Datasheet

LTC6993-1/LTC6993-2
LTC6993-3/LTC6993-4
17
69931234fb
Since 625k is not available as a standard 1% resistor,
substitute 619k if a –0.97% shift in t
OUT
is acceptable.
Otherwise, select a parallel or series pair of resistors such
as 309k and 316k to attain a more precise resistance.
The completed design is shown in Figure 7.
APPLICATIONS INFORMATION
Figure 7. 100µs Negative Pulse Generator
LTC6993-2
TRIG
GND
SET
OUT
V
+
DIV
R1
102k
DIVCODE = 14
69931234 F07
2.25V TO 5.5V
R2
976k
0.1µF
R
SET
625k
Figure 8. Voltage-Controlled Pulse Width
LTC6993
TRIG
GND
SET
OUT
V
+
DIV
R1
C1
0.1µF
69931234 F08
V
+
R2
R
SET
R
MOD
V
CTRL
69931234 F09
LTC6993
TRIG
GND
SET
OUT
V
+
DIV
C1
0.1µF
R1
R2
V
+
R
MOD
R
SET
–
+
V
+
0.1µF
1/2
LTC6078
LTC1659
V
+
V
CC
REF
GND
V
OUT
µP
D
IN
CLK
CS/LD
N
DIV
• R
MOD
50kΩ
t
OUT
=
D
IN
= 0 TO 4095
•
–
1+
R
MOD
R
SET
D
IN
4096
1µs
0.1µF
Voltage-Controlled Pulse Width
With one additional resistor, the LTC6993 output pulse
width can be manipulated by an external voltage. As shown
in Figure 8, voltage V
CTRL
sources/sinks a current through
R
MOD
to vary the I
SET
current, which in turn modulates
the pulse width as described in Equation (3).
t
OUT
=
N
DIV
• R
MOD
50kΩ
•
1µs
1+
R
MOD
R
SET
–
V
CTRL
V
SET
(3)
Digital Pulse Width Control
The control voltage can be generated by a DAC (digital-to-
analog converter), resulting in a digitally-controlled pulse
width. Many DACs allow for the use of an external refer-
ence. If such a DAC is used to provide the V
CTRL
voltage,
the V
SET
dependency can be eliminated by buffering V
SET
and using it as the DAC’s reference voltage, as shown
in
Figure 9. The DAC’s output voltage now tracks any V
SET
variation and eliminates it as an error source. The SET pin
cannot be tied directly to the reference input of the DAC
because the current drawn by the DAC’s REF input would
affect the pulse width.
Figure 9. Digitally Controlled Pulse Width