Accurate PWM LED Dimming without External Signal Generators, Clocks or µControllers

Accurate PWM LED Dimming without External Signal
Generators, Clocks or µControllers
Keith Szolusha
LEDs can be dimmed in two ways: analog and pulse-width modulation (PWM) dimming.
Analog dimming changes LED light output by simply adjusting the DC current in the string,
while PWM dimming acheives the same effect by varying the duty cycle of a constant
current in the string to effectively change the average current in the string. Despite its
attractive simplicity, analog dimming is inappropriate for many applications because it
loses dimming accuracy by about 25%+ at only 10:1 brightness levels, and it skews the
color of the LEDs. In contrast, PWM dimming can produce 3000:1 and higher dimming
ratios (at 100Hz) without any significant loss of accuracy, and no change in LED color.
The LT3761 combines the simplicity of
analog dimming with the accuracy of
PWM dimming by generating its own
PWM signal. High dimming ratios are
possible by adjusting a simple DC signal
at its dimming input—no additional
PWM-generating microcontrollers, oscillators or signal generators are required.
The LT3761’s internal PWM signal
can produce 25:1 dimming, while it
can still deliver up to 3000:1 dimming with an external PWM signal.
HIGH POWER LED DRIVER
The LT3761 is a high power LED driver
similar to the LT3755-2 and LT3756-2
family. It is a 4.5V-to-60V input to
0V-to-80V output single-switch controller IC that can be configured as a
boost, SEPIC, buck-boost mode or buck
mode LED driver. It has a 100kHz to
1MHz switching frequency range, open
LED protection, extra internal logic to
provide short-circuit protection, and can
be operated as a constant voltage regulator with current limit or as a constantcurrent SLA battery or supercap charger.
Figure 1 shows a 94% high efficiency 60V,
1A (60W) 350k Hz automotive headlamp
34 | October 2012 : LT Journal of Analog Innovation
application with PWM dimming. The
LT3761 uses the same high performance
PWM dimming scheme as the LT3755/LT3756
family, but with the additional feature
of the internally generated PWM dimming signal and no additional pins.
INTERNAL PWM DIMMING
GENERATOR
Unlike other high power LED drivers, the
LT3761 can generate its own PWM dimming signal to produce up to 25:1 dimming. This enables it to produce accurate
PWM dimming without the need for
external PWM-generating components. The
Figure 1. 94% efficient boost LED driver for automotive headlamp with 25:1 internal PWM dimming
L1
10µH
VIN
8V TO
60V
CIN
2.2µF
×2
100V
499k
90.9k
EN/UVLO
VREF
1M
VIN
140k
RSENSE
10mΩ
1M
RLED
0.25Ω
1A
16.9k
FB
60W
LED
STRING
ISP
OPENLED
ISN
DIM/SS
PWM
PWMOUT
VC
RT INTVCC
RDIM
124k
CSS
0.01µF
COUT
2.2µF
×4
GND
100k
DIM
M1
SENSE
LT3761
CTRL
INTVCC
GATE
D1
CPWM
47nF
300Hz
RC
5.1k
CC
4.7nF
RT
28.7k
350kHz
M1: INFINEON BSC123N08NS3-G
D1: DIODES INC PDS5100
L1: COILTRONICS HC9-100-R
M2: VISHAY SILICONIX Si2328DS
COUT, CIN: MURATA GRM42-2X7R225K100R
INTVCC
CVCC
1µF
M2
(CURRENT DERATED FOR VIN < 10V)
design ideas
The LT3761 generates its own PWM signal to achieve accurate PWM dimming,
but with the simple control of analog dimming. High dimming ratios are possible
by adjusting a simple DC signal at its dimming input—no additional PWMgenerating microcontrollers, oscillators or signal generators are required.
Figure 2. Internally generated PWM signal and LED
current for the application in Figure 1
VDIM = 7.7V
DCPWM = 96%
VDIM = 4V
DCPWM = 50%
ILED
1A/DIV
VDIM = 1.5V
DCPWM = 10%
VDIM = 0.4V
DCPWM = 4.3%
0.5ms/DIV
LT3761 requires only an external DC voltage, much like analog dimming control,
for high performance PWM dimming at
a chosen frequency. It can still receive a
PWM input signal to drive the LED string
with that signal in standard fashion.
the PWM pin to GND according to the
equation: fPWM = 14kHz • nF/CPWM. The
duty cycle of the signal at PWMOUT is set
by a µ A-scale current into the DIM/SS pin
as shown in Figure 3. Internally generated pull-up and pull-down currents
on the PWM pin are used to charge and
discharge its capacitor 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
The internal PWM dimming signal
generator features programmable
frequency and duty cycle. The frequency of the square wave signal at
PWMOUT is set by a capacitor CPWM from
Figure 3. Setting the duty cycle at the DIM/SS pin
takes a µA-scale signal. This pin can also be used
with an external PWM signal for very high dimming
ratios.
CONCLUSION
The high power and high performance
LT3761 LED driver has its own onboard
PWM dimming signal generator that
is both accurate and easy to use. n
Figure 4. Given a high speed PWM input signal, the
LT3761 still provides a high speed PWMOUT signal.
100
PWMOUT DUTY RATIO (%)
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. For 100% duty cycle operation,
the PWM pin can be tied to INTVCC .
CPWMOUT = 2.2nF
80
PWM
INPUT
60
PWMOUT
5V/DIV
40
20
0
–10
0
10
20
30
DIM/SS CURRENT (µA)
40
50
200ns/DIV
October 2012 : LT Journal of Analog Innovation | 35
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