AN865

4

Rev. 1.0

Figure 3 shows the previous idea in a real-world example. Here, the circuit periodically transmits an identifying

code utilizing a 433 MHz SAW resonator transmitter. The duty-cycling clock provides the timing for the bursts by

enabling a logic pattern generator (possibly a fed-back, shift-register-based code generator) to feed the transmitter

with an OOK (on-off keyed) signal for transmission. As a bonus, the clock provides the transmitter with

approximately doubled voltage (3.3 V) during the clock’s “on time”, enabling the SAW resonator to transmit at a

reasonable transmit power level.

Capacitor C3 serves as the charge pump’s flying capacitor, with the lower side of the cap driven by Output A, which

is fundamentally the duty-cycler clock. The cap charges through Schottky diode D1 when the clock is low, and the

stacked bypass capacitor for the output. The values of C2 and C3 should be chosen to support the load and on-

time duration at OUT. C2 = C3 = 100 µF was chosen for this design, which provided a minimum of 3 V during the

“on time” of 500 µs for a 25 mA load. Q4 provides an optional load cutoff; however, the SAW resonator transmitter

does not need to be disconnected in this way since it draws virtually no current when the logic pattern generator

provides a “zero” level.

Logic gates U2 and U3 add some timing adjustments to the duty-cycling clock. Output B is not enabled until Output

A goes high. Output B falls before Output A, thus ensuring break-before-make timing for Q3. Additionally, this

timing ensures that the pattern generator does not start until the SAW resonator transmitter has had some time to

settle with its boosted supply voltage and that it stops transmission before the boosted voltage is removed from the

transmitter. “AUP” family logic is utilized for low supply currents over temperature.

Figure 4. . “On-Time” Waveforms (for the Circuit in Figure 3)

The “no load” currents measured in the circuit show almost no overhead increase with the added boost circuitry,

and the loaded currents reflect the 3.5 mA average load current drawn by the transmitter during the 500 µs burst

period.

Table 2. No-Load Currents

4.1µA

0.58µA

Radio Burst 

VOUT (no optional 

cutoff switch) 

BURST ON 

˜3.5V

1.8V