Customized Output with Comparator-Controlled Charge Pump
Integrating a comparator and a transistor into the oscillator of a charge pump circuit empowers the pump to produce a precisely regulated output of virtually any desired value. Charge pump ICs have the capability to either invert or double an input voltage, transforming, for instance, 3 V to –3 V or 3 V to 6 V. However, it’s crucial to note that the charge pump alone does not regulate the output voltage. Typically, a charge pump operating at 3 V does not possess the inherent capacity to generate intermediate output voltage levels like 5 V.
Creating Custom Output Levels with Comparator Control
Integrating a comparator and a reference device allows the creation and regulation of specific output levels such as 5 V. The charge pump IC1 (MAX660) incorporates an internal oscillator operating at 45 kHz, transferring charge from C1 to C2, elevating the regulated output. When the feedback voltage (pin 3 of IC2) surpasses 1.18 V, the output of comparator IC2 (MAX921) goes high, deactivating the oscillator via T1.
Control Loop and Output Ripple Management
In this setup, no hysteresis is necessary in the control loop; hence, the comparator hysteresis on IC2 is set to zero. The oscillator generates two cycles when enabled, nudging VOUT slightly above the desired level. Subsequently, feedback deactivates the oscillator. The resultant output ripple predominantly depends on the input voltage and the output load current. To reduce output ripple, a small resistor (e.g., 1 Ω) can be added in series with C1, albeit at the cost of circuit efficiency. Ripple also varies based on C1’s value and equivalent series resistance (ESR) — smaller C1 values transfer less charge to C2, resulting in smaller VOUT fluctuations.
Versatile and Efficient Charge-Pump Voltage Converter
The MAX660 CMOS charge-pump voltage converter operates as an unregulated switched-capacitor inverter or doubler. Working within a broad 1.5 V to 5.5 V supply range. MAX660 utilizes two economical capacitors to deliver 100 mA of output current without the bulk, expense, and electromagnetic interference associated with inductor-based converters. Consuming a mere 120 µA of operating current and boasting an efficiency exceeding 90% at most loads. MAX660 is ideal for battery-powered systems. Additionally, these devices can be operated in parallel to lower output impedance, enabling the provision of more current at a specific voltage level.