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.
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 currentsense resistor is used to set the output current.
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.
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.
Data sheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
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
–40C to 125C junction temperature range
Applications
Architectural, industrial, and ambient lighting
LED bulbs
Indicators and emergency lighting
Street lighting
Channel letters
12V lighting systems (MR-16 bulbs, under-cabinet
lighting, garden/pathway lighting)
_________________________________________________________________________________________________________________________
Typical Application
Normalized Switching Frequency
vs. Input Voltage
NORMALIZED FREQUENCY
2.0
ILED = 1A
RCS = 0.2Ω
1.5
1 LED
L = 22µH
1.0
4 LED
L = 47µH
0.5
10 LED
L = 33µH
6 LED
L = 68µH
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
October 2012
M9999-102312-A
Micrel, Inc.
MIC3205
Ordering Information
Part Number
Junction Temperature Range
Package(1)
MIC3205YML
40°C to 125°C
10-Pin 3mm x 3mm MLF
Note:
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
1
VCC
2
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.
3
VIN
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.”
4
VINS
VIN Sense. Positive input to the current sense comparator. Connect as close as possible to the current
sense resistor.
5
AGND
Analog Ground. Ground for all internal low-power circuitry.
6
EN
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.
7
DIM
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.
8
CTIMER
9
PGND
10
DRV
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.
EP
ePAD
Exposed Pad. Must be connected to a GND plane for best thermal performance.
October 2012
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.
Timer Capacitor. A capacitor is required from CTIMER to ground sets the target switching frequency
using the equation CTIMER=2.22*10-4 / FSW
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.”
2
M9999-102312-A
Micrel, Inc.
MIC3205
Absolute Maximum Ratings (1)
Operating Ratings (2)
VIN to PGND .................................................. 0.3V to 42V
VINS to PGND......................................... 0.3V to (VIN+0.3V)
VCC to PGND ................................................ 0.3V to 6.0V
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)
HBM ...................................................................... 1.5kV
MM .........................................................................200V
Supply Voltage (VIN).......................................... 4.5V to 40V
Enable Voltage (VEN) .............................................. 0V to VIN
Dimming Voltage (VDIM)................................................................. 0V to VIN
Junction Temperature (TJ) ........................ 40C to 125C
Junction Thermal Resistance
10-pin 3x3 MLF (JA).......................................60.7C/W
10-pin 3x3 MLF (JC).......................................28.7C/W
Electrical Characteristics (4)
VIN = VEN = VDIM = 12V; CVCC = 4.7µF; TJ = 25C; bold values indicate 40C TJ 125C, unless noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
40
V
1.3
3
mA
2
µA
4
4.5
V
Input Supply
VIN
Input Voltage Range (VIN)
IS
Supply Current
4.5
DRV = Open
ISD
Shutdown Current
VEN = 0V
UVLO
VIN UVLO Threshold
VIN Rising
UVLOHYS
VIN UVLO Hysteresis
3.2
600
mV
VCC Supply
VCC
VCC Output Voltage
VIN = 12V, ICC = 5mA
Average Current Sense
Threshold
∆VCS =VINS VCS
4.5
5
5.5
V
190
200
210
mV
188
200
212
mV
Current Sense
∆VCS
VCS Rising
50
ns
VCS Falling
70
ns
∆tCS
Current Sense Response
Time
ICS
CS Input Current
VIN = VCS
0.5
Sense Voltage Hysteresis (5)
VIN =12V, VLED =3V,
L=47µH, FSW =250kHz,
VD = 0.7V, ILED = 1A
46
mV
66
µA
1.189
V
∆VHYS
10
µA
Frequency
ITIMER
CTIMER Pull-up Current
VCTREF
CTIMER Threshold
(4*ITIMER)/
VCTREF
Frequency Coefficient (6)
October 2012
1.776 × 10-4
3
2.22 × 10-4
2.664 × 10-4
A/V
M9999-102312-A
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
0.4
V
60
µA
1
µA
Enable Input
ENHI
EN Logic Level High
ENLO
EN Logic Level Low
IEN
EN Bias Current
tSTART
Start-Up Time
2.0
VEN = 12V
V
20
VEN = 0V
From EN pin going high to DRV
going high
65
µs
Dimming Input
DIMHI
DIM Logic Level High
DIMLO
DIM Logic Level Low
IDIM
DIM Bias Current
2.0
VDIM = 12V
V
20
VDIM = 0V
tDIM
DIM Delay Time
From DIM pin going high to DRV
going high
fDIM
Maximum Dimming Frequency
% of switching frequency
0.4
V
50
µA
1
µA
450
ns
2
%
Pull-Up, ISOURCE = 10mA
4
Ω
Pull-Down, ISINK = -10mA
1.5
Ω
Rise Time, CLOAD = 1000pF
13
ns
Fall Time, CLOAD = 1000pF
7
ns
160
C
20
C
External FET Driver
RON
DRV On-Resistance
tDRV
DRV Transition Time
Thermal Protection
TLIM
Overtemperature Shutdown
TLIMHYS
Overtemperature Shutdown Hysteresis
TJ Rising
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, 1.5kΩ 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.
October 2012
4
M9999-102312-A
Micrel, Inc.
MIC3205
Typical Characteristics
Efficiency (ILED = 1A)
vs. Input Voltage
VIN Supply Current
vs. Input Voltage
3.0
VIN SUPPLY CURRENT (mA)
90
4 LED
L = 47µH
85
6 LED
L = 68µH
80
1 LED
L = 22µH
75
10 LED
L = 33µH
70
65
60
2.5
2.0
1.5
1.0
0.5
0.0
0
9
18
27
36
9
18
VCC Output Voltage
vs. Input Voltage
36
5.5
5.0
4.5
4.0
0
36
27
36
1.5
1 LED
L = 22µH
1.0
4 LED
L = 47µH
0.5
10 LED
L = 33µH
6 LED
L = 68µH
TA = 25°C
RCS = 0.2Ω
1.05
1.00
1 LED
L = 22µH
6 LED
L = 68µH
0.95
4 LED
L = 47µH
0.90
0
9
18
27
36
45
0
9
18
27
36
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
CTIMER Current
vs. Input Voltage
Enable Threshold
vs. Input Voltage
Enable Bias Current
vs. Input Voltage
1.5
ENABLE THRESHOLD (V)
64
62
60
1.2
RISING
0.9
FALLING
0.6
0.3
HYST
0.0
9
18
27
INPUT VOLTAGE (V)
October 2012
36
45
45
100
ILED = 1A
TA = 25°C
66
45
1.10
INPUT VOLTAGE (V)
VEN = VIN
TA = 25°C
0
18
ILED Output Current
vs. Input Voltage
ILED = 1A
RCS = 0.2Ω
45
70
68
9
INPUT VOLTAGE (V)
0.0
27
0.2
45
ILED OUTPUT CURRENT (A)
NORMALIZED FREQUENCY
VCC OUTPUT VOLTAGE (V)
27
2.0
TA = 25°C
ILED = 1A
18
0.4
Normalized Switching Frequency
vs. Input Voltage
6.0
9
0.6
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
0
VEN = 0V
ILED = 0A
TA = 25°C
0.8
0.0
0
45
ENABLE BIAS CURRENT (µA)
EFFICIENCY (%)
95
1.0
ILED = 0A
TA = 25°C
VIN SHUTDOWN CURRENT (µA)
100
CTIMER CURRENT (µA)
VIN Shutdown Current
vs. Input Voltage
VEN = VIN
TA = 25°C
ILED = 0A
80
60
40
20
0
0
9
18
27
INPUT VOLTAGE (V)
5
36
45
0
9
18
27
36
INPUT VOLTAGE (V)
M9999-102312-A
45
Micrel, Inc.
MIC3205
Typical Characteristics (Continued)
Enable Bias Current
vs. Enable Voltage
Thermal Shutdown
vs. Input Voltage
200
80
VIN = 42V
60
40
20
0
2.0
RISING
160
FALLING
120
ILED = 1A
80
40
HYST
0
0
9
18
27
36
0
9
VIN Shutdown Current
vs. Temperature
18
27
36
1.2
0.8
0.4
0.0
-25
0
25
50
75
100
0
25
50
75
ILED Output Current
vs. Temperature
Switching Frequency
vs. Temperature
1.01
1.00
490
470
450
0.99
430
-25
0
25
50
75
100
-50
125
-25
0
25
50
75
TEMPERATURE (°C)
TEMPERATURE (°C)
VCC
vs. Temperature
Enable Threshold
vs. Temperature
Enable Bias Current
vs. Temperature
1.6
5.0
4.5
4.0
1.2
RISING
0.8
FALLING
0.4
HYST
0.0
-25
0
25
50
75
TEMPERATURE (°C)
October 2012
100
125
100
125
100
125
30
VIN = 12V
ILED = 1A
EN BIAS CURRENT (µA)
ENABLE THRESHOLD (V)
5.5
125
VIN = 12V
VLED = 3.5V
L = 22µH
CT = 470pF
RCS = 0.2Ω
510
TEMPERATURE (°C)
VIN = 12V
ILED = 1A
100
530
-50
6.0
-50
-25
TEMPERATURE (°C)
VIN = 12V
VLED = 3.5V
RCS = 0.2Ω
1.02
125
1.2
-50
0.98
-50
1.4
INPUT VOLTAGE (V)
FREQUENCY (kHz)
1.6
1.6
45
1.03
VIN = 12V
ILED = 0A
VEN = 0V
ILED OUTPUT CURRENT (A)
SUPPLY CURRENT (µA)
2.0
VIN = 12V
ILED = 0A
1.8
1.0
45
ENABLE VOLTAGE (V)
VCC (V)
SUPPLY CURRENT (mA)
VEN ≠ VIN
TA = 25°C
ILED = 0A
THERMAL SHUTDOWN (°C)
ENABLE BIAS CURRENT (µA)
100
VIN Supply Current
vs. Temperature
VIN = 12V
ILED = 0A
VEN = 12V
25
20
15
10
-50
-25
0
25
50
75
TEMPERATURE (°C)
6
100
125
-50
-25
0
25
50
75
TEMPERATURE (°C)
M9999-102312-A
Micrel, Inc.
MIC3205
Typical Characteristics (Continued)
VIN UVLO Threshold
vs. Temperature
VIN UVLO THRESHOLD (V)
5
RISING
4
FALLING
3
2
1
HYST
0
-50
-25
0
25
50
75
100
125
TEMPERATURE (°C)
October 2012
7
M9999-102312-A
Micrel, Inc.
MIC3205
Functional Characteristics
October 2012
8
M9999-102312-A
Micrel, Inc.
MIC3205
Functional Characteristics (Continued)
October 2012
9
M9999-102312-A
Micrel, Inc.
MIC3205
Functional Diagram
Figure 1. MIC3205 Block Diagram
October 2012
10
M9999-102312-A
Micrel, Inc.
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.
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.
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.
October 2012
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.
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%.
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.
11
M9999-102312-A
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.
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.
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.
LED Current and RCS
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:
200mV
ILED
RCS
Eq. 1
RCS (Ω)
ILED (A)
I2R (W)
Size (SMD)
1.33
0.15
0.03
0603
0.56
0.35
0.07
0805
0.4
0.5
0.1
0805
0.28
0.7
0.137
0805
0.2
1.0
0.2
1206
0.13
1.5
0.3
1206
0.1
2.0
0.4
2010
0.08
2.5
0.5
2010
0.068
3.0
0.6
2010
FSW
2.22 10 -4
CT
Eq. 2
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.
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.
Sense Voltage Hysteresis Range
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.
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:
As seen in Figure 2, for the inductor,
IL
VHYS
RCS
Eq. 3
where:
∆IL = inductor ripple current
∆VHYS = hysteresis on CS pin
For rising inductor current (MOSFET is on):
tr
Table 1. RCS for LED Current
L IL
VL_RISE
Eq. 4
where:
Frequency of Operation
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
October 2012
VL_RISE = VIN ILED × RCS VLED
VLED is the total voltage drop of the LED string
VIN is the input voltage
RCS is the current sense resistor
ILED is the average LED current
12
M9999-102312-A
Micrel, Inc.
MIC3205
tr is the MOSFET ON-time
L is the inductor
For falling inductor current (MOSFET is off):
tf
L I L
VL_FALL
Eq. 5
where:
RCS (Ω)
ILED (A)
VIN (V)
L (µH)
∆VHYS
0.56
0.35
5
22
64.1
0.56
0.35
12
68
57.7
0.28
0.7
5
10
70.5
0.28
0.7
12
33
59.4
0.2
1.0
5
6.8
72.6
0.2
1.0
12
22
62.4
0.1
2.0
5
3.6
68.5
0.1
2.0
12
10
68.6
(mV)
VL_FALL = VD + ILED × RCS VLED
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
Operating frequency and time period are given by:
RCS (Ω)
ILED (A)
VIN (V)
L (µH)
∆VHYS
0.56
0.35
24
150
55.8
0.56
0.35
36
220
56.8
0.28
0.7
24
68
61.6
0.28
0.7
36
100
62.5
0.2
1.0
24
47
62.4
0.2
1.0
36
68
64.3
0.1
2.0
24
22
66.6
0.1
2.0
36
33
66.2
FSW
1
T
T tr tf
Eq. 6
Eq. 7
Using Equations 3, 4, 5, 6, and 7:
VHYS
(VIN - ILED RCS - VLED) (VD ILED RCS VLED) RCS
( VIN VD) L FSW
Eq. 8
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
Inductor
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.
Thus, inductor L is given by:
L
(VIN - ILED RCS - VLED) (VD ILED RCS VLED) RCS
Eq. 9
( VIN VD) VHYS FSW
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:
(mV)
RCS (Ω)
ILED (A)
VIN (V)
L (µH)
∆VHYS
0.56
0.35
36
150
58.4
0.56
0.35
40
220
54.3
0.28
0.7
36
68
64.4
0.28
0.7
40
100
59.6
0.2
1.0
36
47
65.2
0.2
1.0
40
68
61.4
0.1
2.0
36
22
69.6
0.1
2.0
40
33
63.3
(mV)
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
VHYS
IL
200m
October 2012
13
Eq. 11
M9999-102312-A
Micrel, Inc.
MIC3205
With 40mv ≤ ∆VHYS ≤ 100mV:
IL(RMS ) IL2
IL(PK) IL(1
1 2
IL IL
12
VHYS
)
400m
Eq. 12
Eq. 13
where:
IL is the average inductor current
IL(PK) is the peak inductor current
where:
RGATE is total MOSFET gate resistance; Qgs2 and Qgd can be
found in a MOSFET manufacturer data sheet.
A gate resistor can be connected between the MOSFET
gate and the DRV pin to slow down MOSFET switching
edges. A 2Ω resistor is usually sufficient.
The total power loss is:
PLOSS( TOT ) = PLOSS( CON) + PLOSS( TRAN)
The MOSFET junction temperature is given by:
TJ = PLOSS( TOT ) × R θJA + TA
Select an inductor with a saturation current rating at
least 30% higher than the peak current.
For space-sensitive applications, smaller inductors with
higher switching frequency could be used but regulator
efficiency will be reduced.
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.
Logic level MOSFETs are preferred as the drive voltage
is limited to 5V.
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:
PLOSS(CON)
2
IRMS(FET)
R DSON
IRMS(FET) ILED D
D
VLED
VIN
The switching loss occurs during the MOSFET turn-on
and turn-off transition and can be found by:
PLOSS( TRAN) =
IDRV =
VIN × ILED × FSW
× (Qgs2 + Qgd )
IDRV
TJ must not exceed maximum junction temperature under
any conditions.
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:
ID (1 D) ILED
Diode power dissipation (PD) is given by:
PD VD ID
Typically, higher current rating diodes have a lower VD and
have better thermal performance, improving efficiency.
Input Capacitor
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.
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.
VDRV
RGATE
October 2012
14
M9999-102312-A
Micrel, Inc.
PCB Layout Guidelines
NOTE:
To minimize EMI and output noise, follow
these layout recommendations.
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.
Follow these guidelines to ensure proper operation of
the MIC3205.
IC
Use thick traces to route the input and output power
lines.
Keep signal and power grounds separate and
connect them at only one location.
Input Capacitor
MIC3205
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.
MOSFET
To avoid trace inductance, place the N-channel
MOSFET as close as possible to the MIC3205.
Provide sufficient copper area on MOSFET ground to
dissipate the heat.
Freewheeling Diode
Place the Schottky diode on the same side of the board
as the IC and input capacitor.
Keep the connection from the Schottky diode’s anode to
the switching node as short as possible.
Keep the diode’s cathode connection to the RCS as short
as possible.
Place the input capacitors on the same side of the
board and as close to the IC as possible.
Keep both the VIN and PGND traces as short as
possible.
RC Snubber
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.
Use either X7R or X5R dielectric input capacitors.
Do not use Y5V or Z5U type capacitors.
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.
If a tantalum input capacitor is placed in parallel with
the ceramic input capacitor, it must be recommended for switching regulator applications and the
operating voltage must be derated by 50%.
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.
If an RC snubber is needed, place the RC snubber on
the same side of the board and as close to the Schottky
diode as possible. A 1.2Ω resistor in series with a 1nF
capacitor is usually a good choice.
RCS (Current-Sense Resistor)
VINS pin and CS pin must be as close as possible to
RCS.
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 30Ω
resistors and a 1nF capacitor, as shown in Figure 3, to
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.
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.
October 2012
15
M9999-102312-A
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:
L
Figure 3. Input Filter for Low Values of ∆VHYS
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 (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
Keep all analog signal traces away from the switching
node and its connecting traces.
Design Example
SPECIFICATIONS:
FSW = 400 kHz
VSUPPLY = 24V rectified AC
ILED = 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
(VIN - ILED RCS - VLED) (VD ILED RCS VLED) RCS
( 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
Thus, L = 70µH
Chose L = 68µH as closest available value.
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
For this example, VIN = 34V, a 50V rating or greater Nchannel MOSFET is required. A high current rating MOSFET
is a good choice because it has lower RDSON.
A 60V, 12A MOSFET with 10mΩ RDSON 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 VIN = 34V, choose a 2A, 60V Schottky diode with a
forward drop voltage of 0.4V at 1A forward current.
For ILED = 1A
RCS = 0.2Ω
2. SWITCHING FREQUENCY
From Equation 2: FSW
October 2012
2.22 10 -4
CT
16
M9999-102312-A
Micrel, Inc.
MIC3205
Evaluation Board Schematic
October 2012
17
M9999-102312-A
Micrel, Inc.
MIC3205
Bill of Materials
Item
Part Number
12105C475KAZ2A
C1, C2,C3,C4,C11
C5
C10
GRM32ER71H475KA88L
Murata
CGA4J3X7R1H105K
TDK
06035C471K4T2A
AVX
GRM188R60J475KE19J
CGA3E1X5R0J475K
06035C102KAT2A
GRM188R71H102KA01D
C1608X7R1H102K
SK36-TP
D1
SK36
SK36-7-F
L1
SLF10145T-220M1R9-PF
M1
FDS5672
Qty.
4.7µF/50V, Ceramic Capacitor, X7R, Size 1210
5
1µF/50V, Ceramic Capacitor, X7R, Size 0805
1
470pF/50V, Ceramic Capacitor, X7R, Size 0603
1
4.7µF/6.3V, Ceramic Capacitor, X5R, Size 0603
1
1nF/50V, Ceramic Capacitor, X7R, Size 0603
2
60V, 3A, SMC, Schottky Diode
1
22µH, 2.1A, 0.0591Ω, SMT, Power Inductor
1
MOSFET, N-CH, 60V, 12A, SO-8
1
0.2Ω Resistor, 1/2W, 1%, Size 1206
1
(3)
GRM21BR71H105KA12L
06036D475KAT2A
C7,C9
Murata(2)
TDK
GRM188R71H471KA01D
Description
AVX(1)
CGA6P3X7R1H475K
C1608X7R1H471K
C8
Manufacturer
Murata
TDK
AVX
Murata
TDK
AVX
Murata
TDK
MCC(4)
Fairchild(5)
(6)
Diodes, Inc.
TDK
Fairchild
Stackpole
Electronics, Inc.(7)
RCS
CSR1206FKR200
R5, R8
CRCW0603100KFKEA
Vishay Dale(8)
100kΩ Resistor, 1%, Size 0603
2
R2, R3
CRCW060330R0FKEA
Vishay Dale
30Ω Resistor, 1%, Size 0603
2
R1, R9
CRCW06032R00FKEA
Vishay Dale
2Ω Resistor, 1%, Size 0603
2
R4
CRCW060310K0FKEA
Vishay Dale
10kΩ Resistor, 1%, Size 0603
1
R6
CRCW060351R0FKEA
Vishay Dale
51Ω Resistor, 1%, Size 0603
1
R7
CRCW06030000Z0EA
Vishay Dale
0Ω Resistor, Size 0603
1
U1
MIC3205YML
High-Brightness LED Driver Controller with
Fixed Frequency Hysteretic Control
1
Micrel, Inc.(9)
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.
October 2012
18
M9999-102312-A
Micrel, Inc.
MIC3205
PCB Layout Recommendations
Top Assembly
Top Layer
October 2012
19
M9999-102312-A
Micrel, Inc.
MIC3205
PCB Layout Recommendations (Continued)
Bottom Layer
October 2012
20
M9999-102312-A
Micrel, Inc.
MIC3205
Package Information
10-Pin 3mm x 3mm MLF (ML)
October 2012
21
M9999-102312-A
Micrel, Inc.
MIC3205
Recommended Landing Pattern
10-Pin 3mm x 3mm MLF (ML) Land Pattern
October 2012
22
M9999-102312-A
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.
October 2012
23
M9999-102312-A
工商网监
湘ICP备2023018690号
