Application Hints (Continued)

TLH7892 – 4

Available from National Semiconductor Corporation

FIGURE 4 Time Variable FET Attenuator

completely turned ON until all detectable ringing has

stopped The duration of the ring is rarely specified by the

transducer manufacturer and must be experimentally deter-

mined

When designing an ultrasonic ranging system three

trans-

ducer parameters are very important

1) resonant impedance (RT in Figure 3b )

2) maximum peak-to-peak voltage

3) resonant frequency fr

This data used in conjunction with the curves given in

Figure 6 results in a functional output stage design

TRANSMITTER

The transmitter (

Figure 5 ) consists of an oscillator a 1 ms

one-shot and a power amplifier

When the transmitter is keyed ON at pin 8 the L1-C1 tank is

switched to the oscillator mode An on-chip 1 ms one-shot is

triggered with each cycle of the oscillator and in turn drives

a power amplifier This one-shot has a reset time of 2 ms

limiting the maximum operating frequency to about 325 kHz

A transformer couples the transducer to the output stage

The oscillator frequency is set by L1-C1 and can be calcu-

lated from

fO e

1

2q

0L1C1

(1)

The L1-C1 tank must have a minimum Rp of 10 kX where

RP e 2qfOQL1

(2)

and Q e unloaded Q of L1-C1 tank

The output transformer (L6) is designed with the aid of

Fig-

ure 6 Curves are shown for two common frequencies

40 kHz and 200 kHz For a given load impedance (RT Fig-

ure 3b ) a turns ratio for L6 is determined In order not to

exceed the transducer’s specifications the peak-to-peak

output voltage may need to be adjusted using the equation

Vp-p e 2Va

NSN

P

J

(3)

To ensure that the output stage is not overloaded a current

measurement must be made at pin 6 While the first few

pulses of each transmit period may reach 2A or 3A the

steady-state current spikes must not exceed 1A Current

spikes are reduced by decreasing the turns ratio of L6

The secondary of L6 tunes with C6 at the operating frequen-

cy fO

TLH7892 – 5

FIGURE 6 L6 Turns Ratio vs Load Resistance

TLH7892 – 6

FIGURE 5 Transmitter

4