Synchronous Boost Converter with Fault Handling Generates 5V at 500mA in 1cm2 of Board Space

Introduction

Today’s power supply designs must meet a number of stringent and sometimes competing requirements. In many cases the requirement for a small solution is at odds with the need for high conversion efficiency and the need to safely deal with fault conditions. The LTC3529 step-up DC/DC converter is designed to provide a “no compromises” design, offering high efficiency to minimize dissipated heat and maximize battery life while still maintaining a small footprint for size-constrained power applications requiring a 5V supply.

The LTC3529 can detect a shorted output condition, disable the IC, and report the event to a host microprocessor. This feature is important for portable applications where devices communicate with each other directly, or system power applications where voltages on multiple boards must be monitored and maintained. As shown in Figure 1, the LTC3529 offers a compact and efficient solution consisting of only three tiny external components.

Figure 1. A tiny (1cm2) yet complete solution drives USB On-The-Go bus power.

Lithium-Ion to 5V, 2.5W Converter

Figure 2 shows an LTC3529-based solution for converting from a single lithium-ion battery or 3.3V board supply to 5V with up to 500mA of load current. Requiring only an inductor and input/output filter capacitors, the entire converter occupies only about 1cm2 of board space. The IC includes internal compensation, the output divider, and soft-start circuitry to minimize external components. In shutdown, the LTC3529 disconnects the output from the input and draws less than 1µA from the source.

Figure 2. Li-Ion to 5V synchronous boost converter.

In fixed frequency PWM mode, the efficiency for a typical Li-Ion source to 5V peaks at 92%, as shown in Figure 4, and remains above 80% for load currents greater than 30mA. The LTC3529 delivers up to 500mA of current at a 5V output and is therefore suitable for both low and high power USB applications. As with any DC/DC converter, a tradeoff exists between switching frequency, inductor value, output capacitance and output ripple.

To allow the use of tiny external components, the LTC3529 operates at 1.5MHz and is stable with a 4.7µH inductor and output capacitances of 4.7µF (compatible with USB On-The-Go specifications) or greater. The Li-Ion-to-5V converter in Figure 3 utilizes a 10µF output capacitor, and exhibits a peak-to-peak output ripple of only 10mV. Low ESR and ESL ceramic capacitors (such as X5R) are recommended for both VIN and VOUT bypassing.

Figure 3. Efficiency for the circuit in Figure 2.

Fault Detection

The LTC3529 is robust to output short circuits, a problem that arises as the terminals of the IC are exposed to the outside world to facilitate connection between portable devices or system board edge connectors. To defend against output shorts, the LTC3529 shuts down when an excessive current draw is detected through the internal MOSFET switches continuously for 15ms.

Figure 4 illustrates the fault handling protocol of the LTC3529. Based on a pin-selectable setting, the IC can be configured to either periodically attempt to power up (RST pin high, Figure 4a), or remain shut down until power is cycled to the device (RST pin low, Figure 4b). The waveform indicating the fault condition is seen at the Fault pin and is produced by an internal open-drain device whose input is pulled high in the event of a fault. The Fault pin can either be connected to a microprocessor or drive an LED.

(a). RST high: converter attempts power-up every 15ms.

(b). RST low: converter remains shut down until power is cycled.

Figure 4. A fault detection mechanism powers down the converter, providing robustness to output shorts.

Conclusion

High conversion efficiency and the ability to detect and handle output shorts make the LTC3529 an ideal solution for either peer-to-peer portable applications or point-of-load board power with robust fault handling. The 1.5MHz switching frequency and highly integrated design of the LTC3529 yield compact solutions with minimal design effort.

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Eddy Wells