MSP430 datasheet(76/88 Pages) TI

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8.2.1.2.2 Current Loop Control

The AFE88101 sets an output voltage from 0.3 V to 2.5 V if configured in Range 0 with PVDD > 2.7 V. Figure 8-5

shows the feedback circuit that sets the loop current from the DAC output voltage.

–

3V3

1 k

40.2

40.2 k

100 k

LOOP–

100 nF

OPA333

0.3 V to 2.5 V

from

AFE88101

VOUT

LOOP

20 M

3V3

8.2

510

330 k

200 k

10 μF

LOOP+

3V3

Figure 8-5. Current Loop Control for the AFE88101 Transmitter

In this circuit, the VOUT voltage is set across the 100-kΩ resistor by the AFE88101. The opposite end of the

100-kΩ resistor is set to ground by the feedback of the OPA333. The current across the 100-kΩ resistor is VOUT

divided by 100 kΩ. This current continues through the 40.2-kΩ resistor so that the voltage at LOOP– is less than

ground. Equation 11 calculates the voltage at LOOP–.

VLOOP–= – VOUT/100kΩ ×40.2kΩ= –VOUT×0.402

(11)

When the DAC output voltage is set to 0.3 V, the voltage at LOOP– is 0.1206 V less than ground. When the

DAC output voltage is set to 2.5 V, the voltage at LOOP– is 1.005 V less than ground. The LOOP– voltage

sets the loop current that flows from ground to LOOP– through the 40.2-Ω resistor. This current is sourced from

ground but controlled by the current sunk from Q4 coming from the start-up circuit. Equation 12 calculates the

loop current.

ILOOP= –VLOOP/40.2kΩ

(12)

Substituting Equation 12 into Equation 11, Equation 13 is obtained.

ILOOP=VOUT×0.402/40.2Ω=VOUT/100Ω

(13)

When the DAC output voltage is set to 0.3 V, the loop current is 3 mA. When the DAC output voltage is set

to 2.5 V, the loop current is 25 mA. The OPA333 drives the base of transistor Q4 to pull the correct amount of

current to set the feedback loop. The current pulled from LOOP+ powers the board. Excess current greater than

what is required to power the board is shunted through the TLVH431B regulator.

AFE78101, AFE88101

SLASF21 – DECEMBER 2022

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