MIC3263 Datasheet, PDF (23/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
Design Procedure for a LED Driver
Symbol
Input
V_IN
I_IN
Output
LEDs
Chs
V_F
V_IO
V_OUT
I_LED/ch
I_OUT
P_OUT
DIM IN
F_DIM
OVP
F_SW
eff
V_DIODE
Parameter
Input Voltage
Input Current
Number of LEDs
Number of Channels
Forward Voltage of LED
Voltage Drop at the IO Pin
Output Voltage
LED Current/Channel
Output Power
PWM Dimming
Dimming Frequency (internal)
Output Over-Voltage Protection
Switching Frequency
Efficiency
Forward Drop of Schottky Diode
MIC3263
Minimum
8
Nominal
12
Maximum
14
Units
V
8/Channel
6
3.4
1.1
28
30
1
80
8/Channel
6
3.6
1.2
30
30
5
1
85
0.5
8/Channel
6
4.0
2
34
30
0.18
6.2
100
40
Channels
V
V
mA
A
W
%
kHz
V
MHz
%
V
Design Example
In this example, a boost six-channel LED driver
operating off a 12V input is being designed. This design
has been created to drive six channels of eight
LEDs/channels for a total of 48 LEDs. The LED current
will be set at 30mA. One is designing for 80% minimum
efficiency at a switching frequency of 1MHz.
For 34V out:
Let R2=150k,
R1= ^{VCRV ×R2} = ^{1.8V ×150kΩ} = 8.39kΩ
V_OUT - V_CRV 34V - 1.8V
Therefore: V_OVP= 2.4* (R2+R2)/R1=45V. 45V is too high,
meaning V_CRV has to operate at a higher voltage than
1.8V. The CRV loop will charge the CRV capacitor to the
necessary voltage to regulate.
Let V_CRV = 2.2V therefore:
R1 =
V_CRV × R2
2.2V × 150kΩ
=
= 10.4kΩ
V_OUT - V_CRV 34V - 2.2V
Use the closest standard value of 10.5kΩ.
Therefore:
V_OVP = 2.4* (R2+R2)/R1 = 40V
Select R_ISET for a Given ILED
R_ISET
=
60
I_LED
Ω
=
60
30mA
=
2k Ω
Use 2kΩ for R_ISET (R9)
January 2010
23
M9999-012110