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20
MSL2021 [DATASHEET]
42062A–LED–02/2013
11.7 PWM and LED Brightness
The “Block Diagram” on page 11 shows how the MSL2021 controls the brightness of the LEDs. The duty cycle of the
main string equals the duty cycle of the input signal at PWM. The PWM input accepts an input signal frequency of 60Hz
to 10kHz, while the LED dimming frequency, of both the main and color-adjust strings, is 400Hz. The duty cycle of the
color-adjust string is based on the duty cycle of the signal at the PWM input, but compensated for temperature based on
a programmable look-up table, whose defaults are presented in Table 11-2 on page 21. See “Light Color and the THM
Input” on page 20 for temperature adjustment information.
Figure 11-3. LED current and duty cycle control.
11.8 Light Color and the THM Input
The overall color of the light generated by the two LED strings is a blend of the main string’s white LEDs and the color-
adjust string’s color LEDs. Brightness is primarily controlled by the duty cycles of the PWM signals driving the LEDs. The
brightness of white LEDs is relatively constant over temperature, but the brightness of color LEDs may drop significantly
as temperature increases. The main string’s PWM duty cycle is fixed at the duty cycle of the input PWM signal, but the
duty cycle of the color-adjust string is changed as the LED temperature changes, to keep the blended light color
constant.
The thermistor input, THM, monitors the temperature of an external thermistor connected from THM to ground. A fixed
current is forced out THM to generate a voltage that is proportional to the thermistor’s temperature. The THM voltage is
measured by a 8-bit ADC internal to the MSL2021. When used with the suggested thermistor (
ERT-J0EG103FA or
equivalent
) in series with a 1.5k resistor, THM measures temperatures from 18
o
C to 80
o
C with 2
o
C resolution, for 32
different temperature values. When the temperature is below 18
o
C, 18
o
C is returned by the temperature monitor circuit.
When the temperature is above 80
o
C, 80
o
C is returned by the temperature monitor circuit. The temperature information
is fed to the color-adjust string’s duty cycle circuit.
G
D
DRV
CS
S
+
-
+
-
R
D
R
S
R
CS
PWM
PWM ENGINE
400Hz CLOCK
DAC
0x20
DAC
0x21
THM
THERMAL
MONITOR
EN
EN
MSL2021