Here’s a design guidelines for ATtiny43U from Atmel, app note here (PDF!):
The integrated boost converter of ATtiny43U provides the microcontroller and peripherals with a fixed supply voltage, generated from an external supply of lower voltage. The boost converter is a switching type, step-up regulator that requires
some external components to be complete. This includes an external inductor, a diode and some bypass capacitors. The inductor is connected between the VBAT node and the LSW pin, and the Schottky diode between pins LSW and VCC. In addition, an input capacitor and external bypass capacitor from VCC to GND areusually required.