2 Proposed Circuit Principle

Figure 2-1 shows the theoretical circuit of the TPS61022 boost converter circuit in a supercap backup power system. The V_sys is the brief power, coming from other DC/DC converter or the grid. The TPS61022 has a MODE pin to set the operation mode. When the mode is logic high, the device operates at forced pulse width modulation (PWM) mode. At forced PWM mode, the device keeps switching with 1 MHz (V_IN > 1.5 V) and 600 kHz (V_IN < 1.0 V) disregarding the loading. This results in very low efficiency at light load conditions. There may be a reverse current from V_OUT to charge the supercap by setting the MODE pin to logic high if the voltage of V_sys is higher than the TPS61022 setting output voltage. When the MODE is connected to a logic low voltage, the device works in auto PFM mode and there is no reverse current from V_OUT to V_IN by using the following configurations:

• It is recommended to connect the MODE pin to GND to set the device working at auto PFM mode. Efficiency at light load is increased and total supercap discharging time is extended.
• Set the TPS61022 V_OUT 3 to 5% less than V_sys. The device does not switch when V_sys exists.
• The EN pin is connected to V_IN to make the device stay in theenable state. The TPS61022 device consumes very little quiescent current (3.0 µA into V_IN pin and 32 µA into the V_OUT pin).

When the brief power V_sys browns out and drops less than the TPS61022 programmed output voltage V_OUT, the TPS61022 starts switching and maintains V_OUT at the programmed output voltage level.