Datasheet

LT3761
15
3761f
applicaTions inFormaTion
PWM Dimming Signal Generator
The LT3761 features a PWM dimming signal generator with
programmable duty cycle. The frequency of the square
wave signal at PWMOUT is set by a capacitor C
PWM
from
the PWM pin to GND according to the equation:
f
PWM
= 14kHz • nF/C
PWM
The duty cycle of the signal at PWMOUT is set by a µA
scale current into the DIM/SS pin (see Figure 4 and the
Typical Performance Characteristics).
Programming the Switching Frequency
The RT frequency adjust pin allows the user to program
the switching frequency (f
SW
) from 100kHz to 1MHz to
optimize efficiency/performance or external component
size. Higher frequency operation yields smaller compo-
nent size but increases switching losses and gate driving
current, and may not allow sufficiently high or low duty
cycle operation. Lower frequency operation gives better
performance at the cost of larger external component
size. For an appropriate R
T
resistor value see Table 2. An
external resistor from the RT pin to GND is required—do
not leave this pin open.
Table 2. Switching Frequency (f
SW
) vs R
T
Value
f
SW
(kHz) R
T
(kΩ)
100 95.3
200 48.7
300 33.2
400 25.5
500 20.5
600 16.9
700 14.3
800 12.1
900 10.7
1000 8.87
Duty Cycle Considerations
Switching duty cycle is a key variable defining converter
operation, therefore, its limits must be considered when
programming the switching frequency for a particular ap-
plication. The minimum duty cycle of the switch is limited
by the fixed minimum on-time and the switching frequency
(f
SW
). The maximum duty cycle of the switch is limited
by the fixed minimum off-time and f
SW
. The following
equations express the minimum/maximum duty cycle:
Min Duty Cycle = 220ns • f
SW
Max Duty Cycle = 1 – 170ns • f
SW
DIM VOLTAGE (V)
0
PWMOUT DUTY RATIO (%)
100
60
20
80
40
0
2 64
3761 F04
8
C
PWM
= 47nF
Figure 4. PWMOUT Duty Ratio vs DIM Voltage for R
DIM
= 124k
Internally generated pull-up and pull-down currents on
the PWM pin are used to charge and discharge its capaci-
tor between the high and low thresholds to generate the
duty cycle signal. These current signals on the PWM pin
are small enough so they can be easily overdriven by a
digital signal from a microcontroller to obtain very high
dimming performance. The practical minimum duty cycle
using the internal signal generator is about 4% if the DIM/
SS pin is used to adjust the dimming ratio. Consult the
factory for techniques for and limitations of generating a
duty ratio less than 4% using the internal generator. For
always on operation, the PWM pin should be connected
as shown in the application Boost LED Driver with Output
Short Protection.