LTC6992-4 Linear Technology, LTC6992-4 Datasheet - Page 19

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

APPLICATIONS INFORMATION
Basic Operation
The simplest and most accurate method to program the
LTC6992 is to use a single resistor, R
SET and GND pins. The design procedure is a four step
process. After choosing the POL bit setting and proper
LTC6992 version, select the N
the value for the R
Step 1: Selecting the POL Bit Setting
Most applications will use POL = 0, resulting in a positive
transfer function. However, some applications may require a
negative transfer function, where increasing V
the output duty cycle. For example, if the LTC6992 is used
in a feedback loop, POL = 1 may be required to achieve
negative feedback.
Step 2: Selecting the LTC6992 Version
The difference between the LTC6992 versions is observed
at the endpoints of the duty cycle control range. Applica-
tions that require the output never stop oscillating should
use the LTC6992-2. If it is better to allow the output to
rest at GND or V
select the LTC6992-1.
The LTC6992-3 and LTC6992-4 clamp the duty cycle at
only one end of the control range, allowing the output to
stop oscillating at the other extreme. If POL = 1 the clamp
will swap from low duty cycle to high, or vice-versa. Refer
to Table 2 and Figure 4 for assistance in selecting the
proper version.
Step 3: Selecting the N
As explained earlier, the voltage on the DIV pin sets the
DIVCODE which determines both the POL bit and the
N
selected to be within the following range.
To minimize supply current, choose the lowest N
(generally recommended). For faster start-up or decreased
jitter, choose a higher N
as a guide to select the best N
plication.
DIV
62.5kHz
value. For a given output frequency, N
f
OUT
≤ N
DIV
+
(0% or 100% duty cycle, respectively),
SET
≤
1MHz
DIV
resistor.
f
OUT
DIV
setting. Alternatively, use Table 1
Frequency Divider Value
DIV
DIV
value and then calculate
value for the given ap-
SET
, between the
DIV
MOD
should be
DIV
reduces
value
(1a)
With POL already chosen, this completes the selection of
DIVCODE. Use Table 1 to select the proper resistor divider
or V
Step 4: Calculate and Select R
The final step is to calculate the correct value for R
using the following equation.
Select the standard resistor value closest to the calculated
value.
Example: Design a PWM circuit that satisfies the following
requirements:
• f
• Positive V
• Output can reach 100% duty cycle, but not 0%
• Minimum power consumption
Step 1: Selecting the POL Bit Setting
For positive transfer function (duty cycle increases with
V
Step 2: Selecting the LTC6992 Version
To limit the minimum duty cycle, but allow the maximum
duty cycle to reach 100%, choose LTC6992-4. (Note that
if POL = 1 the LTC6992-3 would be the correct choice.)
Step 3: Selecting the N
Choose an N
Equation (1a).
Potential settings for N
the best choice, as it minimizes supply current by us-
ing a large R
DIVCODE = 1. Using Table 1, choose the R1 and R2 values
to program DIVCODE = 1.
MOD
3.125 ≤ N
R
OUT
DIV
SET
), choose POL = 0.
/V
= 20kHz
=
+
LTC6992-1/LTC6992-2/
1MHz • 50k
ratio to apply to the DIV pin.
N
LTC6992-3/LTC6992-4
DIV
MOD
DIV
SET
DIV
• f
≤ 50
resistor. POL = 0 and N
to duty cycle response
value that meets the requirements of
OUT
DIV
DIV
include 4 and 16. N
Frequency Divider Value
SET
www.DataSheet4U.com
DIV
= 4 requires
DIV
19
69921234p
= 4 is
(1b)
SET