Constant Voltage Schematic Circuit Diagram
When batteries are used to power a circuit, there is always the problem that the battery voltage drops during operation. A National Semiconductor type LM3354 IC (www.national.com/ 0ds/LM/LM3354.pdf) can provide a solution to this problem. It contains a dc/dc converter that operates as a buck converter in the step-down mode when the input voltage is too high, and which can also handle input voltages that are too low by operating as a boost converter in step-up mode. All of this works without any coils using switched-capacitor technology.
Several capacitors are charged to the input voltage and then connected by an internal matrix switch either in series (step-up) or parallel (step-down). The output voltage is regulated by varying the duration of the switch-on phase. The LM3354 is available in several versions for various voltages. The IC is clocked at 1 MHz and works with input voltages between +2.5 V and +5.5 V. At maximum output current, the minimum input voltage is 2.9 V (3.4 V for the 5-V version). A Low signal level applied to the Shutdown input can be used to disable the converter. The efficiency of the IC ranges from 75% to 85%. Thermal overload protection prevents damage to the IC from overloads.
The LM3354 is a CMOS-switched capacitor DC/DC converter that produces a regulated output voltage by automatically stepping up (boost) or stepping down (buck) the input voltage. It accepts an input voltage between 2.5V and 5.5V. The LM3354 is available with standard output voltages of 1.8V, 3.3V, 4.1V (ideal for white LED applications), and 5.0V. If other output voltage options between 1.8V and 5.0V are desired, please contact your National Semiconductor representative.
- Regulated VOUT with ±3% (5.0V, 4.1V, and 3.3V options) or ±4% (1.8V option) accuracy
Standard output voltages of 1.8V, 3.3V, 4.1V, and 5.0V
- Custom output voltages available from 1.8V to 5.0V in 100 mV increments with volume order
- 2.5V to 5.5V input voltage range
- Up to 90mA (5.0V, 4.1V, and 1.8V options) or 70mA (3.3V option) output current
- >75% average efficiency
- Uses a few, low-cost external components
- Very small solution size
- 375 µA typical operating current
- 2.3 µA typical shutdown current
- 1 MHz typical switching frequency
- Architecture and control methods provide high load current and good efficiency
- MSOP-10 package
- Over-temperature protection