LTC6992-1 Datasheet, PDF (20/32 Pages) Linear Technology – TimerBlox Voltage-Controlled Pulse Width Modulator (PWM)

LTC6992-1/LTC6992-2/
LTC6992-3/LTC6992-4
APPLICATIONS INFORMATION
Step 4: Select R_SET
Calculate the correct value for R_SET using Equation (1b).
R_SET
=
1MHz • 50k
4 • 20kHz
=
625k
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.
V_MOD
MOD OUT
LTC6992-4
GND
V⁺
SET
R_SET
625k
DIV
6992 F07
2.25V TO 5.5V
R1
976k
DIVCODE = 1
R2
102k
Some applications can eliminate ΔV_SET sensitivity by
making V_MOD proportional to V_SET. For example, Figure 9
shows a simple circuit for generating an arbitrary duty
cycle. The equation for duty cycle does not depend on
V_SET at all.
100
90
80
70
60
50
40
30
20
10
0
0
ΔV_SET = –30mV
ΔV_SET = 0mV
ΔV_SET = 30mV
0.2 0.4 0.6 0.8
1
V_MOD (V)
6992 F08
Figure 8. Duty Cycle Variation Due to ΔV_SET
Figure 7. 20kHz PWM Oscillator
Duty Cycle Sensitivity to ΔV_SET
The output duty cycle is proportional to the ratio of V_MOD/
V_SET. Since V_SET can vary up to ±30mV from 1V it can
effectively gain or attenuate V_MOD, as shown below when
ΔV_SET is added to the equation.
( ) ^{D =} 0.8 •
V_MOD
V_SET + ΔV_SET
−
1
8
The simplifying assumption of ΔV_SET = 0V creates the po-
tential for additional duty cycle error, which increases with
V_MOD, reaching a maximum of 3.4% if ΔV_SET = –30mV.
ΔD ≅
V_MOD
800mV
•
ΔV_SET
V_SET
≅
−
⎛
⎝⎜
D_ideal
+
1_⎞
8 ^⎠⎟
•
ΔV_SET
V_SET
Figure 8 demonstrates the worst-case impact of this varia-
tion (if V_SET is at its 0.97V or 1.03V limits).
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_MOD near 0.9V) is critical.
20
MOD OUT
LTC6992-X
GND
V⁺
2.25V TO 5.5V
R1
SET
DIV
R_SET1
6992 F09
R2
R_SET2
D
5
4
•
R_SET2
R_SET1  R_SET2

1
8
Figure 9. Fixed-Frequency, Arbitrary Duty Cycle Oscillator
I_SET Extremes (Master Oscillator Frequency Extremes)
Pushing I_SET outside of the recommended 1.25μA to 20μA
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_SET < 1.25μA. At approximately 500nA, the oscillator
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.
69921234p