Expand Buck Converter Minimum Input Voltage with External VCC Bias

A typical step down (buck) converter has an internal linear low-drop-out (LDO) power supply for internal logic control and circuit drive capability. Integrating the output pin and capacitor of the LDO can help save IC pin count as well as reduce solution size; however, taking the approach of utilizing an external bias cap can provide stronger capability to drive the circuit. Additionally, using an external VCC bias allows the use of a device in lower input voltage applications while also increasing efficiency.

TPS56C215 is a monolithic 12-A synchronous buck converter with an adaptive on-time D-CAP3™ control mode. Using TPS56C215 as an example, this application note introduces a method of using an external VCC bias to support lower input voltage (VIN) applications. First, this report will describe the features of the TPS56C215 device before an example of a low input voltage application is introduced. Then, a detailed schematic with the external VCC bias configuration is presented, followed by the confirmation of this theory via bench testing and an efficiency comparison.

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1 TPS56C215 Introduction

TPS56C215 is a monolithic 12-A synchronous buck converter with an adaptive on-time D-CAP3™ control mode. The device integrates low RDS(on) power MOSFETs that enable high efficiency and offer ease-of-use with minimum external component count, handy for space-conscious power systems.

The TPS56C215 has a 4.7 V internal LDO that generates a bias for all internal circuitry. There is a feature to overdrive this internal LDO with an external voltage on the VREG5 pin which improves the converter’s efficiency. Figure 1 is the Typical Application Diagram of TPS56C215.

Figure 1. TPS56C215 Typical Application Diagram

2 12 V / 5 V O-ring supply for DDR application

In workstation DDR applications, customers can use 12 V to convert 2.5 V for Vpp supply. When 12 V fails, customers can have another 5 V rail as backup source to supply the Vpp rail. So customers typically want to use their ORing topology summing together 5 V and 12 V inputs to Vin through diodes. Considering the margin, usually a 4 V–13.2 V input range voltage buck converter is employed here. A simplified schematic is shown in Figure 2

Figure 2. Simplified Schematic of DDR Vpp Supply

In the current market, devices with 17 V max-VIN ratings typically have a minimum input voltage in the range of 4.5 V. If the VIN_min goes lower, the internal logic circuit power supply would be difficult to realize or the cost of device will increase significantly, especially for a high-current device. For this DDR application, there is already 5 V rail in the system. When connecting this 5 V to VREG5 for internal circuit supply, the buck converter could support input voltages lower than 4.5 V, meaning the customer could use the current solution to realize 5 V / 12 V ORing topology. The configuration schematic based on TPS56C215 is shown in Figure 3.

Figure 3. Configuration Schematic Based on TPS56C215