September 12, 1997

Li-ion boost circuit uses no inductors

John Parry and Bill McIntyre, National Semiconductor, Grass Valley, CA

To maintain a regulated 3.3V supply for portable applications, most Li-ion batteries require a low-dropout (LDO) regulator and a dc/dc boost circuit. This need arises because the typical voltage range of a Li-ion battery is 4.2V (charged) to 2.5V (before recharge). The regulator alone suffices in the battery range of 4.2 to 3.5V, but below that level, the battery voltage needs boosting to keep the LDO IC in regulation. The circuit in Figure 1 senses the battery voltage via the R₁-R₂ resistor divider at comparator IC₂. The comparator turns off the converter when the battery voltage exceeds 3.6V.

When the converter is off, p-channel MOSFET Q₁ is on, providing a low-impedance path from the battery to IC₃, the 3.3V LDO regulator. When the battery voltage drops below 3.6V, the comparator turns off Q₁ and enables IC₁, a switched-capacitor converter that can supply 500 mA with no significant output-voltage drop. The 3.6V threshold (300 mV above the regulator's output voltage) allows for the output impedance of the converter and the dropout voltage of the regulator. This extra voltage ensures that the output voltage does not drop more than 5% from the target voltage at 50 mA.

Instead of using a common voltage doubler, the circuit uses a 3/2 voltage converter, IC₁, for improved efficiency. This 900-kHz charge pump switches capacitors C₁ and C₂ in two phases to create a voltage of 3/2×V_BATT at output V_HIGH. D₁ is a Schottky diode that charges hold capacitor C₃ during start-up. In normal operation, this diode conducts no significant current. (DI #2081)

| EDN Access | Feedback | Table of Contents |