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LM2681 Arkusz danych(PDF) 7 Page - Texas Instruments

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Numer części LM2681
Szczegółowy opis  LM2681 Switched Capacitor Voltage Converter
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Strona internetowa  http://www.ti.com
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LM2681
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SNVS042B – MARCH 1999 – REVISED MAY 2013
APPLICATION INFORMATION
POSITIVE VOLTAGE DOUBLER
The main application of the LM2681 is to double the input voltage. The range of the input supply voltage is 2.5V
to 5.5V.
The output characteristics of this circuit can be approximated by an ideal voltage source in series with a
resistance. The voltage source equals 2V+. The output resistance Rout is a function of the ON resistance of the
internal MOSFET switches, the oscillator frequency, the capacitance and ESR of C1 and C2. Since the switching
current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the
pumping capacitor C1 will be multiplied by four in the output resistance. The output capacitor C2 is charging and
discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the
output resistance. A good approximation of Rout is:
(1)
where RSW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 12.
The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the
output capacitor C2:
(2)
High capacitance, low ESR capacitors can reduce both the output reslistance and the voltage ripple.
The Schottky diode D1 is only needed for start-up. The internal oscillator circuit uses the OUT pin and the GND
pin. Voltage across OUT and GND must be larger than 1.8V to insure the operation of the oscillator. During start-
up, D1 is used to charge up the voltage at the OUT pin to start the oscillator; also, it protects the device from
turning-on its own parasitic diode and potentially latching-up. Therefore, the Schottky diode D1 should have
enough current carrying capability to charge the output capacitor at start-up, as well as a low forward voltage to
prevent the internal parasitic diode from turning-on. A Schottky diode like 1N5817 can be used for most
applications. If the input voltage ramp is less than 10V/ms, a smaller Schottky diode like MBR0520LT1 can be
used to reduce the circuit size.
SPLIT V+ IN HALF
Another interesting application shown in the Basic Application Circuits is using the LM2681 as a precision voltage
divider. . This circuit can be derived from the voltage doubler by switching the input and output connections. In
the voltage divider, the input voltage applies across the OUT pin and the GND pin (which are the power rails for
the internal oscillator), therefore no start-up diode is needed. Also, since the off-voltage across each switch
equals Vin/2, the input voltage can be raised to +11V.
CAPACITOR SELECTION
As discussed in the POSITIVE VOLTAGE DOUBLER section, the output resistance and ripple voltage are
dependent on the capacitance and ESR values of the external capacitors. The output voltage drop is the load
current times the output resistance, and the power efficiency is
(3)
Where IQ(V+) is the quiescent power loss of the IC device, and IL
2R
out is the conversion loss associated with the
switch on-resistance, the two external capacitors and their ESRs.
The selection of capacitors is based on the specifications of the dropout voltage (which equals Iout Rout), the
output voltage ripple, and the converter efficiency. Low ESR capacitors are recommended to maximize efficiency,
reduce the output voltage drop and voltage ripple.
Copyright © 1999–2013, Texas Instruments Incorporated
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