Synchronous Boost Converter with Output Disconnect Delivers 4W from Two Cells

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Introduction

Portable, battery-powered devices require power supplies that are efficient and small. The LTC3421 synchronous boost converter offers both. It features a low, 12μA quiescent current in Burst Mode operation, greatly improving battery life in applications that spend much of their time in low power mode. The LTC3421 itself is small, available in a small 4mm × 4mm QFN package, and its oscillator frequency can be programmed or synchronized up to 3MHz, which minimizes the size of external components. It can drive power hungry circuits with its 3A guaranteed switch current—up to 4W output power from two NiCd or NiMH cells.

In a conventional synchronous boost converter, the internal body diode of the synchronous rectifier connects the input supply through the inductor to the load. The peak inrush current when the input supply is first applied to the boost converter is only limited by the resistance in the loop consisting of the input source, inductor, diode, and output capacitor. The large surge current during initial plugin can cause sufficient input voltage drop to possibly trigger a low-battery detector. The direct path from the input to the output also leaves the load connected to the input even when the boost converter is in shutdown. This can cause additional power loss due to leakage current. With true output disconnect, by eliminating body diode conduction of the internal PMOS rectifier, the LTC3421 eliminates these problems.

2-Cell to 3.3V/1.2A Synchronous Boost Converter

The circuit in Figure 1 shows a 2-cell to 3.3V converter that can provide up to 1.2A of load current. The switching frequency is set at 1MHz by having 28kΩ at R_T pin. This gives a good trade-off between efficiency and circuit size. The footprint of this converter is about 0.35inch², as shown in Figure 2. The LTC3421 has a bottom metal pad to improve thermal performance. The entire metal pad can be soldered directly to the PC board copper area and through multiple thermal vias to internal and backside copper layers to optimize efficiency and thermal performance.

Figure 1. A 1MHz, 2-cell to 3.3V at 1.2A boost converter.

Figure 2. The circuit of Figure 1 fits in a mere 0.35in².

1.5mm Height, 2-Cell to 3.3V/1A Converter

Low-profile is required in many hand-held devices, such as cellular phones, and MP3 players. Figure 3 shows how to make a 2-NiCd, or 2-NiMH cell to 3.3V output converter with 1.5mm maximum height by using a lower profile inductor and output capacitor. This circuit can provide up to 1A load current for 2V minimum input battery voltage and 900mA load if the battery cutoff voltage is 1.8V. Figure 4 shows the efficiency of this circuit. With Burst Mode enabled, the efficiency stays above 85% over three decades of load current.

Figure 3. A 1.5mm height, 1MHz, 2-cell to 3.3V at 1A boost converter.

Figure 4. Efficiency curves for the converter in Figure 3 (V_IN = 2.4V).

Conclusion

With output disconnect, inrush current limiting and 12μA quiescent current, the LTC3421 synchronous boost converter is an ideal fit for many portable applications. Its guaranteed 1V start-up input voltage works with a large variety of battery configurations. It is available in a small 4mm × 4mm QFN package with exposed copper on the backside, making it possible to provide up to 1.2A at 3.3V from 2-cell input without taking much space.