MIC3205YML-TR [MICROCHIP]
LED DISPLAY DRIVER;
| 型号： | MIC3205YML-TR |
| 厂家： | 
MICROCHIP |
| 描述： | LED DISPLAY DRIVER 驱动 光电二极管 接口集成电路 |
| 文件： | 总23页 (文件大小：958K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
MIC3205
High-Brightness LED Driver Controller
with Fixed-Frequency Hysteretic Control
General Description
Features
The MIC3205 is a hysteretic, step-down, high-brightness
LED (HB LED) driver with a patent pending frequency
regulation scheme that maintains a constant operating
frequency over input voltage range. It provides an ideal
solution for interior/exterior lighting, architectural and
ambient lighting, LED bulbs, and other general illumination
applications.
4.5V to 40V input voltage range
Fixed operating frequency over input voltage range
High efficiency (90%)
5% LED current accuracy
High-side current sense
Dedicated dimming control input
Hysteretic control (no compensation!)
Up to 1.5MHz switching frequency
Adjustable constant LED current
Over-temperature protection
The MIC3205 is well suited for lighting applications
requiring a wide input voltage range. The hysteretic control
provides good supply rejection and fast response during
load transients and PWM dimming. The high-side current
sensing and on-chip current-sense amplifier deliver LED
current with 5% accuracy. An external high-side current-
sense resistor is used to set the output current.
–40C to 125C junction temperature range
Applications
The MIC3205 offers a dedicated PWM input (DIM) which
enables a wide range of pulsed dimming. A high-frequency
switching operation up to 1.5MHz allows the use of smaller
external components minimizing space and cost.
Architectural, industrial, and ambient lighting
LED bulbs
Indicators and emergency lighting
Street lighting
The MIC3205 operates over a junction temperature from
–40°C to +125°C and is available in a 10-pin 3mm x 3mm
MLF® package.
Channel letters
12V lighting systems (MR-16 bulbs, under-cabinet
Data sheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
lighting, garden/pathway lighting)
_________________________________________________________________________________________________________________________
Typical Application
Normalized Switching Frequency
vs. Input Voltage
2.0
ILED = 1A
RCS = 0.2Ω
1.5
1 LED
L = 22µH
1.0
4
LED
10 LED
L = 33µH
6 LED
L = 68µH
L = 47µH
0.5
0.0
0
9
18
27
36
45
INPUT VOLTAGE (V)
MIC3205 Buck LED Driver
MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
M9999-102312-A
October 2012
Micrel, Inc.
MIC3205
Ordering Information
Part Number
MIC3205YML
Note:
Junction Temperature Range
Package(1)
10-Pin 3mm x 3mm MLF
40°C to 125°C
1. MLF is a GREEN RoHS-compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free.
Pin Configuration
10-Pin 3mm x 3mm MLF (ML)
Top View
Pin Description
Pin Number Pin Name Pin Function
Voltage Regulator Output. The VCC pin is the output of a linear regulator powered from VIN, which
supplies power to the internal circuitry. A 4.7µF ceramic capacitor is recommended for bypassing. Place
it as close as possible to the VCC and AGND pins. Do not connect to an external load.
1
2
3
VCC
CS
Current Sense Input. Negative input to the current sense comparator. Connect an external sense
resistor to set the LED current. Connect the current sense resistor as close as possible to the chip.
Input Power Supply. VIN is the input supply pin to the internal circuitry. Due to high frequency switching
noise, a 10µF ceramic capacitor is recommended for bypassing and should be placed as close as
possible to the VIN and PGND pins. See “PCB Layout Guidelines.”
VIN
VIN Sense. Positive input to the current sense comparator. Connect as close as possible to the current
sense resistor.
4
5
VINS
AGND
Analog Ground. Ground for all internal low-power circuitry.
Enable Input. Logic high (greater than 2V) powers up the regulator. A logic low (less than 0.4V) powers
down the regulator and reduces the supply current of the device to less than 2µA. A logic low pulls down
the DRV pin turning off the external MOSFET. Do not drive the EN pin above VIN. Do not leave floating.
6
EN
PWM Dimming Input. A PWM input can be used to control the brightness of the LED. Logic high (greater
than 2V) enables the output. Logic low (less than 0.4V) disables the output regardless of the EN state.
Do not drive the DIM pin above VIN. Do not leave floating.
7
8
9
DIM
Timer Capacitor. A capacitor is required from CTIMER to ground sets the target switching frequency
using the equation CTIMER=2.22*10-4 / FSW
CTIMER
PGND
Power Ground. Ground for the power MOSFET gate driver. The current loop for the power ground
should be as small as possible and separate from the analog ground loop. See “PCB Layout
Recommendations.”
Gate Drive Output. Connect to the gate of an external N-channel MOSFET. The drain of the external
MOSFET connects directly to the inductor and provides the switching current necessary to operate in
hysteretic mode.
10
DRV
EP
ePAD
Exposed Pad. Must be connected to a GND plane for best thermal performance.
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October 2012
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Micrel, Inc.
MIC3205
Absolute Maximum Ratings (1)
Operating Ratings (2)
Supply Voltage (VIN).......................................... 4.5V to 40V
Enable Voltage (VEN) .............................................. 0V to VIN
VIN to PGND .................................................. 0.3V to 42V
VINS to PGND.........................................0.3V to (VIN+0.3V)
Dimming Voltage (VDIM .................................................................0V to VIN
)
VCC to PGND ................................................ 0.3V to 6.0V
Junction Temperature (TJ) ........................ 40C to 125C
Junction Thermal Resistance
CS to PGND........................................ 0.3V to (VIN 0.3V)
EN to AGND........................................ 0.3V to (VIN 0.3V)
DIM to AGND ...................................... 0.3V to (VIN 0.3V)
CTIMER to AGND.............................. 0.3V to (VCC 0.3V)
DRV to PGND ....................................0.3V to (VCC 0.3V)
PGND to AGND .......................................... 0.3V to 0.3V
Junction Temperature ................................................ 150C
Storage Temperature Range ....................60°C to 150C
Lead Temperature (Soldering, 10sec) ....................... 260C
ESD Ratings (3)
10-pin 3x3 MLF (JA).......................................60.7C/W
10-pin 3x3 MLF (JC).......................................28.7C/W
HBM......................................................................1.5kV
MM.........................................................................200V
Electrical Characteristics (4)
VIN = VEN = VDIM = 12V; CVCC = 4.7µF; TJ = 25C; bold values indicate 40C TJ 125C, unless noted.
Symbol
Input Supply
VIN
Parameter
Condition
Min.
Typ.
Max.
Units
Input Voltage Range (VIN)
Supply Current
4.5
40
3
V
IS
DRV = Open
1.3
mA
µA
V
ISD
Shutdown Current
VEN = 0V
2
UVLO
VIN UVLO Threshold
VIN UVLO Hysteresis
VIN Rising
3.2
4.5
4
4.5
UVLOHYS
VCC Supply
VCC
600
mV
VCC Output Voltage
VIN = 12V, ICC = 5mA
5
5.5
V
Current Sense
190
200
200
50
210
mV
mV
ns
Average Current Sense
Threshold
∆VCS
∆VCS =VINS VCS
188
212
VCS Rising
VCS Falling
VIN = VCS
Current Sense Response
Time
∆tCS
70
ns
ICS
CS Input Current
0.5
10
µA
V
IN =12V, VLED =3V,
∆VHYS
Sense Voltage Hysteresis (5)
L=47µH, FSW =250kHz,
VD = 0.7V, ILED = 1A
46
mV
Frequency
ITIMER
CTIMER Pull-up Current
CTIMER Threshold
66
µA
V
VCTREF
1.189
(4*ITIMER)/
VCTREF
Frequency Coefficient (6)
1.776 × 10-4
2.22 × 10-4
2.664 × 10-4
A/V
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October 2012
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Micrel, Inc.
MIC3205
Electrical Characteristics (4) (Continued)
VIN = VEN = VDIM = 12V; CVCC = 4.7µF; TJ = 25C; bold values indicate 40C TJ 125C, unless noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
Enable Input
ENHI
EN Logic Level High
EN Logic Level Low
2.0
V
V
ENLO
0.4
60
1
VEN = 12V
VEN = 0V
20
65
µA
µA
IEN
EN Bias Current
Start-Up Time
From EN pin going high to DRV
going high
tSTART
µs
Dimming Input
DIMHI
DIMLO
DIM Logic Level High
2.0
V
V
DIM Logic Level Low
0.4
50
1
VDIM = 12V
VDIM = 0V
20
µA
µA
IDIM
DIM Bias Current
From DIM pin going high to DRV
going high
tDIM
fDIM
DIM Delay Time
450
2
ns
%
Maximum Dimming Frequency
% of switching frequency
External FET Driver
Pull-Up, ISOURCE = 10mA
Pull-Down, ISINK = -10mA
Rise Time, CLOAD = 1000pF
Fall Time, CLOAD = 1000pF
4
1.5
13
7
ꢀ
ꢀ
RON DRV On-Resistance
ns
ns
tDRV
DRV Transition Time
Thermal Protection
TLIM
Overtemperature Shutdown
Overtemperature Shutdown Hysteresis
TJ Rising
160
20
C
C
TLIMHYS
Notes:
1. Exceeding the absolute maximum rating can damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Devices are ESD sensitive. Handling precautions are recommended. Human body model, 1.5kꢀ in series with 100pF.
4. Specification for packaged product only.
5. See “Sense Voltage Hysteresis Range” in the “Application Information” section.
6. See “Frequency of Operation” in the “Application Information” section.
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October 2012
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Micrel, Inc.
MIC3205
Typical Characteristics
Efficiency (ILED = 1A)
vs. Input Voltage
VIN Supply Current
vs. Input Voltage
VIN Shutdown Current
vs. Input Voltage
100
95
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.0
0.8
0.6
0.4
0.2
0.0
ILED = 0A
A = 25°C
VEN = 0V
ILED = 0A
TA = 25°C
T
90
4
LED
L = 47µH
85
80
75
70
65
60
6 LED
L = 68µH
10 LED
L = 33µH
1 LED
L = 22µH
0
0
0
9
18
27
36
45
45
45
0
9
18
27
36
45
45
45
0
9
18
27
36
45
45
45
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
VCC Output Voltage
vs. Input Voltage
ILED Output Current
vs. Input Voltage
Normalized Switching Frequency
vs. Input Voltage
6.0
5.5
5.0
4.5
4.0
2.0
1.5
1.0
0.5
0.0
1.10
1.05
1.00
0.95
0.90
TA = 25°C
ILED = 1A
ILED = 1A
RCS = 0.2ꢀ
TA = 25°C
RCS = 0.2ꢀ
1 LED
L = 22µH
1 LED
L = 22µH
6 LED
L = 68µH
10 LED
L = 33µH
4
LED
6 LED
L = 68µH
L = 47µH
4
LED
L = 47µH
9
18
27
36
0
9
18
27
36
0
9
18
27
36
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
CTIMER Current
vs. Input Voltage
Enable Threshold
vs. Input Voltage
Enable Bias Current
vs. Input Voltage
70
68
66
64
62
60
1.5
1.2
0.9
0.6
0.3
0.0
100
80
60
40
20
0
ILED = 1A
TA = 25°C
VEN = VIN
TA = 25°C
ILED = 0A
VEN = VIN
TA = 25°C
RISING
FALLING
HYST
9
18
27
36
0
9
18
27
36
0
9
18
27
36
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
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October 2012
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Micrel, Inc.
MIC3205
Typical Characteristics (Continued)
Enable Bias Current
vs. Enable Voltage
Thermal Shutdown
vs. Input Voltage
VIN Supply Current
vs. Temperature
100
80
60
40
20
0
200
160
120
80
2.0
1.8
1.6
1.4
1.2
1.0
VEN ≠ VIN
TA = 25°C
ILED = 0A
VIN = 12V
LED = 0A
VIN = 42V
I
RISING
FALLING
ILED = 1A
40
HYST
0
0
9
18
27
36
45
125
125
0
9
18
27
36
45
125
125
-50
-25
0
25
50
75
100
125
INPUT VOLTAGE (V)
TEMPERATURE (°C)
ENABLE VOLTAGE (V)
Switching Frequency
vs. Temperature
VIN Shutdown Current
vs. Temperature
ILED Output Current
vs. Temperature
530
510
490
470
450
430
2.0
1.6
1.2
0.8
0.4
0.0
1.03
1.02
1.01
1.00
0.99
0.98
VIN = 12V
VLED = 3.5V
RCS = 0.2ꢀ
VIN = 12V
ILED = 0A
VEN = 0V
VIN = 12V
VLED = 3.5V
L = 22µH
CT = 470pF
R
CS = 0.2ꢀ
-50
-25
0
25
50
75
100
125
-50
-25
0
25
50
75
100
-50
-25
0
25
50
75
100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
VCC
Enable Threshold
vs. Temperature
Enable Bias Current
vs. Temperature
vs. Temperature
6.0
5.5
5.0
4.5
4.0
1.6
1.2
0.8
0.4
0.0
30
25
20
15
10
VIN = 12V
ILED = 1A
VIN = 12V
LED = 1A
VIN = 12V
ILED = 0A
VEN = 12V
I
RISING
FALLING
HYST
-50
-25
0
25
50
75
100
-50
-25
0
25
50
75
100
-50
-25
0
25
50
75
100
125
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
M9999-102312-A
October 2012
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Micrel, Inc.
MIC3205
Typical Characteristics (Continued)
VIN UVLO Threshold
vs. Temperature
5
RISING
4
FALLING
3
2
1
HYST
0
-50
-25
0
25
50
75
100
125
TEMPERATURE (°C)
M9999-102312-A
October 2012
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Micrel, Inc.
MIC3205
Functional Characteristics
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October 2012
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Micrel, Inc.
MIC3205
Functional Characteristics (Continued)
M9999-102312-A
October 2012
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Micrel, Inc.
MIC3205
Functional Diagram
Figure 1. MIC3205 Block Diagram
M9999-102312-A
October 2012
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Micrel, Inc.
MIC3205
The MIC3205 has an EN pin that gives the flexibility to
enable and disable the output with logic high and low
signals. The maximum EN voltage is VIN.
Functional Description
The MIC3205 is a hysteretic step-down driver that
regulates the LED current with a patent pending
frequency regulation scheme. This scheme maintains a
fixed operating frequency over a wide input voltage
range.
Theory of Operation
The device operates from a 4.5V to 40V input MOSFET
voltage. At turn-on, after the VIN input voltage crosses
4.5V, the DRV pin is pulled high to turn on an external
MOSFET. The inductor and series LED current builds up
linearly. This rising current results in a rising differential
voltage across the current sense resistor (RCS). When
this differential voltage reaches an upper threshold, the
DRV pin is pulled low, the MOSFET turns off, and the
Schottky diode takes over and returns the series LEDs
and inductor current to VIN. Then, the current through the
inductor and series LEDs starts to decrease. This
decreasing current results in a decreasing differential
voltage across RCS. When this differential voltage
reaches a lower threshold, the DRV pin is pulled high,
the MOSFET is turned on, and the cycle repeats. The
average of the CS pin voltage is 200mV below VIN
voltage. This is the average current sense threshold
(∆VCS). Thus, the CS pin voltage switches about VIN
–
200mV with a peak-to-peak hysteresis that is the product
of the peak-to-peak inductor current times the current
sense resistor (RCS). The average LED current is set by
RCS, as explained in the “Application Information”
section.
Figure 2. Theory of Operation
LED Dimming
The MIC3205 LED driver can control the brightness of the
LED string through the use of pulse width modulated (PWM)
dimming. A DIM pin is provided, which can turn on and off
the LEDs if EN is in an active-high state. This DIM pin
controls the brightness of the LED by varying the duty cycle
of DIM pin from 1% to 99%.
MIC3205
dynamically
adjusts
hysteresis
to
accommodate fixed-frequency operation. Average
frequency is programmed using an external capacitor
connected to the CTIMER pin, as explained in the
“Frequency of Operation” subsection in the “Application
Information” section. The internal frequency regulator
dynamically adjusts the inductor current hysteresis every
eight switching cycles to make the average switching
frequency a constant. If the instantaneous frequency is
higher than the programmed average value, the
hysteresis is increased to lower the frequency and vice
versa. In other hysteretic control systems, current sense
hysteresis is constant and frequency can change with
input voltage, inductor value, series LEDs voltage drop,
or LED current. However, with this patent pending
frequency regulation scheme, the MIC3205 changes
inductor current hysteresis and keeps the frequency
fixed even upon changing input voltage, inductor value,
series LEDs voltage drop, or LED current.
An input signal from DC up to 20kHz can be applied to the
DIM pin (see “Typical Application”) to pulse the LED string
on and off. A logic signal can be applied on the DIM pin for
dimming, independent of input voltage (VIN). Using PWM
dimming signals above 120Hz is recommended to avoid any
recognizable flicker by the human eye. Maximum allowable
dimming frequency is 2% of operating frequency that is set
by the external capacitor on the CTIMER pin (see
“Frequency of Operation”). See “Functional Characteristics”
on page 9 for PWM dimming waveforms. Maximum DIM
voltage is VIN.
PWM dimming is the preferred way to dim an LED to prevent
color/wavelength shifting. Color/wavelength shifting occurs
with analog dimming. By using PWM dimming, the output
current level remains constant during each DIM pulse. The
hysteretic buck converter switches only when the DIM pin is
high. When the DIM pin is low, no LED current flows and the
DRV pin is low turning the MOSFET off.
The MIC3205 has an on-board 5V regulator, which is for
internal use only. Connect a 4.7µF capacitor on VCC pin
to analog ground.
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October 2012
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Micrel, Inc.
MIC3205
CTIMER pin, gives the average frequency of operation, as
seen in the following equation:
Application Information
The internal block diagram of the MIC3205 is shown in
Figure 1. The MIC3205 is composed of a current-sense
comparator, voltage reference, frequency regulator, 5V
regulator, and MOSFET driver. Hysteretic mode control,
also called bang-bang control, is a topology that does
not use an error amplifier, instead using an error
comparator.
2.22 10-4
FSW
Eq. 2
CT
The actual average frequency can vary depending on the
variation of the frequency co-efficient and the parasitic board
capacitances in parallel to the external capacitor CT. As
shown in the Electrical Characteristics table, part to part
variation for the frequency co-efficient is ±20% over
temperature, from the target frequency co-efficient of
2.22 × 10-4.
The frequency regulator dynamically adjusts hysteresis
for the current sense comparator to regulate frequency.
The inductor current is sensed by an external sense
resistor (RCS) and controlled within a hysteretic window.
It is a simple control scheme with no oscillator and no
loop compensation. The control scheme does not need
loop compensation. This makes design easy, and avoids
instability problems.
Switching frequency selection is based on the trade-off
between efficiency and system size. Higher frequencies
result in smaller, but less efficient, systems and vice versa.
The operating frequency is independent of input voltage,
inductor value, series LEDs voltage drop, or LED current, as
long as 40mv ≤ ∆VHYS ≤ 100mV is maintained as explained
in the next sections.
Transient response to load and line variation is very fast
and depends only on propagation delay. This makes the
control scheme very popular for certain applications.
Sense Voltage Hysteresis Range
LED Current and RCS
The frequency regulation scheme requires that the
hysteresis remain in a controlled window. Components and
operating conditions must be such that the hysteresis on the
CS pin is between 40mV and 100mV.
The main feature in MIC3205 is that it controls the LED
current accurately within 5% of set current. Choosing a
high-side RCS resistor is helpful for setting constant LED
current regardless of wide input voltage range. The
following equation and Table 1 give the RCS value for
required LED current:
Hysteresis less than 40mV or more than 100mV can result in
loss of frequency regulation.
After average LED current (ILED) has been set by RCS and
operating frequency has been set by external capacitor CT,
the hysteresis ∆VHYS is calculated as follows:
200mV
RCS
Eq. 1
ILED
As seen in Figure 2, for the inductor,
RCS (Ω)
ILED (A)
0.15
0.35
0.5
I2R (W)
0.03
0.07
0.1
Size (SMD)
0603
1.33
0.56
0.4
VHYS
RCS
IL
Eq. 3
0805
0805
where:
0.28
0.2
0.7
0.137
0.2
0805
∆IL = inductor ripple current
∆VHYS = hysteresis on CS pin
1.0
1206
0.13
0.1
1.5
0.3
1206
2.0
0.4
2010
For rising inductor current (MOSFET is on):
0.08
0.068
2.5
0.5
2010
L IL
VL_RISE
3.0
0.6
2010
tr
Eq. 4
Table 1. RCS for LED Current
where:
VL_RISE = VIN ILED × RCS VLED
LED is the total voltage drop of the LED string
VIN is the input voltage
CS is the current sense resistor
LED is the average LED current
Frequency of Operation
V
The patent pending frequency regulation scheme allows
for operating frequency to be programmed by an
external capacitor from the CTIMER pin to AGND. The
frequency co-efficient (typically 2.22 × 10-4 A/F) divided
by the value of this external capacitor connected to the
R
I
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October 2012
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Micrel, Inc.
MIC3205
tr is the MOSFET ON-time
L is the inductor
∆VHYS
RCS (ꢀ)
ILED (A)
VIN (V)
L (µH)
(mV)
0.56
0.56
0.28
0.28
0.2
0.35
0.35
0.7
5
12
5
22
68
10
33
6.8
22
3.6
10
64.1
57.7
70.5
59.4
72.6
62.4
68.5
68.6
For falling inductor current (MOSFET is off):
L IL
VL_FALL
0.7
12
5
tf
Eq. 5
1.0
1.0
2.0
2.0
0.2
12
5
where:
VL_FALL = VD + ILED × RCS VLED
0.1
0.1
12
VD is the freewheeling diode forward drop
tf is the MOSFET OFF-time
Table 2. Inductor for FSW = 400 kHz, VD = 0.4V, 1 LED
∆VHYS
Operating frequency and time period are given by:
1
RCS (ꢀ)
ILED (A)
VIN (V)
L (µH)
(mV)
55.8
56.8
61.6
62.5
62.4
64.3
66.6
66.2
0.56
0.56
0.28
0.28
0.2
0.35
0.35
0.7
0.7
1.0
1.0
2.0
2.0
24
36
24
36
24
36
24
36
150
220
68
FSW
Eq. 6
T
100
47
T tr tf
Eq. 7
Using Equations 3, 4, 5, 6, and 7:
0.2
68
(VIN -ILED RCS - VLED)(VD ILED RCS VLED)RCS
VHYS
0.1
22
Eq. 8
( VIN VD)L FSW
0.1
33
The value of ∆VHYS calculated in this way must be
between 40mV and 100mV to ensure frequency
regulation.
Table 3. Inductor for FSW = 400 kHz, VD = 0.4V, 4 LED
∆VHYS
RCS (ꢀ)
ILED (A)
VIN (V)
L (µH)
(mV)
58.4
54.3
64.4
59.6
65.2
61.4
69.6
63.3
Inductor
0.56
0.56
0.28
0.28
0.2
0.35
0.35
0.7
0.7
1.0
1.0
2.0
2.0
36
40
36
40
36
40
36
40
150
220
68
According to the above equations, the inductor value can
be calculated once average LED current, operating
frequency and an appropriate hysteresis ∆VHYS value
have been chosen.
100
47
Thus, inductor L is given by:
0.2
68
(VIN -ILED RCS - VLED) (VD ILED RCS VLED) RCS
Eq. 9
L
0.1
22
( VIN VD) VHYS FSW
0.1
33
Table 2, Table 3, and Table 4 give reference inductor
values for an operating frequency of 400 kHz, for a given
LED current, freewheeling diode forward drop, and
number of LEDs. By selecting ∆VHYS in the 55mV to
75mV range, we get the following inductor values:
Table 4. Inductor for FSW = 400 kHz, VD = 0.4V, 8 LED
Given an inductor value, the size of the inductor can be
determined by its RMS and peak current rating.
Because LEDs are in series with the inductor,
IL ILED
Eq. 10
From Equations 1, 3, and 10:
IL
IL
VHYS
200m
Eq. 11
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Micrel, Inc.
MIC3205
With 40mv ≤ ∆VHYS ≤ 100mV:
where:
RGATE is total MOSFET gate resistance; Qgs2 and Qgd can be
found in a MOSFET manufacturer data sheet.
1
IL(RMS) IL2
IL2 IL
Eq. 12
12
A gate resistor can be connected between the MOSFET
gate and the DRV pin to slow down MOSFET switching
edges. A 2ꢀ resistor is usually sufficient.
VHYS
400m
IL(PK) IL(1
)
Eq. 13
The total power loss is:
P
LOSS(TOT) =PLOSS(CON) + P
where:
IL is the average inductor current
L(PK) is the peak inductor current
LOSS(TRAN)
The MOSFET junction temperature is given by:
TJ = PLOSS(TOT) ×RθJA + TA
I
Select an inductor with a saturation current rating at
least 30% higher than the peak current.
TJ must not exceed maximum junction temperature under
any conditions.
For space-sensitive applications, smaller inductors with
higher switching frequency could be used but regulator
efficiency will be reduced.
Freewheeling Diode
The freewheeling diode should have a reverse voltage rating
that is at least 20% higher than the maximum input supply
voltage. The forward voltage drop should be small to get the
lowest conduction dissipation for high efficiency. The forward
current rating should be at least equal to the LED current.
Schottky diodes with low forward voltage drop and fast
reverse recovery are ideal choices and give the highest
efficiency. The freewheeling diode average current (ID) is
given by:
MOSFET
N-channel MOSFET selection depends on the maximum
input voltage, output LED current, and switching
frequency.
The selected N-channel MOSFET should have 30%
margin on maximum voltage rating for high reliability
requirements.
The MOSFET channel resistance (RDSON) is selected
such that it helps to get the required efficiency at the
required LED currents and meets the cost requirement.
ID (1 D)ILED
Diode power dissipation (PD) is given by:
Logic level MOSFETs are preferred as the drive voltage
is limited to 5V.
PD VD ID
The MOSFET power loss has to be calculated for proper
operation. The power loss consists of conduction loss
and switching loss. The conduction loss can be found
by:
Typically, higher current rating diodes have a lower VD and
have better thermal performance, improving efficiency.
Input Capacitor
PLOSS(CON) IR2MS(FET) RDSON
IRMS(FET) ILED D
The ceramic input capacitor is selected by voltage rating and
ripple current rating. A 10µF ceramic capacitor is usually
sufficient. Select a voltage rating that is at least 30% larger
than the maximum input voltage.
VLED
D
VIN
LED Ripple Current
The LED current is the same as inductor current ∆IL. A
ceramic capacitor should be placed across the series LEDs
to pass the ripple current. A 4.7µF capacitor is usually
sufficient for most applications. Voltage rating should be the
same as the input capacitor.
The switching loss occurs during the MOSFET turn-on
and turn-off transition and can be found by:
V ×ILED ×FSW
IN
P
=
×(Qgs2 + Qgd)
LOSS(TRAN)
IDRV
VDRV
IDRV
=
RGATE
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Micrel, Inc.
MIC3205
PCB Layout Guidelines
NOTE: To minimize EMI and output noise, follow
these layout recommendations.
LED Ripple Current Carrying Capacitor
Place this ceramic capacitor as close to the LEDs as
possible.
Use either X7R or X5R dielectric capacitors. Do not use
Y5V or Z5U type capacitors.
PCB layout is critical to achieve reliable, stable, and
efficient performance. A ground plane is required to
control EMI and minimize the inductance in power,
signal, and return paths.
MOSFET
To avoid trace inductance, place the N-channel
MOSFET as close as possible to the MIC3205.
Follow these guidelines to ensure proper operation of
the MIC3205.
Provide sufficient copper area on MOSFET ground to
dissipate the heat.
IC
Use thick traces to route the input and output power
lines.
Freewheeling Diode
Place the Schottky diode on the same side of the board
as the IC and input capacitor.
Keep signal and power grounds separate and
connect them at only one location.
Keep the connection from the Schottky diode’s anode to
the switching node as short as possible.
Input Capacitor
Place the input capacitors on the same side of the
board and as close to the IC as possible.
Keep the diode’s cathode connection to the RCS as short
as possible.
Keep both the VIN and PGND traces as short as
possible.
RC Snubber
If an RC snubber is needed, place the RC snubber on
If the application requires vias to the ground plane,
place them close to the input capacitor ground
terminal, but not between the input capacitors and
IC pins.
the same side of the board and as close to the Schottky
diode as possible. A 1.2ꢀ resistor in series with a 1nF
capacitor is usually a good choice.
Use either X7R or X5R dielectric input capacitors.
Do not use Y5V or Z5U type capacitors.
RCS (Current-Sense Resistor)
VINS pin and CS pin must be as close as possible to
RCS.
Do not replace the ceramic input capacitor with any
other type of capacitor. Any type of capacitor can be
placed in parallel with the ceramic input capacitor.
Make a Kelvin connection to the VINS and CS pin,
respectively, for current sensing. For low values of ∆VHYS
(around 40mV) the switching noise could cause faulty
switching on the DRV pin. If this occurs, place two 30ꢀ
resistors and a 1nF capacitor, as shown in Figure 3, to
If a tantalum input capacitor is placed in parallel with
the ceramic input capacitor, it must be recom-
mended for switching regulator applications and the
operating voltage must be derated by 50%.
filter out switching noise for low values of ∆VHYS
.
Alternatively, as seen in Equation 8, a smaller inductor
value can be used to increase ∆VHYS and make the
system more noise tolerant.
In “Hot-Plug” applications, place a tantalum or
electrolytic bypass capacitor in parallel to the
ceramic capacitor to limit the overvoltage spike seen
on the input supply when power is suddenly applied.
In this case, an additional tantalum or electrolytic
bypass input capacitor of 22µF or higher is required
at the input power connection.
Inductor
Keep the inductor connection to the switch node
(MOSFET drain) short.
Do not route any digital lines underneath or close to
the inductor.
To minimize noise, place a ground plane underneath
the inductor.
M9999-102312-A
October 2012
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Micrel, Inc.
MIC3205
For FSW = 400 kHz
CT = 550pF
The actual frequency may vary as explained in “Frequency
of Operation” in the “Application Information” section.
3. INDUCTOR SELECTION
From Equation 9:
(VIN -ILED RCS - VLED) (VD ILED RCS VLED) RCS
L
( VIN VD) VHYS FSW
Given VSUPPLY = 24V rectified AC
The peak voltage = √2 x VSUPPLY
Thus for MIC3205, VIN ≈ 34V
VLED = 3.5 x 4 = 14, VD = 0.4V
Select ∆VHYS = 60mV
Figure 3. Input Filter for Low Values of ∆VHYS
Thus, L = 70µH
Trace Routing Recommendation
Chose L = 68µH as closest available value.
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 (CIN),
RCS, the LEDs, the inductor, the MOSFET, and back to
CIN. There is another current flowing loop during the
MOSFET OFF-time; the traces for this loop connect RCS,
the LED, the inductor, the freewheeling diode, and back
to RCS. These two loop areas should kept as small as
possible to minimize noise interference
As a side note, for this example, L = 68µH can be used even
if VSUPPLY = 24V DC. This is because ∆VHYS calculates to
around 44mV (with VIN = VSUPPLY = 24V) which is acceptable.
From Equations 12 and 13:
IL(PK) = 1.15A
Thus, we choose L = 68µH with an RMS saturation current
of 1.5A or higher.
4. MOSFET SELECTION
Keep all analog signal traces away from the switching
node and its connecting traces.
For this example, VIN = 34V, a 50V rating or greater N-
channel MOSFET is required. A high current rating MOSFET
is a good choice because it has lower RDSON
Design Example
.
A 60V, 12A MOSFET with 10mꢀ RDSON is a good choice.
SPECIFICATIONS:
5. CAPACITOR SELECTION
FSW = 400 kHz
Use a 10µF/50V X7R type ceramic capacitor for the input
capacitor.
V
SUPPLY = 24V rectified AC
ILED = 1A
Use a 4.7µF/50V X5R type ceramic capacitor for the LED
ripple current carrying capacitor connected across the series
connection of 4 LEDs
Voltage drop per LED = 3.5V
Number of LEDs = 4
Schottky diode drop at 1A = 0.4V
6. FREEWHEELING DIODE SELECTION
1. CURRENT SENSE RESISTOR
With VIN = 34V, choose a 2A, 60V Schottky diode with a
forward drop voltage of 0.4V at 1A forward current.
200mV
From Equation 1: RCS
ILED
For ILED = 1A
RCS = 0.2ꢀ
2. SWITCHING FREQUENCY
2.22 10-4
From Equation 2: FSW
CT
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October 2012
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Micrel, Inc.
MIC3205
Evaluation Board Schematic
M9999-102312-A
October 2012
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Micrel, Inc.
MIC3205
Bill of Materials
Item
Part Number
Manufacturer
AVX(1)
Description
Qty.
12105C475KAZ2A
GRM32ER71H475KA88L
CGA6P3X7R1H475K
C1, C2,C3,C4,C11
Murata(2)
TDK(3)
4.7µF/50V, Ceramic Capacitor, X7R, Size 1210
1µF/50V, Ceramic Capacitor, X7R, Size 0805
470pF/50V, Ceramic Capacitor, X7R, Size 0603
5
1
1
GRM21BR71H105KA12L
Murata
C5
CGA4J3X7R1H105K
06035C471K4T2A
GRM188R71H471KA01D
C1608X7R1H471K
06036D475KAT2A
GRM188R60J475KE19J
CGA3E1X5R0J475K
06035C102KAT2A
GRM188R71H102KA01D
C1608X7R1H102K
SK36-TP
TDK
AVX
C10
Murata
TDK
AVX
C8
4.7µF/6.3V, Ceramic Capacitor, X5R, Size 0603
1nF/50V, Ceramic Capacitor, X7R, Size 0603
60V, 3A, SMC, Schottky Diode
1
2
1
Murata
TDK
AVX
C7,C9
D1
Murata
TDK
MCC(4)
Fairchild(5)
Diodes, Inc.(6)
TDK
SK36
SK36-7-F
L1
SLF10145T-220M1R9-PF
FDS5672
22µH, 2.1A, 0.0591ꢀ, SMT, Power Inductor
1
1
M1
Fairchild
MOSFET, N-CH, 60V, 12A, SO-8
Stackpole
RCS
CSR1206FKR200
0.2ꢀ Resistor, 1/2W, 1%, Size 1206
1
Electronics, Inc.(7)
R5, R8
R2, R3
R1, R9
R4
CRCW0603100KFKEA
CRCW060330R0FKEA
CRCW06032R00FKEA
CRCW060310K0FKEA
CRCW060351R0FKEA
CRCW06030000Z0EA
Vishay Dale(8)
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
100kꢀ Resistor, 1%, Size 0603
30ꢀ Resistor, 1%, Size 0603
2ꢀ Resistor, 1%, Size 0603
10kꢀ Resistor, 1%, Size 0603
51ꢀ Resistor, 1%, Size 0603
0ꢀ Resistor, Size 0603
2
2
2
1
1
1
R6
R7
High-Brightness LED Driver Controller with
Fixed Frequency Hysteretic Control
U1
MIC3205YML
Micrel, Inc.(9)
1
Notes:
1. AVX: www.avx.com.
2. Murata: www.murata.com.
3. TDK: www.tdk.com.
4. MCC: www.mccsemi.com.
5. Fairchild: www.fairchildsemi.com.
6. Diodes Inc.: www.diodes.com.
7. Stackpole Electronics: www.seielect.com.
8. Vishay Dale: www.vishay.com.
9. Micrel, Inc.: www.micrel.com.
M9999-102312-A
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Micrel, Inc.
MIC3205
PCB Layout Recommendations
Top Assembly
Top Layer
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Micrel, Inc.
MIC3205
PCB Layout Recommendations (Continued)
Bottom Layer
M9999-102312-A
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Micrel, Inc.
MIC3205
Package Information
10-Pin 3mm x 3mm MLF (ML)
M9999-102312-A
October 2012
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Micrel, Inc.
MIC3205
Recommended Landing Pattern
10-Pin 3mm x 3mm MLF (ML) Land Pattern
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October 2012
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Micrel, Inc.
MIC3205
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
Micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in this data sheet. This
information is not intended as a warranty and Micrel does not assume responsibility for its use. Micrel reserves the right to change circuitry,
specifications and descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual
property rights is granted by this document. Except as provided in Micrel’s terms and conditions of sale for such products, Micrel assumes no liability
whatsoever, and Micrel disclaims any express or implied warranty relating to the sale and/or use of Micrel products including liability or warranties
relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant
into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 2012 Micrel, Incorporated.
M9999-102312-A
October 2012
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