Intro to Multi-function Pins and their Applications in TI Step-down Converters
Abstract
Today, more and more engineers are asking for smaller components when designing their systems. This application report explains the Multi-function pin present in some of TI step-down converters (VSET/VID for TPS62864/6/8/9, VSET/MODE for TPS62865/7 and TPSM82864/6A and VSEL/MODE for TPS62800/1/2/6/7/8). Several applications can benefit from the Multi-function pins; they enable the engineers to introduce additional features into their design while assuring a minimal solution size.
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1 Introduction
Today, more and more engineers are asking for smaller components when designing their systems.
Smaller parts lead to a reduced board size, with the benefit of space critical applications (wearables, personal electronics, and so on) and reducing the cost.
Also, more devices can be incorporated on the same boards, leading to an increased complexity and additional capabilities per board area.
In the past, power managements DC-DC Buck ICs had an independent pin for every function, as power good, output voltage setting, mode of operation, and so on.
This limitation led to an intrinsic tradeoff; for space critical applications, the designer had to choose the simplest component, without any additional features, to assure the smallest package possible. For more complex designs, where more features are required, the only choice were bulky components with a big package and an elevated number of pins.
To overcome this tradeoff, Multi-function pins were introduced. A Multi-function pin is simply a single pin where more than one features are integrated.
This application note considers the TPS6280x, TPS6286x and TPSM8286xA family, that are capable of offering many features in a small size package.
With this intent, an input pin is multiplexed to provide two different functions (VSET/VID for TPS62864/6/8/9, VSET/MODE for TPS62865/7 and TPSM82864/6A and VSEL/MODE for TPS62800/1/2/6/7/8), usually separated in time domain as shown in Figure 1-1.
Figure 1-1 TPS62864/6 Typical Application
SchematicsAt the beginning, directly after enabling the startup (t_startup_delay), the ICs use the multiplexed pin for resistance measurement (R2D conversion, see Benefits of a Resistor-to-Digital Converter in Ultra-Low Power Supplies for reference), where the result allows to correctly set the output voltage value. During operation, the pin acts as a digital input as shown in Figure 1-2, to correctly configure the corresponding setting.
Figure 1-2 VSET/MODE Pin Time MultiplexingThis application note describes the Multi-function pin behavior and it proposes some driving circuitry to effectively multiplex between the different features while avoiding measurement errors.
2 Standard Device Operation: Resistance Measurement and Digital Input
During t_startup_delay, the IC needs to perform a resistance measurement on the Multi-function pin.
The resistance measurement is done by injecting a small current IMEAS in the external resistor RVSET, and reading the correspondent voltage VMEAS as shown in Figure 2-1 The value of RVSET can be computed as: RVSET = VMEAS / IMEAS;
Figure 2-1 R2D Conversion on VSET PinThis operation has to be as precise as possible, since any measurement errors could cause an erroneous output voltage setting, with possible damages to the load.
For example, referring to TPS62864/6 2.4-V to 5.5-V Input, 4-A and 6-A Synchronous Step-Down Converter with I2C Interface in WCSP Package data sheet: The R2D converter has an internal current source which applies current through the external resistor, and an internal ADC which reads back the resulting voltage level. Depending on the level, the correct start-up output voltage and I2C slave address are set.... Ensure that there is no additional current path or capacitance greater than 30 pF from this pin to GND during R2D conversion. Otherwise a false value is set.
The previous limits must to be considered when designing the driving circuit.
The capacitance limit is expressly given in the data sheet, whereas the maximum additional current is not specifically defined. In most cases it is okay to consider 40nA as maximum additional current.
