MIC3205 Datasheet, PDF (16/23 Pages) Micrel Semiconductor – High-Brightness LED Driver Controller with Fixed-Frequency Hysteretic Control

Micrel, Inc.
MIC3205
Figure 3. Input Filter for Low Values of ∆V_HYS
Trace Routing Recommendation
Keep the power traces as short and wide as possible.
There is one current flowing loop during the MOSFET
ON-time; the traces connect the input capacitor (C_IN),
R_CS, the LEDs, the inductor, the MOSFET, and back to
C_IN. There is another current flowing loop during the
MOSFET OFF-time; the traces for this loop connect R_CS,
the LED, the inductor, the freewheeling diode, and back
to R_CS. These two loop areas should kept as small as
possible to minimize noise interference
Keep all analog signal traces away from the switching
node and its connecting traces.
Design Example
SPECIFICATIONS:
F_SW = 400 kHz
V_SUPPLY = 24V rectified AC
I_LED = 1A
Voltage drop per LED = 3.5V
Number of LEDs = 4
Schottky diode drop at 1A = 0.4V
1. CURRENT SENSE RESISTOR
From
Equation
1:
RCS

200mV
ILED
For I_LED = 1A
R_CS = 0.2Ω
2. SWITCHING FREQUENCY
From
Equation
2:
FSW

2.22  10^-4
CT
For F_SW = 400 kHz
C_T = 550pF
The actual frequency may vary as explained in “Frequency
of Operation” in the “Application Information” section.
3. INDUCTOR SELECTION
From Equation 9:
L

(VIN
-
ILED

RCS - VLED)  (VD  ILED  RCS
( VIN  VD)  VHYS  FSW

V LED)

RCS
Given V_SUPPLY = 24V rectified AC
The peak voltage = √2 x V_SUPPLY
Thus for MIC3205, V_IN ≈ 34V
V_LED = 3.5 x 4 = 14, V_D = 0.4V
Select ∆V_HYS = 60mV
Thus, L = 70µH
Chose L = 68µH as closest available value.
As a side note, for this example, L = 68µH can be used even
if V_SUPPLY = 24V DC. This is because ∆V_HYS calculates to
around 44mV (with V_IN = V_SUPPLY = 24V) which is acceptable.
From Equations 12 and 13:
I_L(PK) = 1.15A
Thus, we choose L = 68µH with an RMS saturation current
of 1.5A or higher.
4. MOSFET SELECTION
For this example, V_IN = 34V, a 50V rating or greater N-
channel MOSFET is required. A high current rating MOSFET
is a good choice because it has lower R_DSON.
A 60V, 12A MOSFET with 10mΩ R_DSON is a good choice.
5. CAPACITOR SELECTION
Use a 10µF/50V X7R type ceramic capacitor for the input
capacitor.
Use a 4.7µF/50V X5R type ceramic capacitor for the LED
ripple current carrying capacitor connected across the series
connection of 4 LEDs
6. FREEWHEELING DIODE SELECTION
With V_IN = 34V, choose a 2A, 60V Schottky diode with a
forward drop voltage of 0.4V at 1A forward current.
October 2012
16
M9999-102312-A