MSP430 datasheet(75/88 Pages) TI

75 / 88 page

8.2.1.2.1 Start-Up Circuit

When the loop is applied to the terminals, the loop power starts up the board. Transistor Q4 from Figure 8-2 pulls

current from the start-up and current-shunt regulator sections of the transmitter. The start-up circuit is shown in

Figure 8-4.

8.2

510

249 k

1 k

330 k

200 k

3V3

10 μF

3.6-V Zener

LOOP+

Current

sourced to

loop control

Figure 8-4. Start-Up Circuit

In the start-up circuit, the 3.6-V Zener diode sets the voltage at the base of Q1. If the TLVH431B shunt regulator

has not started, apply voltage to LOOP+ and LOOP– to turn on Q1 and source current to the shunt regulator.

As the shunt regulator turns on and approaches the set voltage of 3.3 V, the base-emitter voltage (VBE) of

Q1 becomes smaller. The collector current of Q2 drives the current of the shunt regulator to set the LOOP

current going through the 40.2-Ω resistor in the current loop control circuit shown in the following section. After

the start-up circuit has started, Q1 stops supplying current because the VBE is restricted. Q1 shuts off, leaving

several microamps of current flowing through the 3.6-V Zener diode.

Take care when selecting the Zener diode. The voltage across the Zener diode varies with the loop voltage and

the temperature of the circuit. This variance can change the VBE across Q1 and change the total current going

through the start-up circuit. If the voltage is too high, the Zener diode sets Q1 to continue to source current after

the circuit starts up. If the voltage is too low, the Zener diode prevents the TLVH431B from turning on. Verify

proper start up by checking that the 3.3-V supply starts up, and that Q1 turns off when in operation.

When the circuit starts up and the 3V3 line comes up to the desired 3.3-V supply level, the current through

the TLVH431B is primarily sourced through Q2. The Q2 transistor must be able to dissipate enough power to

handle the high current (> 20 mA) and the high voltage (> 30 V) in the loop. Because the biased transistor, Q2,

is responsible for sourcing most of the output current, choose the components in the path of this current flow with

appropriate power ratings. In this case, the 8.2-Ω resistor is rated to 0.25 W.

The current mirror is set up so that the current gain from Q3 to Q2 is approximately a factor of 60 ×. The exact

current gain is not important as long as the current through Q3 is low.

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AFE78101, AFE88101

SLASF21 – DECEMBER 2022

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