Datasheet

LM2767
SNVS069C –FEBRUARY 2000–REVISED MAY 2013 www.ti.com
Figure 14. Voltage Doubling Principle
POSITIVE VOLTAGE DOUBLER
The main application of the LM2767 is to double the input voltage. The range of the input supply voltage is 1.8V
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, and the capacitance and ESR of C
1
and C
2
. Since the
switching current charging and discharging C
1
is approximately twice 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 resistances of the internal MOSFET switches shown in Figure 14. R
SW
is
typically 4.5Ω for the LM2767.
The peak-to-peak output voltage ripple is determined by the oscillator frequency as well as 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 to protect the device from turning-on its own parasitic diode and potentially
latching-up. During start-up, D
1
will also quickly charge up the output capacitor to V
IN
minus the diode drop
thereby decreasing the start-up time. 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.
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.
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