LTC6992-4 Linear Technology, LTC6992-4 Datasheet - Page 20

• Manufacturer Part Number: LTC6992-4
• Description: TimerBlox Voltage-Controlled Pulse Width Modulator (PWM)
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LTC6992-1/LTC6992-2/
LTC6992-3/LTC6992-4
Step 4: Select R
Calculate the correct value for R
Since 625k is not available as a standard 1% resistor,
substitute 619k if a 0.97% frequency shift 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.
Duty Cycle Sensitivity to ΔV
The output duty cycle is proportional to the ratio of V
V
effectively gain or attenuate V
ΔV
The simplifying assumption of ΔV
tential for additional duty cycle error, which increases with
V
Figure 8 demonstrates the worst-case impact of this varia-
tion (if V
This error is in addition to the inherent PWM duty cycle
accuracy spec ΔD (±4.5%), so care should be taken if ac-
curacy at high duty cycles (V
20
SET
MOD
R
SET
D =
ΔD ≅
. Since V
SET
, reaching a maximum of 3.4% if ΔV
is added to the equation.
0.8 • V
V
MOD
=
SET
800mV
V
1MHz • 50k
MOD
4 • 20kHz
(
is at its 0.97V or 1.03V limits).
R
625k
SET
SET
Figure 7. 20kHz PWM Oscillator
SET
V
MOD
MOD
GND
SET
SET
•
LTC6992-4
can vary up to ±30mV from 1V it can
+ ΔV
ΔV
V
SET
SET
= 625k
6992 F07
OUT
SET
DIV
V
+
≅ − D
)
SET
−
MOD
MOD
⎛
⎝ ⎜
8
1
SET
ideal
, as shown below when
SET
near 0.9V) is critical.
2.25V TO 5.5V
using Equation (1b).
= 0V creates the po-
+
8
1
DIVCODE = 1
R1
976k
R2
102k
⎞
⎠ ⎟
SET
•
ΔV
V
= –30mV.
SET
SET
MOD
/
I
Pushing I
range forces the master oscillator to operate outside of the
62.5kHz to 1MHz range in which it is most accurate.
The oscillator will still function with reduced accuracy
for I
output will be frozen in its current state. The output could
halt in a high or low state. This avoids introducing short
pulses while frequency modulating a very low frequency
output.
Some applications can eliminate ΔV
making V
shows a simple circuit for generating an arbitrary duty
cycle. The equation for duty cycle does not depend on
V
SET
SET
Figure 9. Fixed-Frequency, Arbitrary Duty Cycle Oscillator
SET
Extremes (Master Oscillator Frequency Extremes)
at all.
< 1.25μA. At approximately 500nA, the oscillator
MOD
SET
Figure 8. Duty Cycle Variation Due to ΔV
100
90
80
70
60
50
40
30
20
10
0
outside of the recommended 1.25μA to 20μA
0
proportional to V
R
R
SET1
SET2
0.2
MOD
GND
SET
LTC6992-X
D
5
4
0.4
V
ΔV
•
6992 F09
MOD
OUT
R
DIV
SET
SET1
V
+
R
(V)
ΔV
= –30mV
SET2
0.6
SET
SET
R
SET2
= 30mV
ΔV
. For example, Figure 9
SET
0.8
8
1
2.25V TO 5.5V
www.DataSheet4U.com
= 0mV
SET
6992 F08
R1
R2
1
sensitivity by
SET
69921234p