LM2765_15 Datasheet, PDF (6/14 Pages) Texas Instruments – LM2765 Switched Capacitor Voltage Converter

LM2765
SNVS070C – MARCH 2000 – REVISED MAY 2013
www.ti.com
CIRCUIT DESCRIPTION
The LM2765 contains four large CMOS switches which are switched in a sequence to double the input supply
voltage. Energy transfer and storage are provided by external capacitors. Figure 13 illustrates the voltage
conversion scheme. When S₂ and S₄ are closed, C₁ charges to the supply voltage V+. During this time interval,
switches S₁ and S₃ are open. In the next time interval, S₂ and S₄ are open; at the same time, S₁ and S₃ are
closed, the sum of the input voltage V+ and the voltage across C₁ gives the 2V+ output voltage when there is no
load. The output voltage drop when a load is added is determined by the parasitic resistance (R_ds(on) of the
MOSFET switches and the ESR of the capacitors) and the charge transfer loss between capacitors. Details will
be discussed in the following application information section.
Figure 13. Voltage Doubling Principle
POSITIVE VOLTAGE DOUBLER
The main application of the LM2765 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 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 R_out is:
(1)
where R_SW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 13. R_SW is typically
8Ω for the LM2765.
The peak-to-peak output voltage ripple is determined by the oscillator frequency as well as the capacitance and
ESR of the output capacitor C₂:
(2)
High capacitance, low ESR capacitors can reduce both the output resistance and the voltage ripple.
The Schottky diode D₁ is only needed to protect the device from turning-on its own parasitic diode and potentially
latching-up. During start-up, D₁ 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 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.
SHUTDOWN MODE
A shutdown (SD) pin is available to disable the device and reduce the quiescent current to 0.1 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.
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