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78M6612 Hardware Design Guidelines

AN_6612_007

Rev 2

Revision History

REVISION

NUMBER

REVISION

DATE

DESCRIPTION

1.0

3/23/2010

First publication.

1.1

8/30/2010

Added the

Timing Reference

section; fixed VB connection in Figure 1.

1.2

10/29/2010

Added “The V3P3 connection to AC-Neutral can be eliminated when using
current transformers (CT) as the current sensing elements. Refer to the
section on Isolated Connections for designing with CTs.” to

Section 2

Section 2.2

, added a note about the values used for the anti-aliasing

filters.
Added

Section 3.3, Voltage Transformers

11/2/2011

Corrected

Figure 7

Содержание 78M6612

Страница 1: ...tage Resistor Divider Selection o Shunt Selection and Connections Isolated Configuration o Current Transformers o Other Connections o Voltage Transformers Calibration Considerations Basic Configuratio...

Страница 2: ...to the 78M6612 will be subject to this 3 3 V ground reference disparity External test equipment must be floated from earth ground to avoid equipment damage due to this ground reference disparity An ad...

Страница 3: ...e voltage IA input is used to measure the load current VB input is used to flag a Line Neutral polarity reversal If this feature is not desired terminate A2 to V3P3 through a resistor capacitor filter...

Страница 4: ...o the V3P3A pin of the 78M6612 An additional 22 F bulk capacitor is placed in the vicinity of the V3P3SYS pin to provide decoupling for the external DIO circuitry Connect the VBAT pin to the V3P3SYS p...

Страница 5: ...re coefficient TCR in ppm C Figure 3 Input Voltage Divider and Filtering Initial component tolerance can easily be compensated for during calibration Depending on the system accuracy requirements the...

Страница 6: ...s the ADC input range This shunt value produces a dissipated power of 1 85 W at maximum load current In order to ensure more ADC signal margin due to transients and to lower the power dissipation in t...

Страница 7: ...e target load current might be restrictively too large and expensive for its purpose Figure 5 Current Transformer CT Basic Connections Usually current transformers have turns ratios ranging from 10 1...

Страница 8: ...er components with top and bottom printed circuit board layer V3P3 plane surfaces Insert multiple V3P3 vias to interconnect the top and bottom V3P3 structures for a low impedance shield Refer to the 7...

Страница 9: ...ary output voltage range in conjunction with the manufacturer s recommended burden resistor value must meet the 78M6612 s signal input range of 176 78 mVrms 250 mVpk Higher output VT secondary voltage...

Страница 10: ...stors for the voltage sensor The Full Calibration compensates for both the voltage divider plus the current sensor tolerances If the system does not require a high level of accuracy relative to the in...

Страница 11: ...the reset signal If an external reset is not required connect the Reset pin to GND GNDD An external reset is recommended only during the development phase of a project It is recommended that the RESET...

Страница 12: ...comparator output is low 0 signaling battery mode operation via an external battery attached to the VBAT pin Connect the voltage divider shown in Figure 9 to the V1 pin to enable normal WDT enabled 7...

Страница 13: ...If the external ICE cables exceed 12 inches insert the series resistors to control signal reflections Connect the ICE_EN pin to GND on production boards using pre programmed 78M6612 devices Otherwise...

Страница 14: ...gital Output 5 7 Connecting I2 C EEPROMs Connect I 2 C EEPROMs or other I 2 C compatible devices to DIO pins DIO4 and DIO5 as shown in Figure 12 Add pull up resistors of roughly 10 k to V3P3 for both...

Страница 15: ...3 Wire EEPROM Connection 5 9 UART0 TX RX Attach a 10 k pull down resistor to the RX input pin Additionally include a 100 pF ceramic capacitor for EMI protection as shown in Figure 14 Figure 15 Connec...

Страница 16: ...f V3P3 power in non isolated configurations 5 11 1 Capacitive Figure 16 Simple Circuit Design for Low Power Applications 5 11 2 Transformer Figure 17 No High Voltage Components for Higher Power Applic...

Страница 17: ...llators and to achieve high reliability it is important to pay attention to the components and their values and the layout This section shows how these elements affect such factors as stability temper...

Страница 18: ...ong connections from these capacitors that make a large loop on the PCB which behaves like an antenna and can collect surrounding high frequency radiation 6 2 3 Layout The reference ground of the osci...

Страница 19: ...ard ring surrounding the crystal oscillator One way to accomplish this is by surrounding the circuit with a wide grounded trace For this to work the grounded trace must have zero current flowing throu...

Страница 20: ...3 4 Oscillator Start up Time In general the startup time for a low frequency crystal oscillator is longer than for at high frequency crystal For a 32 768 kHz crystal startup should be within the range...

Страница 21: ...operate The most critical corner is at the highest temperature and lowest supply voltage This condition leads to minimum loop gain and could result in a slow or no start up To minimize undesirable tem...

Страница 22: ...78M6612 Hardware Design Guidelines AN_6612_007 22 Rev 2 XIN XOUT 32 768 kHz 36K 20K 3 3V External Canned Oscillator Or Clock Divider XIN Attenuator Figure 25 78M6612 with External Canned Oscillator...

Страница 23: ...nput LINE voltage Not to exceed 250 mVpp Line Voltage Divider Resistor Rating Select package based on its maximum voltage rating and isolation clearance requirements CT Turns Ratio and Power Rating Se...

Страница 24: ...ed VB connection in Figure 1 1 2 10 29 2010 Added The V3P3 connection to AC Neutral can be eliminated when using current transformers CT as the current sensing elements Refer to the section on Isolate...

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