Operating TPS6287X-Q1 Devices in a Stacked Configuration

1 Abstract

Non isolated inductive step-down converters designed with integrated solutions similar to TPS6287x are using only a few passive components to complete the circuit. Usually this is already a small design. In some cases, the passive components are still too large or just too high to fit in the application environment or the current rating of suitable components is not sufficient. To address this, multiple converters can be operated in parallel to supply the same load. Devices of the TPS6287x family support this type of configuration. Those devices have functions integrated which facilitate operating the devices in a stack in parallel. This application note describes in detail how such a stacked configuration compares to a single converter solution.

Trademarks

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1 Introduction

Although it sounds simple to just use converters in parallel, there are certain important details to take care of. To avoid beat frequencies causing unpredictable input and output voltage ripple and noise, the switching frequency of the converters in a stack must be synchronized. Adding a phase shift to the synchronization is also beneficial to ensure that the converters are not switching at the same time. This process minimizes the peak input current of the converter design. The process is also beneficial if the load current always is equally shared among the converters in a stack to use each converter and its directly attached passive components to their maximum capability. This way, the stack can be designed to be most area and cost efficient. The TPS6287x devices have features implemented to properly support such configurations in a way which is easy to design with. More details can be found in the TPS6287x Data Sheet. In this application note a single stage converter design is compared to a dual stage converter design supplying the same load. Both designs are based on converters of the TPS6287x family with different output current ratings.

2 Configurations

For the comparison in this application note, the TPS62873EVM-143 is used for the measurements at the single stage converter design and the TPS62873EVM-144 is used for the measurements at the dual converter design. Both circuits are configured to supply a 12-A load at 0.75 V. The circuits are also configured to support a fast load transient from 4.5 A to 12 A in 1 μs, allowing an output voltage deviation of less than 3.3%.

The TPS62873EVM-143 is already configured for this specification, so no modifications are needed for the measurements. The device was used with its default configuration. Figure 2-1 shows an overview of the critical components of the schematic and details of the BOM are listed in Table 2-1.

Figure 2-1 Single Converter Schematic

Table 2-1 Single Converter BOM

Ref	Qty	Value	Description	Size	Part Number
CIN	2	0.47 µF	Ceramic Capacitor, 10V, X7S	0402	GCM155C71A474KE36D
	2	2.2 µF	Ceramic Capacitor, 10V, X7S	0603	CGA3E3X7S1A225K080AB
	2	10 µF	Ceramic Capacitor, 10V, X7R	0805	GCM21BR71A106KE22
U1	1		2.7-V to 6-V Input, 12A, Fast Transient Synchronous Step-Down Converter with I2C Interface, Remote Sense and Stackability	WQFN-FCRLF16	TPS62872QWRXSRQ1
L1	1	110 nH	Inductor	4.0 × 4.0 × 2.0 mm	XGL4020-111MEC
C1	1	1800 pF	Ceramic Capacitor, 50V, X7R	0402
R1	1	2.43 kΩ	Resistor 1%, 0.1 W	0402
C2	1	10pF	Ceramic Capacitor, 50V, COG/NPO	0402
COUT	2	0.47 µF	Ceramic Capacitor, 10V, X7S	0402	GCM155C71A474KE36D
COUT	2	22 µF	Ceramic Capacitor, 10V, X7R	1206	GCM31CR71A226KE02
CLOAD	2	47 µF	Ceramic Capacitor, 6.3V, X7R	1210	GCM32ER70J476ME19L
	2	100 µF	Ceramic Capacitor, 6.3V, X5R	1210	GRT32ER60J107ME13L
	2	0.47 µF	Ceramic Capacitor, 10V, X7S	0402	GCM155C71A474KE36D

The TPS62873EVM-144 by default is configured for an output current of 30 A. For the measurements for this application note the TPS62873 devices are changed to the lower current rated TPS62870 devices of the same device family. Since the current rating of the inductor can be lower as well a smaller inductor, the VCTA32252E-R11 was used to replace the larger XGL4020-110. With this modification the required PCB area for the two inductors in the stack is about the same as the required area for the single inductor in the single converter design. The amount of capacitors in the load side capacitor bank is also reduced from the default assembly which was designed to support higher output currents to the same configuration as it is used on the TPS62873EVM-143. Figure 2-2 shows an overview of the critical components of the dual converter schematic. Details of the BOM are listed in Table 2-2.

Figure 2-2 Dual Converter Schematic

Table 2-2 Dual Converter BOM

Ref	Qty	Value	Description	Size	Part Number
CIN1, CIN2	2 × 2	0.47 µF	Ceramic Capacitor, 10V, X7S	0402	GCM155C71A474KE36D
	2 × 2	2.2 µF	Ceramic Capacitor, 10V, X7S	0603	CGA3E3X7S1A225K080AB
	2 × 2	10 µF	Ceramic Capacitor, 10V, X7R	0805	GCM21BR71A106KE22
U1,U2	2 × 1		2.7-V to 6-V Input, 6 A, Fast Transient Synchronous Step-Down Converter with I2C Interface, Remote Sense and Stackability	WQFN-FCRLF16	TPS62870QWRXSRQ1
L1, L2	2 × 1	110 nH	Inductor	3.2 × 3.2 × 2.5 mm	VCTA32252E-R11
C1	1	6800 pF	Ceramic Capacitor, 50V, X7R	0402
R1	1	820 Ω	Resistor 1%, 0.1 W	0402
C2, C3	2	10 pF	Ceramic Capacitor, 50 V, COG/NPO	0402
COUT1, COUT2	2 × 2	0.47 µF	Ceramic Capacitor, 10 V, X7S	0402	GCM155C71A474KE36D
COUT1, COUT2	2 × 2	22 µF	Ceramic Capacitor, 10 V, X7R	1206	GCM31CR71A226KE02
CLOAD	2	47 µF	Ceramic Capacitor, 6.3V, X7R	1210	GCM32ER70J476ME19L
	2	100 µF	Ceramic Capacitor, 6.3 V, X5R	1210	GRT32ER60J107ME13L
	2	0.47 µF	Ceramic Capacitor, 10 V, X7S	0402	GCM155C71A474KE36D

More details of the test boards can be found in the EVM Users Guides for the TPS62873EVM-143 and TPS62873EVM-144.