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SBAA646 October 2024 AMC0380D-Q1 , AMC0381D-Q1 , AMC0386-Q1

2.4 Fully Integrated Resistors vs. Additional External Resistor Example

Accurate voltage measurement and performance over temperature is crucial in onboard charger (OBC) applications. Achieving full state of charge on the battery is necessary for the battery to fully charge after years of use. Ergo, increased accuracy and low lifetime drift directly contribute to the continued success of these systems. These principles can extend to other HEV, Energy Infrastructure, and Motor Drive applications as well.

Some applications can alternatively consider including an external resistor to manually adjust the gain of the internal resistor divider. This is feasible; however, the caveat is reintroducing temperature drift and gain error that is virtually foregone when using integrated resistor devices. With integrated resistors, the gain drift of the HV and LV resistors can drift in the same direction and remain stable over temperature, effectively going unmeasured. When introducing an external resistor, R_EXT, the gain drift of the internal resistors and R_EXT can shift in opposite directions in the worst case and add secondary error to the system. For example, if a user wanted to sense 1200V on a 1000V device, the user can consider the following demonstration:

Figure 2-7 Gain Error Resistor Divider Variation Schematics

Case 1: Sensing 1000V on a 1000V Device (AMC0381R10):

For 1000V Devices: R_HV = 12.5MΩ; R_SNS = 12.5kΩ

Integrated resistors have a tolerance of ±20%. Both the HV and LV resistors, R_HV and R_SNS, drift in the same direction.

Nominal Resistor Divider Voltage at SNSP Pin:

Equation 5.

V_{N O M} = V_{P E A K} \times \frac{R_{S N S}}{R_{H V} + R_{S N S}}

Equation 6.

V_{N O M} = 1000 V \times \frac{12.5 k Ω}{12.5 M Ω + 12.5 k Ω} = 0.999 V

Maximum Resistor Divider Voltage at SNSP Pin:

Equation 7.

V_{M A X} = V_{P E A K} \times \frac{R_{S N S + 20 %}}{R_{H V + 20 %} + R_{S N S + 20 %}}

Equation 8.

V_{M A X} = 1000 V \times \frac{15.0 k Ω}{15.0 M Ω + 15.0 k Ω} = 0.999 V

Gain Error Output Referred:

Equation 9.

V_{G A I N E R R O R O U T P U T} = (V_{M A X} - V_{N O M}) \times V_{O U T P U T}

Equation 10.

V_{G A I N E R R O R O U T P U T} = (0.999 V - 0.999 V) \times 2 V = 0 V

Equation 11.

G a i n E r r o r % = \frac{V_{M A X} - V_{N O M}}{V_{N O M}} \times 100

Equation 12.

G a i n E r r o r % = \frac{0.999 V - 0.999 V}{0.999 V} \times 100 = 0 %

Not maximizing full scale input range can result in the offset error contributing to a larger portion of the full scale error. Please refer to the isolated voltage sensing calculator for more information.

Case 2: Sensing 1200V using a 1000V Device (AMC0381R10):

For 1000V Devices: R_HV = 12.5MΩ; R_SNS = 12.5kΩ

This design requires including an external resistor, R_EXT, from SNSP to AGND. This can introduce secondary error to the system and is unadvised. The absolute maximum ratings of the device must not be exceeded.

Equation 13.

\frac{R_{E X T} ∥ 12.5 k Ω}{12.5 M Ω + R_{E X T} ∥ 12.5 k Ω} = \frac{1}{1200}

Equation 14.

R_{E X T} = 62.8 k Ω

Integrated resistors have a tolerance of ±20% and external resistors have a tolerance of 0.1%. In the worst case scenario, R_EXT can drift in the opposite direction of R_HV and R_SNS.

Nominal Resistor Divider Voltage with External Resistor at SNSP Pin:

Equation 15.

V_{N O M} = V_{P E A K} \times \frac{R_{S N S} ∥ R_{E X T}}{R_{H V} + R_{S N S} ∥ R_{E X T}}

Equation 16.

R_{S N S} ∥ R_{E X T} = \frac{12.5 k Ω \times 62.8 k Ω}{12.5 k Ω + 62.8 k Ω} = 10.4 k Ω

Equation 17.

V_{N O M} = 1200 V \times \frac{10.4 k Ω}{12.5 M Ω + 10.4 k Ω} = 1.00 V

Maximum Resistor Divider Voltage with External Resistor at SNSP Pin:

Equation 18.

V_{M A X} = V_{P E A K} \times \frac{R_{S N S - 20 %} ∥ R_{E X T + 0.1 %}}{R_{H V - 20 %} + R_{S N S - 20 %} ∥ R_{E X T + 0.1 %}}

Equation 19.

R_{S N S - 20 %} ∥ R_{E X T + 0.1 %} = \frac{10.0 k Ω \times 62.9 k Ω}{10.0 k Ω + 62.9 k Ω} = 8.63 k Ω

Equation 20.

V_{M A X} = 1200 V \times \frac{8.63 k Ω}{10.0 M Ω + 8.63 k Ω} = 1.03 V

Gain Error Output Referred:

Equation 21.

V_{G A I N E R R O R O U T P U T} = (1.03 V - 1.00 V) \times 2 V = 0.069 V

Equation 22.

G a i n E r r o r % = \frac{1.03 V - 1.00 V}{1.00 V} \times 100 = 3.44 %

Using the integrated resistor devices as is does not incorporate any measurable gain drift. Adding an external resistor to manually adjust the gain of these devices can introduce an additional worst case scenario gain drift error of 3.44% to the total system error and is therefore not recommended.