LT1714IGN datasheet(16/16 Pages) LINER

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LT1713/LT1714

©LINEAR TECHNOLOGY CORPORATION 2000

171314f LT/TP 0501 4K • PRINTED IN USA

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forte of this particular implementation is that it is rail-to-

rail allowing a full-scale analog input. Once the triangle

wave is achieved, the remainder of the pulse width modu-

lator is easy, and is constituted by doing a simple compari-

son using the second half of the LT1714. The triangle wave

and the relatively slow moving analog signal (the one to be

modulated or to do the modulation, depending on how you

look at it) are fed into the inputs of comparator B, whose

output is then the PWM representation of the analog input

voltage. The higher the analog input voltage, the wider the

output pulse. The time averaged output level is thus

proportional to the analog input voltage. This binary

output can then be fed into power transistors with direct

control over motor or speaker winding current, for

TYPICAL APPLICATIO

–

1/2 LT1714

–

V+

0.01

µF

V+ = 2.7V TO 7V

1MHz

TRIANGLE

WAVE

V+

26.1

Ω

499

Ω

10k

500pF

–

1/2 LT1714

171314 F09

V+

100pF

LT1809

0.001

µF

0.01

µF

0.001

µF

ANALOG

INPUT

ANTIALIASING

FILTER

16kHz ANALOG FILTER

FOR LINEARITY MEASUREMENT

COMPLEMENTARY

1MHz PWM

OUTPUTS

Figure 9. Rail-to-Rail 1MHz Pulse Width Modulator

example, with their inherent lowpass characteristic. Care

must be taken to avoid cross conduction in the output

power transistors.

The linearity of the pulse width modulated signal can easily

be ascertained by putting a simple 2-pole RC filter at the

output (as shown in Figure 9). This demodulates the signal

which can then be viewed and compared with the original

input signal on an oscilloscope. Using a spectrum analyzer

and a 1kHz reference signal, this circuit’s distortion prod-

ucts were measured as better than – 50dBc (0.3%) to

about 3.5VP-P, degrading to – 30dBc (3%) as the circuit

clips at 5VP-P on a single 5V supply.

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