Microchip MCP19122 Handleiding

Microchip Niet gecategoriseerd MCP19122

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 2015 Microchip Technology Inc. DS00001882A-page 1

AN1882

INTRODUCTION

In today's highly competitive and highly technological

world, analog signal measurements and regulation

accuracies are ever increasing. The MCP19114/5

Digitally Enhanced Power Analog Synchronous Low-Side

Pulse-Width Modulation Controller from Microchip

Technology Inc. is an analog controller capable of

implementing several switch-mode power supply

topologies. This analog device has the distinct advantage

of a built-in PIC® core microcontroller. The microcontroller

can be used to improve overall performance without the

penalties of additional cost or device area. This

application note provides several explanations complete

with firmware examples on how to improve ratiometric

and non-ratiometric analog signal measurements.

As a digitally enhanced power analog controller, the

MCP19114/5 has a built-in 10-bit Analog-to-Digital

Converter (ADC). When using the ADC in practice,

several factors will affect the accuracy of the

measurement. These factors include noise, offset

errors, DNL/INL errors and variations in the ADC

reference voltage. These error sources will have some

initial errors specified at room temperature (+25°C).

Also, the error due to temperature variation of these

parameters should not be overlooked. This application

note will provide a means of alleviating these errors.

Utilizing factory calibration is advantageous when

attempting to compensate for measurement errors.

However, reference voltage inaccuracies and

temperature drift will directly affect the ADC

measurement result and cannot be easily corrected.

The unique feature of an integral 8-bit PIC

microcontroller allows MCP19114/5 to implement

software solutions to help resolve these issues. To

address high-accuracy requirements of ADC

measurements and the temperature drift issues, two

methods are discussed in this application note. Both

methods require software coding and hardware

configurations. Examples are provided to better

understand these methods.

ASSUMPTIONS

This application note assumes the user:

• is familiar with MCP19114/5 digitally enhanced

analog controllers;

• has basic knowledge of Analog-to-Digital

Converters (ADC);

• has working knowledge of C programming

language.

NON-RATIOMETRIC MEASUREMENT

CORRECTION

Non-ratiometric measurements are measurements

where the signal being measured is not related to the

ADC reference. The reference voltage of MCP19114/5

ADC is AVDD. Non-ratiometric measurement correction

can be achieved by measuring a signal of known

accuracy and using this measurement to correct for

other signal measurements. This method eliminates

the need to consider variations in the ADC reference

(AVDD). The MCP19114/5 internal signal VBGR (1.23V)

is factory trimmed to within 1% and has an

overtemperature tolerance of ±2.5%. This V

BGR signal

can be read internally at the MCP19114/5 ADC through

configuration of the analog test MUX (refer to registers

ABECON and ADCON0). Non-ratiometric correction is

a mathematical approach to eliminate the ADC

reference (AVDD) tolerance errors. This method is

presented in Figure 1.

FIGURE 1: Non-ratiometric

Measurement Correction.

Author: Yiwei Xiong

Microchip Technology Inc.

MCP19114/5

ADC

VBGR

VSIGNAL

ADC Measurement Correction and Optimization

for MCP19114/5

AN1882

DS00001882A-page 2  2015 Microchip Technology Inc.

First, use the ADC to measure VBGR. The measured

value V

ADC_BGR is calculated in Equation 1.

EQUATION 1:

Then, use the ADC to measure V

SIGNAL.

The measured value V

ADC_SIGNAL can be represented

as shown in Equation 2.

EQUATION 2:

Dividing Equation 1 by Equation 2 will cancel AVDD(V),

as shown in Equation 3.

EQUATION 3:

The accuracy impact of AVDD and the drift over

temperature is nullified in exchange for the more

precise VBGR signal accuracy over temperature. V

BGR

is factory trimmed to 1.23V ±1% (±2.5% over

temperature). VSIGNAL can be calculated as follows

from measured signals VADC_BGR

and VADC_SIGNAL by

rearranging Equation 3, as shown in Equation 4.

EQUATION 4:

If VSIGNAL threshold detection is desired, rearranging

Equation 4 and utilizing the known V

BGR value will pro-

vide a signal value in counts, as shown in Equation 5.

EQUATION 5:

In Equation 4 and Equation 5, the reference voltage

AVDD is replaced by VBGR so the measurement accuracy

will incur the tolerance of VBGR.

RATIOMETRIC MEASUREMENT

CORRECTION

Another means to reduce measurement error is to

implement ratiometric measurements. Typically, a

ratiometric ADC measurement is accomplished by

utilizing the same voltage source to excite the target

circuit as is used for the ADC reference. If a signal level

is beyond the ADC maximum measurement range, a

resistor divider is applied to lower the input signal with

a K-factor. The measured signal is proportional to the

reference voltage, thus, the accuracy of the

measurement depends on the sensor resistors instead

of the reference voltage. In the case of the

MCP19114/5, the ADC reference is powered from

AVDD (4V) and not the VDD (5V) regulator. The VDD

(5V) source is available externally to power-up target

circuits. To assist users desiring to make ratiometric

ADC measurements with the MCP19114/5, a factory-

stored ADC measurement value of VDD is available.

EQUATION 6:

FIGURE 2: Ratiometric Measurement

Correction.

VADC_BGR(counts)

VBGR V 

AVDD V 

------------------------- 1024



VADC_SIGNAL(counts)

VSIGNAL V 

AVDD V 

---------------------------------- 1024



VADC_BGR counts 

VADC_SIGNAL counts 

--------------------------------------------------------------

VBGR V 

VSIGNAL V 

----------------------------------=

VSIGNAL V  VBGR V 

VADC_SIGNAL counts 

VADC_BGR counts 

--------------------------------------------------------------=

VADC_SIGNAL counts  VADC_BGR counts 

VSIGNAL V 

BGR V 

-----------------------------------=

KR2

R1 R2+ 

-------------------------=

K =

VDD

VX = (VDD × K ) MCP19114/5

ADC

R1 + R2

 2015 Microchip Technology Inc. DS00001882A-page 3

AN1882

Calibration Word 11 stores the internal ADC reading (in

counts) of VDD/2 or half VDD (HFVDD).This is a factory-

stored value at +25°C. This value can be used to cali-

brate ratiometric ADC measurements powered from

VDD. VDD and AVDD temperature drift track to within

1.1% over temperature.

EQUATION 7:

At x°C, the ADC measurement of signal Vx°C is shown

in Equation 8.

EQUATION 8:

VDD and AVDD voltage drift will track respectively

together over temperature. Assume at temperature

x°C, VDD and AVDD drift tracks to y%.

EQUATION 9:

Divide Equation 7 by Equation 9:

EQUATION 10:

EQUATION 11:

Once KX°C is known, the R2 value can be calculated

from Equation 6. The reference voltage AV

DD is elimi-

nated from the calculation. Since the tracking tolerance

is neglected during calculation, a tracking error will be

expected in the result. See “V

DD vs. AVDD Temperature

Drift Tracking” figure in Section 2.0 Typical Perfor-

mance Curves of the MCP19114/5 Data Sheet

(DS20005281).

VOLTAGE THRESHOLD LEVEL

CHECKING USING

NON-RATIOMETRIC MEASUREMENT

Systems requiring detection of several operation points

or thresholds can implement a comparison table. Using

Equation 5, the VADC_SIGNAL (counts) can be com-

pared to the predetermined table values to perform

system operation decisions. The example shown in

Figure 3 describes the technique.

FIGURE 3: Voltage Threshold Level

Checking Using Non-ratiometric Measurement.

In Figure 3, assume a system wants to detect three

operating thresholds of input voltage V

IN:

• VIN op1 = 8V (low)

• VIN op2 = 12V (normal)

• VIN op3 = 18V (high)

Calculate the divider K based on the hardware config-

uration. In Example 1, the divider K is set to 0.14977.

EXAMPLE 1:

Ratio Reference (R) can be defined as shown in

Equation 12

EQUATION 12:

The relationship between VSIGNAL and VIN is shown in

Equation 13.

EQUATION 13:

HFVDD counts 

VDD25



---



AVDD25



----------------------------------- 1024





Ccounts 

VDDx



CKx





AVDDx



------------------------------------------- 1024





Ccounts 

VDD25



C1 y%+ 







AVDD25



C1 y%+ 



--------------------------------------------------------------------------- 1024



VDD25



CKx





AVDD25



---------------------------------------------- 1024



HFVDD counts 



Ccounts 

--------------------------------------------

---



--------------=



Ccounts 

HFVDD counts 

-------------------------------------------- 1

---



K =

VIN

(VIN × K ) MCP19114/5

ADC

R1 + R2

KR2

R1 R2+ 

------------------------- 0.14977==

VSIGNAL V 

VBGR

----------------------------------=

VSIGNAL V  VIN V  K



Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	MCP19122

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