Switching Waveforms
2-4
Setup and Test Results
2.5
Switching Waveforms
Figure 2−3 shows the switching waveform at the SW pin of the TPS61040.
When the internal FET turns on, the voltage at the SW pin is pulled to ground
until the inductor current reaches 450 mA. When the inductor current reaches
450 mA, the FET turns off and the voltage at the SW pin rises to the output
voltage plus the forward voltage drop of the diode. During this time, the
inductor transfers its stored energy to the load and the output capacitor. When
the inductor current decays to zero, the SW node rings at a frequency
determined by the output inductor and the drain capacitance of the internal
FET. This ringing is indicative of a discontinuous boost power supply topology.
Figure 2−3. SW Waveform
2.6
Input Voltage Ripple
Figure 2−4 shows the input voltage and current ripple of the supply with
Vin = 3.6 V and I
O
= 20 mA. The input capacitance of the TPS61040 and the
impedance of the input voltage supply largely determine the input voltage and
current ripple. Increasing the input capacitance or lowering the source
impedance reduces the input voltage and current ripple.
