Datasheet

LM2665
www.ti.com SNVS009F –NOVEMBER 1999–REVISED MAY 2013
APPLICATION INFORMATION
POSITIVE VOLTAGE DOUBLER
The main application of the LM2665 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 R
out
is a function of the ON resistance of the
internal MOSFET switches, the oscillator frequency, the capacitance and ESR of C
1
and C
2
. Since the switching
current charging and discharging C
1
is approximately twice as the output current, the effect of the ESR of the
pumping capacitor C
1
will be multiplied by four in the output resistance. The output capacitor C
2
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 R
out
is:
(1)
where R
SW
is the sum of the ON resistance of the internal MOSFET switches shown in Figure 13.
The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the
output capacitor C
2
:
(2)
High capacitance, low ESR capacitors can reduce both the output resistance and the voltage ripple.
The Schottky diode D
1
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, D
1
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 D
1
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 LM2665 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 V
in
/2, the input voltage can be raised to +11V.
SHUTDOWN MODE
A shutdown (SD) pin is available to disable the device and reduce the quiescent current to 1 µA. In normal
operating mode, the SD pin is connected to ground. The device can be brought into the shutdown mode by
applying to the SD pin a voltage greater than 40% of the V+ pin voltage.
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 I
Q
(V+) is the quiescent power loss of the IC device, and I
L
2
R
out
is the conversion loss associated with the
switch on-resistance, the two external capacitors and their ESRs.
Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 7
Product Folder Links: LM2665