AN232

DS00232A-page 8

 2002 Microchip Technology Inc.

BASE STATION CIRCUIT

DESCRIPTION

The drive circuitry (see Appendix A) consists of

TC4422 (U2) and TC4421 (U3). U3 is an inverting FET

driver and U2 is a non-inverting FET driver. U3 is the

main 1/2 bridge drive device and U2 is only used during

start-up as explained previously. During the first 5

cycles of start-up, both U2 and U3 work in tandem as a

full bridge drive circuit. In full bridge mode, U2 and U3

are driven from the PWM output via RB3. At this time,

RB4 is defined as an input and thus has no influence

on the input signal to U2.

The driver is converted to a 1/2 bridge made after the 5

start-up cycles by driving point A to ground. This is

done by changing RB4 from an input to a low level (i.e.,

zero) output. This has no influence on the signal to U3,

but it grounds U2. One can remove U2 and physically

ground point A if a fast turn-on is not required. Remov-

ing U2 also increases the efficiency and output voltage

slightly.

Turn-On Timing

Figure 11 shows the proper turn-on sequence refer-

enced from the PWM output cycles.

Turn-Off Clamp

The turn-off clamp is realized by discharging the reso-

nant tank through R1, R2 and a 600 V rated triac Q1.

Q1 is a Q6x3 surface mound triac from Teccor Elec-

tronics. The high tank voltages necessitate the need for

two small serial resistors. Alternatively, one can use a

single high-voltage resistor. A single PNP transistor T1

forms a gated drive circuit and is activated by driving

RB7 low. Refer to the previous section for timing

requirements. The clamp circuit ensures that no

residual energy is left in the tank, thus simplifying

µs

or more).

Power Supply

The circuit consumes roughly 0.5 A

ting continuously in 1/2 bridge mode. It consumes

approximately double that amount of current during the

40

µs start-up. The average transmitting power con-

sumption is 0.25 A

RMS

for Manchester encoding.

Instantaneous peak current is about 1.2 A. The overall

current consumption when transmitting data continu-

ously is roughly 3 Watts, but adequate tank capaci-

tance is needed to supply the relatively high peak

current and reduce supply ripple. C4 is the main

smoothing capacitor. A Panasonic FC series 25V

560

µF is used because of its 1.2 A ripple current

specification. C2 and C3 are local DC coupling capaci-

tors for U2 and U3, and are 20V 0.68

µF tantilum

capacitors.

Communications

Serial communications is performed via the UART on

the PIC16F628, and the voltage translation is done with

a MAX232. Flow control using CTS and RTS is imple-

mented on RB6 and RB5 respectively.

CONCLUSION

The choice of a PIC16F628 with a PWM lends itself

towards an effective design of a low frequency mag-

netic transmitter circuit. In addition, the TC4421/

TC4422’s are well suited as FET driver for this type of

application. The main advantages of using low fre-

quency magnetic transmitter design are:

• Good field penetration

• Precise range control

• Low power consumption

• Overall low cost

FIGURE 11:

TURN-ON TIMING

0

1

2

3

4

567

PWM (RB3)

TRISB.4

A

B

Full Bridge Start-Up

Half Bridge Mode