Datasheet

17
Atmel MSL2010 [DATASHEET]
42072A–AVR–05/2013
Assure that the inductor saturation current rating is greater than I
L
, the peak inductor current from the R
CS
equation
above.
12.2.6 The Drain Resistor, R
D
The drain resistor, R
D
in Figure 12-1 on page 15, connects the MSL2010 to the Drain of the LED string external
MOSFET. Use a 100k for R
D
.
12.2.7 The Input and Output Capacitors
The input and output capacitors carry the high frequency current of the Boost regulator switching. The input capacitor
prevents this high frequency current from travelling back to the input voltage source, reducing conducted and radiated
noise. The output capacitor prevents high frequency current to the load, in this case the LEDs, and also prevents
conducted and radiated noise. The output capacitors also have a large effect on the Boost regulator loop stability and
transient response, and so are critical to optimal Boost regulator operation. Use capacitors that keep their rated values at
their expected operating voltages. The “Typical Application Circuit” on page 14 shows recommended values for these
capacitors.
12.2.8 The Compensation Network
The compensation components, R
C
, C
C1
and C
C2
in Figure 12-1 on page 15, effectively dampen the oscillation-producing
high frequency response of the Boost regulator error amplifier. The “Typical Application Circuit” on page 14 shows
recommended values for these capacitors.
12.3 The LED string
12.3.1 Selecting the LED String MOSFET
The LED string MOSFET sinks the LED current to ground through current sense resistor R
S
. Output G drives the gate of
the MOSFET with at up to at least V
AVIN
– 1.5V. Select a FET with a maximum drain-source voltage of at least V
OUT(MAX)
(from above), and a low R
DS(ON)
.
12.3.2 Setting LED String On-Current with R
S
The LED string on-current regulates by monitoring the voltage at S, the MOSFET source resistor connection. Connect
the current sense resistor R
S
from the MOSFET source to ground. The default feedback voltage for S is 0.2V. Choose
the string current sense resistor R
S
using:
,
where I
LED
is the LED string regulation current. The LED String Reference Voltage (SREF), register 0x20, sets the 0.2V
feedback voltage, at 2mV per LSB. For more information about SREF see the “LED String Reference Voltage register
(SREF, 0x20)” on page 20.
12.3.3 PWM and LED Brightness
MSL2010 uses the input signal at PWM to directly control the duty cycle (brightness) and frequency of the LED string
driver output G. The PWM input accepts a signal of 60Hz to 10kHz, 1% to 100% duty cycle; output G operates with a
minimum on time of 2µs.

LED
S
I
R
2.0