500mA Output Current Low Noise Dual Mode Charge Pump

Introduction

Charge pump (inductorless) DC/DC converters are popular in space-constrained applications with low to moderate load current (10mA–500mA) requirements. The devices in the LTC3203 family are low noise, high efficiency regulating charge pumps that can supply up to 500mA of output current from a single 2.7V to 5.5V supply. The LTC3203-1 and LTC3203B-1 produce a selectable fixed 4.5V or 5V output. The LTC3203B produces an adjustable output voltage. The LTC3203-1 features automatic Burst Mode operation at light load to achieve low supply current whereas the LTC3203B and LTC3203B-1 operate at constant frequency to minimize both input and output noise. High switching frequency (1MHz) makes it possible to use only four tiny low cost ceramic capacitors and two resistors for operation. The device also has two user selectable conversion modes for optimizing the efficiency of the charge pump. Additional features include low shutdown current (<1µA), soft-start at power-on and short circuit protection. The LTC3203 family is available in a 10-lead thermally enhanced DFN package, making it possible to build a complete converter in less than 0.04in2. A typical application circuit is shown in Figure 1.

Figure 1. Typical application.

Low Noise Operation

The constant frequency architecture achieves regulation by sensing the output voltage and regulating the amount of charge transferred per cycle. This method of regulation provides much lower input and output voltage ripple than that of burst mode regulated switched capacitor charge pumps. The LTC3203B and LTC3203B-1 make filtering input and output noise less demanding than burst mode switched capacitor charge pumps where switching frequencies depend on load current and can range over several orders of magnitude. The charge pump operates on two phases, where a break-before-make circuit prevents switch cross-conduction. The higher frequency noise due to the non-overlap “notches” is easily filtered by a small input capacitor and PCB parasitic inductance. Figures 2 and 3 shows the low input and output ripple with a 300mA load. The device is powered from a 3.6V input and produces a regulated 4.5V output. The input voltage source has 0.1Ω impedance.

Figure 2. Input and output noise in 1.5× mode.

Figure 3. Input and output noise in 2× mode.

Dual Mode Conversion

The LTC3203 family offers both 1.5× and 2× boost modes—selected by the mode pin. In the 2× mode, the chip works as a dual-phase regulated voltage doubler. The flying capacitors are charged on alternate clock phases from VIN. While one capacitor is being charged from VIN, the other is stacked on top of VIN and connected to the output. The two flying capacitors operate out of phase to minimize both input and output ripple. Alternatively, in 1.5× mode, it uses a split-capacitor technique rather than doubling. The flying capacitors are charged in series during the first clock phase, and stacked in parallel on top of VIN on the second clock phase. With this technique, the input current is reduced from more than twice the load current to just over 1.5 times the load current, resulting in approximately 25% less input current than what would be required for operating in 2× mode charge pump to drive the same load. Therefore, the efficiency at higher VIN is increased to approximately 90% with VIN at somewhere between 3V and 4V. Figure 4 shows the conversion efficiency at 300mA load current for 4.5V VOUT and 5V VOUT, respectively.

Figure 4. Efficiency vs VIN at 300mA load current.

The conversion mode should be chosen based on considerations of efficiency, available output current and VOUT ripple. With a given VIN, the 1.5× mode gives a higher efficiency at lower available output current. The 2× mode gives a higher available output current at lower efficiency. Moreover, the output voltage ripple in the 2× mode is lower due to the out-of-phase operation of the two flying capacitors. Typically, at low VIN, the 2× mode should be selected, and at higher VIN, the 1.5× mode should be selected.

The MODE pin has a precision comparator. By connecting a resistive divider from VIN to the MODE input pin, the user can accurately program the VIN threshold at which the charge pump will switch from 1.5× mode to 2× mode as VIN falls and vice versa. The 10% hysteresis on the MODE pin prevents the chip from hunting between the two modes.

Conclusion

With low operating current, low external parts count and robust protection features, the LTC3203 family is well suited for low power step-up/step-down DC/DC conversion. The shutdown, dual mode conversion, selectable output voltage and low noise operation features provide additional value and functionality. The simple and versatile LTC3203 family is ideal for moderate power DC/DC conversion applications.

Author

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Yang Wen