOD output that
indicates when the output voltage is within ±10% of the
regulation point. The LTC3611 also has a VON pin that
allows the on-time to be adjusted. Tying the VON pin high
results in lower values for RON which is useful in high VOUT
applications. The VON pin also provides a means to adjust
the on-time to maintain constant frequency operation in
applications where VOUT changes and to correct minor
frequency shifts with changes in load current.
VRNG Pin and ILIMIT Adjust
The VRNG pin is used to adjust the maximum inductor
valley current, which in turn determines the maximum
average output current that the LTC3611 can deliver. The
maximum output current is given by:
IOUT(MAX) = IVALLEY(MAX) + 1/2 ΔIL,
The IVALLEY(MAX) is shown in the figure “Maximum Valley
Current Limit vs VRNG Voltage” in the Typical Performance
Characteristics.
An external resistor divider from INTVCC can be used to
set the voltage on the VRNG pin from 1V to 1.4V, or it can
be simply tied to ground force a default value equivalent
to 0.7V. Do not float the VRNG pin.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3611 applications is de-
termined implicitly by the one-shot timer that controls the
on-time tON of the top MOSFET switch. The on-time is set
by the current into the ION pin and the voltage at the VON
pin according to:
tON
=
VVON
IION
(10pF)
Tying a resistor RON from VIN to the ION pin yields an
on-time inversely proportional to VIN. The current out of
the ION pin is:
IION
=
VIN
RON
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
f=
VVON
VOUT
RON(10pF)
[Hz]
To hold frequency constant during output voltage changes,
tie the VON pin to VOUT or to a resistive divider from VOUT
when VOUT > 2.4V. The VON pin has internal clamps that
limit its input to the one-shot timer. If the pin is tied below
0.7V, the input to the one-shot is clamped at 0.7V. Similarly,
if the pin is tied above 2.4V, the input is clamped at 2.4V.
In high VOUT applications, tying VON to INTVCC so that
the comparator input is 2.4V results in a lower value for
RON. Figures 1a and 1b show how RON relates to switching
frequency for several common output voltages.
3611fb
11
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