Datasheet
16
Atmel MSL2010 [DATASHEET]
42072A–AVR–05/2013
where 2.5V is V
FB
, the feedback regulation voltage of the FB input. The Efficiency Optimizer (EO) output, FBO, connects
to FB and pulls current from the node to force the converter’s output voltage up to the proper level to keep the LEDs in
current regulation with the minimum power loss.
12.2.2 The Boost MOSFET and Schottky Diode
Output DRV drives the gate of an external N-Channel MOSFET at up to V
PVIN
– 1.5V. Select a MOSFET with a maximum
drain-source voltage of at least 1V above V
OUT(MAX)
(from above), a low gate charge and low R
DS(ON)
. Use a Schottky
diode with a maximum reverse voltage of at least V
OUT(MAX)
.
12.2.3 The Off-Time Resistor, R
TOFF
The Boost driver uses a constant off-time algorithm. The MOSFET turns off when V
CS
= 0.2V (Figure 12-1 on page 15).
Control off-time with external resistor R
TOFF
using:
,
where t
OFF
is in seconds. For example, an off-time of 4.4s results in a resistor value of 402k (to the nearest 1% value).
12.2.4 Setting the Inductor Current with R
CS
The Boost inductor peak current regulates by way of the voltage at CS, the Boost MOSFET source resistor connection.
Connect the current sense resistor R
CS
from the MOSFET source to ground. The default feedback voltage for CS is 0.2V.
Choose the current sense resistor using:
,
where I
L
is the inductor peak current. For example, for 350mA I
L
, use R
CS
of 0.56 Ohms. The 0.2V in this equation is set
in the Current Sense Reference Voltage (CSREF) register 0x21. The default value for CSREF is 0x64, which sets the
feedback voltage to 0.2V. For information about CSREF see “Boost Reference Voltage register (CSREF, 0x21)” on page
20.
12.2.5 Sizing The Inductor, L
Current increases through the inductor until the voltage at CS reaches 0.2V. The MOSFET then turns off for the off-time
programmed by R
TOFF
(above). Estimate the inductor value using:
H,
where v(t) = the acceptable ripple voltage of V
LED
, di(t) = acceptable peak to peak inductor ripple current, and dt = the off-
time set by R
TOFF
.
For example:
With an inductor ripple voltage of 0.5% of V
OUT(MAX)
(from above), v(t) = 35.2 (0.005) = 176mV.
With an average load current of 350mA and an inductor ripple current of 4%, di(t) = 0.35 0.04 = 0.014A.
And, with a t
OFF
of 4.4µs, then:
.
9
109.90
OFFTOFF
tR
L
CS
I
R
2.0
dt
tdi
tv
L
)(
)(
HL
55
104.4
014.0
176.0
6