SNVS009H November 1999 – March 2016 LM2665
PRODUCTION DATA.
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers must validate and test their design implementation to confirm system functionality.
The LM2665 provides a simple and efficient means of creating an output voltage level equal to twice that of the input voltage. Without the need of an inductor, the application solution size can be reduced versus the magnetic DC-DC converter solution.
The main application of the LM2665 is to double the input voltage. The range of the input supply voltage is 2.5 V to 5.5 V.
Example requirements for LM2665 device applications:
DESIGN PARAMETER | EXAMPLE VALUE |
---|---|
Input voltage range | 2.5 V to 5.5 V |
Output current | 0 mA to 40 mA |
Boost switching frequency | 80 kHz |
The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistance. The voltage source equals 2 V+. The output resistance ROUT is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance and equivalent series resistance (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 when in the output resistance. A good approximation of ROUT is:
where
The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor C2:
High capacitance, low-ESR capacitors can reduce both the output resistance 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 1.8 V 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 must 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 10 V/ms, a smaller Schottky diode like MBR0520LT1 can be used to reduce the circuit size.
As discussed in Positive Voltage Doubler, 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:
where
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.
Any number of LM2665 devices can be paralleled to reduce the output resistance. Each device must have its own pumping capacitor C1, while only one output capacitor COUT is needed as shown in Figure 12. The composite output resistance is:
Cascading the LM2665 devices is an easy way to produce a greater voltage (a two-stage cascade circuit is shown in Figure 13).
The effective output resistance is equal to the weighted sum of each individual device:
Note that the increasing of the number of cascading stages is pracitically limited since it significantly reduces the efficiency, increases the output resistance and output voltage ripple.
It is possible to regulate the output of the LM2665 by use of a low dropout regulator (such as LP2980-5.0). The whole converter is depicted in Figure 14.
A different output voltage is possible by use of LP2980-3.3, LP2980-3.0, or LP2980-ADJ.
Note that the following conditions must be satisfied simultaneously for worst-case design:
Another interesting application shown in Splitting Vin in Half 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 VIN/2, the input voltage can be raised to 11 V.