16
LT1713/LT1714
LINEAR TECHNOLOGY CORPORATION 2000
171314f LT/TP 0501 4K • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
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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
U
TYPICAL APPLICATIO
–
+
A
1/2 LT1714
2
1
–
+
2
V
+
V
+
3
3
15
16
C2
0.01µF
R6
1k
V
+
= 2.7V TO 7V
1MHz
TRIANGLE
WAVE
R5
1k
13
6
1
4
7
14
4
V
+
V
+
R1
26.1Ω
R4
499Ω
21Ω
1k 10k
1k
C1
500pF
R2
2k
R3
2k
–
+
B
1/2 LT1714
7
8
6
9
10
171314 F09
12
11
5
V
+
C3
100pF
LT1809
C4
0.001µF
C5
0.01µF
C6
0.001µF
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 3.5V
P-P
, degrading to –30dBc (3%) as the circuit
clips at 5V
P-P
on a single 5V supply.
