ltc6993-1 Linear Technology Corporation, ltc6993-1 Datasheet - Page 17

• Manufacturer Part Number: ltc6993-1
• Description: Electrical Specifications Subject To Change
• Manufacturer: Linear Technology Corporation

applicaTions inForMaTion
Since 625k is not available as a standard 1% resistor,
substitute 619k if a –0.97% shift in t
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.
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
R
the pulse width as described in Equation (3).
MOD
t
OUT
to vary the I
=
Figure 7. 100µs Negative Pulse Generator
N
DIV
R
625k
50
SET
•
k
R
TRIG
GND
SET
Ω
MOD
LTC6993-2
CTRL
SET
current, which in turn modulates
•
sources/sinks a current through
OUT
1
DIV
µP
V
+
+
R
R
MOD
SET
2.25V TO 5.5V
1
µs
D
CLK
CS/LD
–
IN
V
V
R1
102k
R2
976k
DIVCODE = 14
CTRL
69931234 F07
OUT
SET
V
LTC1659
V
CC
+
Figure 9. Digitally Controlled Pulse Width
is acceptable.
GND
REF
V
OUT
LTC6078
V
+
1/2
(3)
+
–
R
MOD
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
the V
and using it as the DAC’s reference voltage, as shown in
Figure 9. The DAC’s output voltage now tracks any V
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.
R
SET
SET
V
t
D
CTRL
OUT
TRIG
GND
SET
IN
dependency can be eliminated by buffering V
= 0 TO 4095
=
Figure 8. Voltage-Controlled Pulse Width
LTC6993
N
R
LTC6993-1/LTC6993-2
LTC6993-3/LTC6993-4
DIV
MOD
50k
• R
OUT
DIV
V
MOD
+
R
SET
•
1+
TRIG
GND
SET
R
R
MOD
SET
69931234 F09
LTC6993
C1
0.1µF
1µs
–
4096
D
OUT
IN
DIV
V
V
+
+
R1
R2
C1
0.1µF
V
+
CTRL
R1
R2
69931234 F08
voltage,

69931234p
SET
SET