Datasheet

15
Atmel MSL2010 [DATASHEET]
42072A–AVR–05/2013
12.2 The Boost Regulator
MSL2010 includes a Constant Off-Time DC/DC Boost Controller to generate V
LED
, the voltage for the LED string. The
Boost components are shown in Figure 12-1 on page 15, and include a voltage setting resistor divider, a MOSFET, a
Schottky diode, a resistor to set the Off-Time, an Inductor, a current sense resistor, a Drain resistor, a few capacitors and
a compensation network. The following sections discuss selecting these components.
Figure 12-1. Boost Regulator Components
12.2.1 Setting the Boost Output Voltage, V
LED
A resistor divider sets the nominal Boost output voltage, V
LED
. Select the two feedback resistors by first determining the
minimum output voltage using:
V,
where V
fMIN
is the minimum LED forward voltage at the expected LED current, N is the number of LEDs in the string, and
0.2V is the minimum overhead required for the current sense resistor and the FET. Then determine the maximum output
voltage using:
V,
where V
fMAX
is the maximum LED forward voltage at the expected LED current, N is the number of LEDs in the string,
and 1.2V is the maximum overhead required for the current sense resistor and the FET.
Determine the value for the upper voltage setting resistor using:
,
where 340A is the guaranteed maximum FBO current from the “Electrical Characteristics” on page 4. Determine the
lower resistor using:
,
V
DRV
MSL2010
LED DRIVER
CS
+
FBO
TO
LEDS
R
TOP
R
BOTTOM
TOFF
R
TOFF
L
C
IN
R
CS
D
FBEAO
Q1
R
C
C
C2
C
C1
C
OUT
-
IN
V
LED


2.0
)(
 NVV
fMINMINOUT


2.1
)(
 NVV
fMAXMAXOUT
6
)()(
10340




MINOUTMAXOUT
TOP
VV
R
5.2
5.2
)(


MINOUT
TOPBOTTOM
V
RR