7

LTC1426

microprocessor has “quasi-bidirectional” ports that elimi-

pull-up resistors should be used if the microprocessor

doesn’t pull the port pins high during reset.

In pulse mode, each clock pulse applied to the CLK1 or

CLK2 input increments the respective counter by one

count. When the counter increases beyond full scale

(111111B), the counter rolls over and becomes zero scale

(000000B). In this way, a single pulse applied to the CLK1

or CLK2 input increases the respective counter by one

count, and 63 pulses decrease that counter by one count.

Pushbutton Mode

Figure 5 shows how to use the LTC1426 in a typical

pushbutton application. In pushbutton mode, a logic 1

pulse applied to the CLK1 or CLK2 input increments the

respective counter by one count, and stops incrementing

when the counter reaches full scale (111111B). A logic 0

pulse applied to the CLK1 or CLK2 input decrements the

respective counter by one count, and stops decrementing

when the counter reaches zero scale (000000B). An on-

chip debouncing circuit has a debounce time of 12.8ms to

prevent unintended counts with bouncing pushbuttons.

After a time delay of 410ms, the counter will begin to

increment/decrement at a repeat rate of 19.5Hz if the

pushbutton remains pressed.

Care should be taken to avoid running the CLK and PWM

traces close to one another. Since the CLK pins are high

impedance input nodes in pushbutton mode, current

spikes caused by the switching of the PWM outputs

feedthrough via any stray capacitance between PWM and

CLK lines if not properly routed. Use of proper grounding

techniques and spacing of these lines are highly recom-

mended for optimal performance.

Figure 6 shows a dual digitally programmable current

source using the LT

NPN transistors (2N3904). After the lowpass filter combi-

nation of R1, C1 (R2, C2), its output swings from 0V to

4.93V. In the configuration shown, this voltage will be

to be 493

Ω, the output current will range from 0mA at zero

source can be as low as 5V.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Figure 6. Dual Digitally Programmable Current Source

1

2

3

4

8

7

6

5

CLK1

CLK2

GND

PWM1

SHDN

PWM2

LTC1426

PWM1

1426 F05

PWM2

0.1

µF

0V TO 5.5V

2.7V TO 5.5V

2.7V TO 5.5V

SHDN

DOWN

DOWN

UP

UP

RR

BUTTONS ARE SIMULTANEOUSLY PUSHED. THIS RESISTOR CAN BE PLACED

BOTH BUTTONS ARE PUSHED. VALUE OF R < 50k

Figure 5. Pushbutton Mode Interface

1

2

3

4

8

7

6

5

CLK1

CLK2

GND

PWM1

SHDN

PWM2

P1.0

P1.1

LTC1426

1

2

3

4

8

7

6

5

OUT A

–IN A

+IN A

OUT B

– IN B

+ IN B

LT1013

MPU

(e.g. 8051)

0.1

µF

0.1

µF

493

Ω

R1

10k

5V

10V

C1

0.1

µF

SHDN

2N3904

2N3904

R2

10k

C2

0.1

µF

1426 F06

493

Ω