Datasheet

19Atmel LED Driver-MSL1060
IADJ Input - LED Temperature Compensation
and/or Ambient Light Sensing
The MSL1060 analog control input, IADJ, allows reduction
of the full-scale static LED current from the current set by
resistor R5. IADJ controls the LED current proportionally
to the voltage on IADJ, between 0V and 1.22V. For
example, setting IADJ to 0V reduces the full-scale static
LED current to zero. Setting IADJ to 0.61 V reduces the
full-scale static LED current to half. IADJ voltage greater
than 1.22V retains full-scale static LED current. If analog
control is not used, connect IADJ to VDD.
For automatic temperature compensation, connect a
negative-temperature-co-efficient (NTC) thermistor
from VDD to IADJ and a resistor from IADJ to GND. As
temperature rises, the voltage at IADJ also increases,
effectively increasing the LED current.
For automatic ambient light compensation, connect a
photodiode or integrated ambient light sensor (ALS) to
the IADJ input. As ambient light intensives, the voltage at
IADJ also increases, effectively increasing the LED current.
Boost Regulator Components
The internally compensated boost regulator, includes an
internal, high-voltage FET power switch, and requires
only an inductor, rectifier, and bypass capacitors to
operate. The current mode boost regulator operates in
continuous conduction mode (CCM) or discontinuous
conduction mode (DCM). In CCM, the inductor current
does not fall to zero when operating at full power, which
reduces inductor ripple current and switching noise. The
boost regulator and internal compensation switches at
1.1MHz. Use 115kΩ for resistor R4 from OSC to GND
(Figure 16) to set the oscillator frequency. Select the
inductor, rectifier diode, and output capacitors according
to the following guidelines.
To select inductor use the following equations
( )
( ) ( ) ( )
swin
out
in
in
fI
V
V
V
L
**3.0
1*








−
=
Where
Vout = Forward voltage of LED string + 0.6V
Vin = Input voltage
f
sw
= Switching frequency of boost controller, this is
1.1MHz for MSL1060
and
in
outtotalLED
in
V
VI
I
*9.0
*
)(
=
Choose an inductor with a peak current rating > 2A.
Common inductor values can range from 4.7µH to 22µH.
To minimize losses in the rectifier, choose a fast-
switching diode with low forward drop. Ensure that the
rectifier can withstand a reverse voltage equal to the
regulator output voltage. The average forward current
is equal to the total LED string current (for example 6
strings x 30mA = 180mA), while the peak current is
equal to the inductor peak current (2A).
The boost output capacitor holds the voltage at the
output of the boost regulator while the internal power
switch is on. Use ceramic boost output capacitors
because of their small size and high ripple current
capacity. Derate ceramic capacitors for operating voltage
because the voltage coefficient decreases the effective
capacitance with increased operating voltage. Use two
parallel-connected 1µF 100V X7R ceramic capacitors and
a 10µH inductor with a 1.7A peak current rating.
Over-voltage Protection (OVP) - R8 and R9
The OVP input sets the boost regulator’s output voltage
upper limit, and protects the boost regulator from
common faults such as open circuit LEDs. Resistors R8
and R9 (Figure 16) set the OVP voltage V
TRIP
is set by:
9
)98(
R
RR
VV
OVPTRIP
+
=
where V
OVP
=1.28V, nominal.
Atmel LED Driver-MSL1060
6-string PWM LED Driver with Digitally Compensated, 1.1MHz,
48V Boost Regulator and ±1.5% LED String Current Balance