Buck, Boost and LDO Regulators Combined in a 4mm × 4mm QFN

Introduction

The LT3570 simplifies complex multi-rail power supply designs by integrating three DC/DC regulators into a single package: a current mode buck regulator, a current mode boost regulator, and an LDO controller.

The buck and boost regulators each have a current limit of 1.5A. The LDO controller has an output current capability of 10mA and combines with an external NPN transistor to create a linear regulator. The frequency of the switching regulators can be set from 500kHz to 3MHz by an external resistor or synchronized to an external oscillator. The independent input voltages for each regulator offers a wide operating range from 2.5V up to 40V. Each regulator also has its own shutdown circuitry and the buck and boost regulators have their own soft-start circuitry.

The typical application shown in Figure 1 generates 3.3V at 1A from the buck regulator, 2.5V at 40mA from the LDO controller and 12V at 275mA from the boost regulator, all from a 5V input supply voltage and with an overall efficiency around 85%.

Figure 1. A typical 5V input to 3.3V, 2.5V and 12V application.

Features

Available in either a 24-lead 4mm × 4mm QFN or a 20-pin TSSOP package, the LT3570 is a constant frequency current mode regulator. If all SHDN pins are held low, zero quiescent current is drawn from the input supplies and the part is turned off. Any SHDN pin voltage exceeding 1.5V will turn on the corresponding regulator. A precise shutdown pin threshold allows for easy integration of input supply undervoltage lockout. All three regulators share the same internal 800mV reference voltage. For each regulator, an external resistor divider programs the output voltage to any value above the part’s reference voltage. The switching frequency is set with an external resistor from the RT pin to GND. This allows a trade off between minimizing component size (by using higher switching frequencies) and maximizing efficiency (by using lower switching frequencies). Additionally, running at a low switching frequency allows for applications that require larger VIN-to-VOUT ratios. The adjustable and synchronizable switching frequency also allows the user to keep the switching noise out of critical wireless and audio bands.

Both the buck and boost regulators control the slew rate of the output voltage during start-up. A controlled ramp reduces inrush current on the input supply and minimizes output overshoot. An external capacitor connected between the SS pin and ground programs the slew rate. The voltage on the SS pin overrides the internal reference voltage to the error amplifier and is charged by a 4.5μA internal current source.

The BIAS pin allows the internal circuitry to draw current from a lower voltage supply than the input, reducing power dissipation and increasing efficiency. Normally, the quiescent current is supplied from VIN2, but when the voltage on the BIAS pin exceeds 2.5V the current is supplied from the BIAS pin. The BIAS pin is only available on the 24-lead QFN package.

Applications

“Dying Gasp” Application

The LT3570 provides an ideal solution for any “dying gasp” system. Figure 2 shows a typical application powering an airbag controller. In an automobile accident, the battery may get disconnected from the shock sensors yet the airbag must still fire. In this application, the battery supplies power to the boost regulator. VOUT1 is set to 36V and drives VIN2 and VIN3, the inputs to the buck regulator and the LDO controller, respectively. Even after the input supply is removed, the buck regulator and the LDO continue to function properly for more than 3ms, as the energy continues to be supplied from the output capacitor of the boost regulator. The buck regulator turns off when VIN2 approaches the input undervoltage lockout of 2.3V (see Figure 3).

Figure 2. Dying gasp system keeps power even when battery is disconnected.

Figure 3. Output waveforms when power is removed from the circuit in Figure 2.

DSL Modem

Figure 4 shows an application for a DSL modem or set-top box. The supply voltage for VIN2 comes from a wall adapter that can range from 8V to 30V. This voltage is stepped down to 5V at 100mA for VOUT2, which then supplies the power to drive both the boost regulator and LDO controller. VOUT1 is set to 8V at 200mA and VOUT3 is set to 3.3V at 500mA. Figure 5 shows the load step response of VOUT1 and VOUT2 with a 200mA load step on VOUT1.

Figure 4. DSL modem application.

Figure 5. Step response of Figure 4 with boost current stepped from 200mA to 400mA.

Conclusion

The LT3570 is a monolithic dual output switching regulator (buck and boost) with a NPN LDO controller and is ideal for a broad variety of applications. Because the LT3570 offers a high level of system integration, it greatly simplifies board design for complex applications that need multiple voltage supply rails. With the flexibility of independent supply inputs and adjustable frequency, the user can set a wide array of custom output voltages. The LT3570 is a feature rich solution that satisfies the needs for multiple output voltages in a compact solution.

作者

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Chris Falvey