4

Test Circuit

In production the HA7210 is tested with a 32kHz and a

1MHz crystal. However for characterization purposes data

was taken using a sinewave generator as the frequency

is a smaller amplitude than what a typical crystal would

generate so the transitions are slower. In general the

duty cycle, and rise/fall time. The Generator test method is

useful for testing a variety of frequencies quickly and

provides curves which can be used for understanding

performance trends. Data for the HA7210 using crystals has

also been taken. This data has been overlaid onto the

generator data to provide a reference for comparison.

Application Information

Theory Of Operation

The HA7210 is a Pierce Oscillator optimized for low power

consumption, requiring no external components except for a

bypass capacitor and a Parallel Mode Crystal. The Simplified

Block Diagram shows the Crystal attached to pins 2 and 3, the

Oscillator input and output. The crystal drive circuitry is detailed

showing the simple CMOS inverter stage and the P-channel

operate mostly in its linear region increasing the amplitude of

the oscillation until limited by its transconductance and voltage

center the oscillating waveform at the input threshold. Do not

interfere with this bias function with external loads or excessive

frequency range to 7M

Ω in the highest frequency range.

The HA7210 optimizes its power for 4 frequency ranges

selected by digital inputs Freq1 and Freq2 as shown in the

Block Diagram. Internal pull up resistors (constant current

0.4

µA) on Enable, Freq1 and Freq2 allow the user simply to

leave one or all digital inputs not connected for a

corresponding “1” state. All digital inputs may be left open for

10kHz to 100kHz operation.

A current source develops 4 selectable reference voltages

buffered and used as the negative supply rail for the

oscillator section of the circuit. The use of a current source in

the reference string allows for wide supply variation with

minimal effect on performance. The reduced operating

voltage of the oscillator section reduces power consumption

and limits transconductance and bandwidth to the frequency

range selected. For frequencies at the edge of a range, the

higher range may provide better performance.

The OSC OUT waveform on pin 3 is squared up through a series

of inverters to the output drive stage. The Enable function is

implemented with a NAND gate in the inverter string, gating the

signal to the level shifter and output stage. Also during Disable

the output is set to a high impedance state useful for minimizing

power during standby and when multiple oscillators are OR’ed to

a single node.

Design Considerations

The low power CMOS transistors are designed to consume

power mostly during transitions. Keeping these transitions

short requires a good decoupling capacitor as close as

possible to the supply pins 1 and 4. A ceramic 0.1

µF is

recommended. Additional supply decoupling on the circuit

board with 1

µF to 10µF will further reduce overshoot, ringing

and power consumption. The HA7210, when compared to a

crystal and inverter alone, will speed clock transition times,

reducing power consumption of all CMOS circuitry run from

that clock.

Power consumption may be further reduced by minimizing the

capacitance on moving nodes. The majority of the power will

be used in the output stage driving the load. Minimizing the

load and parasitic capacitance on the output, pin 5, will play

the major role in minimizing supply current. A secondary

source of wasted supply current is parasitic or crystal load

capacitance on pins 2 and 3. The HA7210 is designed to work

with most available crystals in its frequency range with no

external components required. Two 15pF capacitors are

internally switched onto crystal pins 2 and 3 on the HA7210 to

compensate the oscillator in the 10kHz to 100kHz frequency

range.

The supply current of the HA7210 may be approximately

calculated from the equation:

EXAMPLE #1:

EXAMPLE #2:

1

2

3

4

8

7

6

5

HA7210

+5V

18pF

0.1

µF

1000pF

50

Ω

ENABLE

FREQ 2

FREQ 1

FIGURE 1.

HA7210