ADD5207ACPZ [ADI]
Four-String, White LED Driver for LCD Backlight Applications; 四串白光LED驱动器，用于LCD背光应用
| 型号： | ADD5207ACPZ |
| 厂家： | 
ADI |
| 描述： | Four-String, White LED Driver for LCD Backlight Applications |
| 文件： | 总16页 (文件大小：428K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Four-String, White LED Driver
for LCD Backlight Applications
Data Sheet
ADD5207
FEATURES
GENERAL DESCRIPTION
White LED driver based on inductive boost converter
Integrated 40 V MOSFET with 1.5 A peak current limit
Input voltage range: 6 V to 21 V
Maximum output adjustable up to 36 V
600 kHz to 1 MHz adjustable operating frequency
Typical 39 V fixed overvoltage protection (OVP)
Built-in soft start for boost converter
Drives up to 4 LED current strings
LED current adjustable up to 25 mA for each channel
Headroom control to maximize efficiency
Fixed LED dimming frequency: 8 kHz
LED open fault protection
Brightness control with PWM input
Dimming controls
4-channel operation: 90 degree phase shift between
channels
3-channel operation: 120 degree phase shift between
channels
General
The ADD5207 is a white LED driver for backlight applications
based on high efficiency, current mode, step-up converter tech-
nology. It is designed with a 0.15 Ω, 1.5 A internal switch and a
pin-adjustable operating frequency between 600 kHz and 1 MHz.
The ADD5207 contains four regulated current sources for
uniform LED brightness. Each current source can drive up to
25 mA and the LED-driving current is pin adjustable by an
external resistor. The ADD5207 drives up to four parallel
strings of multiple series-connected LEDs with a 1.5% current
matching between strings.
The ADD5207 provides phase shift PWM brightness control
methods. LED dimming control is achieved through the PWM
input. The device includes an 8 kHz LED-dimming oscillator
for driving each current source. The ADD5207 operates over an
input voltage range of 6 V to 21 V, but the device can function
with a voltage as low as 5.6 V.
The ADD5207 also has multiple safety protection features to
prevent damage during fault conditions. If any LED is open, the
device automatically disables the faulty current source. The
internal soft start circuit prevents a high inrush current during
startup. Thermal shutdown protection prevents thermal damage.
Thermal shutdown
Undervoltage lockout
14-lead, 4 mm × 3 mm LFCSP
APPLICATIONS
The ADD5207 is available in a low profile, thermally enhanced,
4 mm × 3 mm × 0.75 mm, 14-lead, RoHS-compliant lead frame
chip scale package (LFCSP) and is specified over the industrial
temperature range of −25°C to +85°C.
Notebook PCs, UMPCs, and monitor displays
TYPICAL APPLICATION CIRCUIT
V
IN
L1
10µH
C
10µF
D1
IN
C
OUT
4µF
+
–
14
13
1
9
VIN
SW
OVP
C
0.1µF
IN2
ADD5207
PWM
10
8
OFF ON
SHDN
VDD
C
4
5
6
7
FB1
FB2
FB3
BYPASS
1µF
2
FSLCT
R
F
100kΩ
FB4
GND
12
ISET
3
COMP
11
R
6.8kΩ
C2
OPEN
R
C
SET
180kΩ
C
C
2.2nF
Figure 1.
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rightsof third parties that may result fromits use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
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One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2009–2012 Analog Devices, Inc. All rights reserved.
ADD5207
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Typical Performance Characteristics ..............................................9
Theory of Operation ...................................................................... 11
Current Mode, Step-Up Switching Regulator Operation ..... 11
Internal 3.3 V Regulator............................................................ 11
Boost Converter Switching Frequency.................................... 11
Dimming Frequency (fPWM) ...................................................... 11
Current Source............................................................................ 11
PWM Dimming Mode .............................................................. 11
Safety Features ............................................................................ 11
External Component Selection Guide..................................... 12
Layout Guidelines....................................................................... 13
Typical Application Circuits ......................................................... 15
Outline Dimensions....................................................................... 16
Ordering Guide .......................................................................... 16
Applications....................................................................................... 1
General Description ......................................................................... 1
Typical Application Circuit ............................................................. 1
Revision History ............................................................................... 2
Functional Block Diagram .............................................................. 3
Specifications..................................................................................... 4
Step-Up Switching Regulator Specifications............................. 4
LED Current Regulation Specifications .................................... 5
General Specifications ................................................................. 6
Absolute Maximum Ratings............................................................ 7
Thermal Resistance ...................................................................... 7
ESD Caution.................................................................................. 7
Pin Configuration and Function Descriptions............................. 8
REVISION HISTORY
2/12—Rev. Sp0 to Rev. A
Changes to Overvoltage Protection (OVP) Section .................. 11
Changes to Open-Loop Protection (OLP) Section,
Undervoltage Lockout (UVLO) Section, and Thermal
Protection Section .......................................................................... 12
Changes to Layout Guidelines Section........................................ 13
Replaced Block Diagram with Typical Application Circuit........ 1
Changes to Features Section and General Description Section . 1
Changes to Current Mode, Step-Up Switching Regulator
Operation Section, Boost Converter Switching Frequency
Section, PWM Dimming Mode Section, Phase Shift PWM
Dimming Section, and Safety Features Section.......................... 11
7/09—Revision Sp0: Initial Version
Rev. A | Page 2 of 16
Data Sheet
ADD5207
FUNCTIONAL BLOCK DIAGRAM
VIN
1
VDD
8
SHDN
10
OVP
13
SW
14
THERMAL
LINEAR
SHUTDOWN
REGULATOR
500kΩ
GND
SHUTDOWN
VOUT_FB
VOLTAGE
REFERENCE
ADD5207
GND
OVP
REF
UVP
COMP
UVP
REF
LL COMP
LL
REF
ERROR
AMP
R
S
Q
VOUT_FB
PWM
COMP
gm
11
COMP
2
OSC
FSLCT
DREF
DCOMP
+
CURRENT SENSE
SOFT START
+
HEADROOM CONTROL
12
GND
VDD
LED OPEN/SHORT
FAULT DETECTOR
CURRENT SOURCE 1
CURRENT SOURCE 2
CURRENT SOURCE 3
CURRENT SOURCE 4
4
5
FB1
FB2
REF
3
ISET
6
7
FB3
FB4
PWM
DUTY
EXTRACTOR
CURRENT SOURCE DRIVER
FPWM OSCILLATOR
9
PWM
500kΩ
GND
Figure 2. Functional Block Diagram
Rev. A | Page 3 of 16
ADD5207
Data Sheet
SPECIFICATIONS
STEP-UP SWITCHING REGULATOR SPECIFICATIONS
SHDN
VIN = 12 V,
= high, TA = −25°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.
Table 1.
Parameter
SUPPLY
Input Voltage Range
BOOST OUTPUT
Symbol
VIN
Test Conditions/Comments
Min
Typ
Max
21
Unit
V
6
Output Voltage
VOUT
36
V
SWITCH
On Resistance
Leakage Current
Peak Current Limit
OSCILLATOR
RDS(ON)
ILKG
ICL
VIN = 12 V, ISW = 100 mA
Duty cycle (D) = DMAX
150
300
1
mΩ
µA
A
1.5
Switching Frequency
Maximum Duty Cycle
SOFT START
fSW
DMAX
RF = 97 kΩ
RF = 97 kΩ
800
84
1000
90
1200
kHz
%
Soft Start Time
tSS
1.1
ms
OVERVOLTAGE PROTECTION
Overvoltage Rising Threshold on OVP Pin
Overvoltage Hysteresis on OVP Pin
VOVPR
VOVP_HYS
36.5
0.1
39
0.7
40
1.4
V
V
Rev. A | Page 4 of 16
Data Sheet
ADD5207
LED CURRENT REGULATION SPECIFICATIONS
SHDN
VIN = 12 V,
= high, TA = −25°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.
Table 2.
Parameter
Symbol
Test Conditions/Comments
Min
Typ
Max
Unit
CURRENT SOURCE
ISET Pin Voltage
VSET
ILED
ILED20
VHR20
6 V ≤ VIN ≤ 21 V
1.14
0
19.4
1.18
1.22
25
20.6
0.9
+1.5
+3
V
Adjustable LED Current1
Constant Current Sink of 20 mA2
Minimum Headroom Voltage2
Current Matching Between Strings2
LED Current Accuracy2
Current Source Leakage Current
FPWM GENERATOR
mA
mA
V
%
%
RSET = 180 kΩ
RSET = 180 kΩ
RSET = 180 kΩ
RSET = 180 kΩ
20
0.66
−1.5
−3
1
µA
Dimming Frequency
fPWM
6.8
8.0
9.2
6.5
kHz
µs
LED FAULT DETECTION
Open Fault Delay1
tD_OPENFAULT
1 This electrical specification is guaranteed by design.
2 Tested at TA = +25°C.
Rev. A | Page 5 of 16
ADD5207
Data Sheet
GENERAL SPECIFICATIONS
SHDN
VIN = 12 V,
= high, TA = −25°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.
Table 3.
Parameter
SUPPLY
Symbol
Test Conditions/Comments
Min
Typ
Max
Unit
Input Voltage Range
Quiescent Current
VIN
IQ
6
21
7
V
mA
µA
6 V ≤ VIN ≤ 21 V, SHDN = high
6 V ≤ VIN ≤ 21 V, SHDN = low
3.5
Shutdown Supply Current
ISD
2
VDD REGULATOR
VDD Regulated Output
PWM INPUT
VVDD_REG
6 V ≤ VIN ≤ 21 V
3.1
2.0
100
3.3
3.5
V
PWM Voltage High
PWM Voltage Low
PWM Input Range
THERMAL SHUTDOWN
Thermal Shutdown Threshold1
Thermal Shutdown Hysteresis1
UVLO
VPWM_HIGH
VPWM_LOW
5.5
0.8
10,000
V
V
Hz
TSD
TSDHYS
160
30
°C
°C
VIN Falling Threshold
VIN Rising Threshold
SHDN CONTROL
VUVLOF
VUVLOR
VIN falling
VIN rising
4
4.2
5.0
V
V
5.6
Input Voltage High
Input Voltage Low
SHDN Pin Input Current
VIH
VIL
ISHDN
2.5
5.5
0.5
V
V
µA
SHDN = 3.3 V
6
1 This electrical specification is guaranteed by design.
Rev. A | Page 6 of 16
Data Sheet
ADD5207
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
THERMAL RESISTANCE
Table 4.
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
Parameter
Rating
VIN
SW
SHDN, PWM
−0.3 V to +23 V
−0.3 V to +40 V
−0.3 V to +6 V
−0.3 V to +3.5 V
−0.3 V to +3.7 V
−0.3 V to +40 V
−0.3 V to +40 V
150°C
Table 5. Thermal Resistance
Package Type
θJA
θJC
Unit
ISET, FSLCT, COMP
VDD
FB1, FB2, FB3, FB4
OVP
Maximum Junction Temperature (TJ max)
Operating Temperature Range (TA)
Storage Temperature Range (TS)
Reflow Peak Temperature (20 sec to 40 sec)
14-Lead LFCSP
33.24
2.42
°C/W
ESD CAUTION
−25°C to +85°C
−65°C to +150°C
260°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
Rev. A | Page 7 of 16
ADD5207
Data Sheet
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
1
2
3
4
5
6
7
14
13
12
11
10
9
VIN
FSLCT
ISET
FB1
SW
OVP
GND
COMP
SHDN
PWM
VDD
ADD5207
FB2
FB3
8
FB4
TOP VIEW
(Not to Scale)
NOTES
1. CONNECT THE EXPOSED PADDLE
TO GROUND.
Figure 3. Pin Configuration
Table 6. Pin Function Descriptions
Pin No. Mnemonic Description
1
VIN
Supply Input. Must be locally bypassed with a capacitor to ground.
2
3
4
5
6
7
8
9
FSLCT
ISET
FB1
FB2
FB3
Frequency Select. A resistor from this pin to ground sets the boost switching frequency from 600 kHz to 1 MHz.
Full-Scale LED Current Set. A resistor from this pin to ground sets the LED current up to 25 mA.
Regulated Current Sink. Connect the bottom cathode of the LED string to this pin.
Regulated Current Sink. Connect the bottom cathode of the LED string to this pin.
Regulated Current Sink. Connect the bottom cathode of the LED string to this pin.
Regulated Current Sink. Connect the bottom cathode of the LED string to this pin. If unused, connect FB4 to GND.
Internal Linear Regulator Output. This regulator provides power to the ADD5207.
PWM Signal Input.
FB4
VDD
PWM
SHDN
COMP
10
11
Shutdown Control for PWM Input Operation Mode. Active low.
Compensation for the Boost Converter. Two capacitors and a resistor are connected in series between ground and
this pin for stable operation.
12
13
14
GND
OVP
SW
Ground.
Overvoltage Protection. The boost converter output is connected to this pin directly.
Drain Connection of the Internal Power FET.
EP
Exposed Paddle. Connect the exposed paddle to ground.
Rev. A | Page 8 of 16
Data Sheet
ADD5207
TYPICAL PERFORMANCE CHARACTERISTICS
25
I
= 20mA
LED
fSW = 800kHz
BRIGHTNESS = 100%
LEDs = 10 SERIES × 4 PARALLEL
90
88
86
84
82
80
78
20
15
10
5
0
5
10
15
INPUT VOLTAGE (V)
20
5
10
15
20
INPUT VOLTAGE (V)
Figure 7. LED Current vs. Input Voltage (ILED = 20 mA)
Figure 4. Boost Converter Efficiency vs. Input Voltage
28
BRIGHTNESS = 100%
1.5
1.2
26
24
22
20
18
LEDs = 10 SERIES × 4 PARALLEL
I
= 20mA
LED
0.9
0.6
0.3
0
16
14
12
10
8
–0.3
–0.6
–0.9
–1.2
–1.5
6
4
6
8
10
12
14
16
18
20
22
135 150 165 180 195 210 225 240 255 270
INPUT VOLTAGE (V)
R
(kΩ)
SET
Figure 5. LED Current vs. RSET
Figure 8. LED Current Matching vs. Input Voltage
V
OUT
20V/DIV
20
15
10
5
0V
0V
0V
V
SW
20V/DIV
SHDN
5V/DIV
I
L
600mA/DIV
0A
V
= 12V
IN
BRIGHTNESS = 100%
LEDs = 10 SERIES × 4 PARALLEL
0
5ms/DIV
PWM DUTY CYCLE (%)
Figure 6. LED Current vs. PWM Input Duty Cycle
Figure 9. Start-Up Waveforms (Brightness = 100%)
Rev. A | Page 9 of 16
ADD5207
Data Sheet
V
PWM
OUT
0V
0V
100mV/DIV
AC
2V/DIV
V
FB1
5V/DIV
SW
20V/DIV
0V
0A
0V
0A
I
L
I
FB1
10mA/DIV
500mA/DIV
V
= 6V, fSW = 800kHz
V
= 12V
IN
IN
BRIGHTNESS = 100%
BRIGHTNESS = 1.5%
LEDs = 10 SERIES × 4 PARALLEL
LEDs = 10 SERIES × 4 PARALLEL
1µs/DIV
100µs/DIV
Figure 10. Switching Waveforms (VIN = 6 V)
Figure 12. LED Current Waveforms (Brightness = 1.5%)
FB1
7V/DIV
V
OUT
0V
0V
100mV/DIV
AC
FB2
7V/DIV
V
SW
20V/DIV
0V
FB3
7V/DIV
0V
0A
0V
0V
I
L
500mA/DIV
FB4
7V/DIV
V
= 21V, fSW = 800kHz
V
= 12V
IN
IN
BRIGHTNESS = 100%
BRIGHTNESS = 25%
LEDs = 10 SERIES × 4 PARALLEL
LEDs = 10 SERIES × 4 PARALLEL
1µs/DIV
50µs/DIV
Figure 11. Switching Waveforms (VIN = 21 V)
Figure 13. LED FBx Waveforms (Brightness = 25%)
Rev. A | Page 10 of 16
Data Sheet
ADD5207
THEORY OF OPERATION
CURRENT SOURCE
CURRENT MODE, STEP-UP SWITCHING
REGULATOR OPERATION
The ADD5207 contains four current sources to provide accu-
rate current sinking for each LED string. String-to-string
tolerance is kept within 1.5% at 20 mA. Each LED string
current is adjusted up to 25 mA using an external resistor.
The ADD5207 uses a current mode PWM boost regulator to
generate the minimum voltage needed to drive the LED string
at the programmed LED current. The current mode regulation
system allows a fast transient response while maintaining a
stable output voltage. By selecting the proper resistor-capacitor
network from COMP to GND, the regulator response is
optimized for a wide range of input voltages, output voltages,
and load conditions. The ADD5207 can provide a 36 V maxi-
mum output voltage and drive up to 10 LEDs (3.4 V/25 mA
type of LEDs) for each channel.
The ADD5207 contains an LED open fault protection circuit
for each channel. If the headroom voltage of the current source
remains below 150 mV while the boost converter output reaches
the OVP level, the ADD5207 recognizes that the current source
has an open-load fault for the current source, and the current
source is disabled.
If an application requires three LED strings, each LED string
should be connected using FB1 to FB3. The unused FB4 pin
should be tied to GND.
INTERNAL 3.3 V REGULATOR
The ADD5207 contains a 3.3 V linear regulator that
is used for biasing internal circuitry. The internal regulator
requires a 1 μF bypass capacitor. Place this bypass capacitor
between Pin VDD (Pin 8) and GND, as close as possible to
Pin VDD.
The ADD5207 contains hysteresis to prevent the LED current
change that is caused by a 0.195% jitter of the PWM input.
Programming the LED Current
As shown in the Figure 2, the ADD5207 has an LED current set
pin (ISET). A resistor (RSET) from this pin to ground adjusts the
LED current up to 25 mA. LED current level can be set with
following equation:
BOOST CONVERTER SWITCHING FREQUENCY
The ADD5207 boost converter switching frequency is user
adjustable, between 600 kHz to 1 MHz, by using an external
resistor, RF. A frequency of 600 kHz is recommended to optim-
ize the regulator for high efficiency, and a frequency of 1 MHz
is recommended to minimize the size of external components.
3600
RSET
ILED
=
(A)
PWM DIMMING MODE
See Figure 14 for considerations when selecting a switching
frequency and an adjustment resistor (RF).
The ADD5207 supports 8-bit resolution to control brightness.
However, each current source has a minimum on time require-
ment for LED current regulation such that the dimming is in
the range of 1.5% to 100%. Accordingly, even when the PWM
input duty cycle is more than 0% and less than 1.5%, the LED
duty cycle is held at 1.5%.
1000
900
800
700
600
500
400
300
Phase Shift PWM Dimming
There is a phase delay between each current source channel that is
programmed by the number of current sources in operation. If the
application requires four separate LED strings, each string has a
90 degree phase delay between channels. If three LED strings are
connected at the FB1 to FB3 pins (FB4 = GND), each string has a
120 degree phase delay.
80
100
120
140
160
180
200
220
SAFETY FEATURES
R
(kΩ)
F
Figure 14. Switching Frequency vs. RF
The ADD5207 contains several safety features to provide stable
and reliable operation.
DIMMING FREQUENCY (fPWM
)
Soft Start
The ADD5207 contains an internal oscillator to generate the
PWM dimming signal for LED brightness control. The LED
dimming frequency (fPWM) is fixed at 8 kHz internally.
The ADD5207 contains an internal soft start function to reduce
inrush current at startup. The soft start time is typically 1.1 ms.
Overvoltage Protection (OVP)
The ADD5207 contains OVP circuits to prevent boost converter
damage if the output voltage becomes excessive for any reason.
To keep a safe output level, the integrated OVP circuit monitors
Rev. A | Page 11 of 16
ADD5207
Data Sheet
the output voltage. When the OVP pin voltage reaches the OVP
rising threshold, the boost converter stops switching, which causes
the output voltage to drop. When the OVP pin voltage drops below
the OVP falling threshold, the boot converter begins switching
again, causing the output to rise. There is about 0.8 V hysteresis
between the rising and falling thresholds. The OVP level is fixed
at 39 V (typical).
The inductor ripple current (ΔIL) in a steady state is:
VIN ×tON
∆IL
=
L
Solve for the inductance value (L):
VIN ×tON
L =
∆IL
Open-Load Protection (OLP)
Make sure that the peak inductor current (that is, the maximum
input current plus half of the inductor ripple current) is less
than the rated saturation current of the inductor. In addition,
ensure that the maximum rated rms current of the inductor is
greater than the maximum dc input current to the regulator.
The ADD5207 contains a headroom control circuit to minimize
power loss at each current source. Therefore, the minimum
feedback voltage is achieved by regulating the output voltage of
the boost converter. If any LED string is open circuit during
normal operation, the current source headroom voltage (VHR) is
pulled to GND. In this condition, OLP is activated if VHR is less
than 150 mV until the boost converter output voltage rises up to
the OVP level.
For duty cycles greater than 50% that occur with input voltages
greater than half the output voltage, slope compensation is required
to maintain stability of the current mode regulator. The inherent
open-loop stability causes subharmonic instability when the
duty ratio is greater than 50%. To avoid subharmonic instability,
the slope of the inductor current should be less than half of the
compensation slope.
Undervoltage Lockout (UVLO)
An undervoltage lockout circuit is included with built-in hysteresis.
The ADD5207 turns on when VIN rises above 5.0 V (typical) and
shuts down when VIN falls below 4.2 V (typical).
Inductor manufacturers include: Coilcraft, Inc., Sumida
Corporation, and Toko.
Thermal Protection
Input and Output Capacitor Selection
Thermal overload protection prevents excessive power dissipa-
tion from overheating and damaging the ADD5207. When the
junction temperature (TJ) exceeds 160°C, a thermal sensor
immediately activates the fault protection, which shuts down
the device and allows it to cool. The device self-starts when the
junction temperature (TJ) of the die falls below 130°C.
The ADD5207 requires input and output bypass capacitors to
supply transient currents while maintaining a constant input
and output voltage. Use a low effective series resistance (ESR)
10 μF or greater capacitor for the input capacitor to prevent noise
at the ADD5207 input. Place the input between VIN and GND,
as close as possible to the ADD5207. Ceramic capacitors are
preferred because of their low ESR characteristics. Alternatively,
use a high value, medium ESR capacitor in parallel with a
0.1 μF low ESR capacitor as close as possible to the ADD5207.
EXTERNAL COMPONENT SELECTION GUIDE
Inductor Selection
The inductor is an integral part of the step-up converter. It stores
energy during the switch’s on time and transfers that energy to
the output through the output diode during the switch’s off
time. An inductor in the range of 4.7 µH to 22 µH is
recommended. In general, lower inductance values result in
higher saturation current and lower series resistance for a given
physical size. However, lower inductance results in higher peak
current, which can lead to reduced efficiency and greater input
and/or output ripple and noise. Peak-to-peak inductor ripple
current at close to 30% of the maximum dc input current
typically yields an optimal compromise.
The output capacitor maintains the output voltage and supplies
current to the load while the ADD5207 switch is on. The value
and characteristics of the output capacitor greatly affect the
output voltage ripple and stability of the regulator. Use a low
ESR output capacitor; ceramic dielectric capacitors are preferred.
For very low ESR capacitors, such as ceramic capacitors, the
ripple current due to the capacitance is calculated as follows.
Because the capacitor discharges during the on time (tON), the
charge removed from the capacitor (QC) is the load current
multiplied by the on time. Therefore, the output voltage ripple
(ΔVOUT) is
The input (VIN) and output (VOUT) voltages determine the
switch duty cycle (D), which, in turn, is used to determine the
inductor ripple current.
QC
COUT
IL ×tON
COUT
∆VOUT
=
=
VOUT −VIN
D =
VOUT
where:
OUT is the output capacitance.
IL is the average inductor current.
C
Use the duty cycle and switching frequency (fSW) to determine
the on time.
D
fSW
tON
=
Rev. A | Page 12 of 16
Data Sheet
ADD5207
Using the duty cycle and switching frequency (fSW), users can
determine the on time with the following equation:
Loop Compensation
The external inductor, output capacitor, and the compensation
resistor and capacitor determine the loop stability. The induc-
tor and output capacitor are chosen based on performance, size,
and cost. The compensation resistor (RC) and compensation
capacitor (CC ) at COMP are selected to optimize control loop
stability. For typical LED application of the ADD5207, a 6.8 kΩ
compensation resistor in series with a 2.2 nF compensation
capacitor at COMP is adequate.
D
fSW
tON
=
The input (VIN) and output (VOUT) voltages determine the
switch duty cycle (D) with the following equation:
VOUT −VIN
D =
VOUT
VOUT_FB
gm
HEADROOM CONTROL
Choose the output capacitor based on the following equation:
IL ×
(
VOUT −VIN
)
R
C
C
COUT
≥
C2
fSW ×VOUT ×∆VOUT
C
Capacitor manufacturers include: Murata Manufacturing Co.,
Ltd., AVX, Sanyo, and Taiyo Yuden Co., Ltd.
Figure 15. Compensation Components
Diode Selection
A step-up converter produces an undesirable right-half plane
zero in the regulation feedback loop. Capacitor C2 is chosen
to cancel the zero introduced by output capacitance ESR.
Solving for C2,
The output diode conducts the inductor current to the output
capacitor and loads while the switch is off. For high efficiency,
minimize the forward voltage drop of the diode. Schottky diodes
are recommended. However, for high voltage, high temperature
applications, where the Schottky diode reverse leakage current
becomes significant and degrades efficiency, use an ultrafast
junction diode. The output diode for a boost regulator must be
chosen depending on the output voltage and the output current.
The diode must be rated for a reverse voltage equal to or greater
than the output voltage used. The average current rating must
be greater than the maximum load current expected, and the peak
current rating must be greater than the peak inductor current.
Using Schottky diodes with lower forward voltage drop decreases
power dissipation and increases efficiency. The diode must be
rated to handle the average output load current. Many diode
manufacturers derate the current capability of the diode as a
function of the duty cycle. Verify that the output diode is rated
to handle the average output load current with the minimum
duty cycle.
ESR×COUT
C2 =
RC
For low ESR output capacitance, such as with a ceramic
capacitor, C2 is optional.
LAYOUT GUIDELINES
When designing a high frequency, switching, regulated power
supply, layout is very important. Using a good layout can solve
many problems associated with these types of supplies. The
main problems are loss of regulation at high output current
and/or large input-to-output voltage differentials, excessive
noise on the output and switch waveforms, and instability.
Using the following guidelines helps minimize these problems.
Make all power (high current) traces as short, direct, and thick
as possible. It is good practice on a standard printed circuit
board (PCB) to make the traces an absolute minimum of 15 mil
(0.381 mm) per ampere. The inductor, output capacitors, and
output diode should be as close to each other as possible. This
helps reduce EMI radiated by the power traces that carry high
switching currents. Close proximity of the components also
reduces lead inductance and resistance, which in turn reduce noise
spikes, ringing, and resistive losses that produce voltage errors.
The minimum duty cycle of the ADD5207 is:
VOUT −VIN_MAX
DMIN
=
VOUT
where VIN_MAX is the maximum input voltage.
For example, DMIN is 0.5 when VOUT is 30 V and VIN_MAX is 15 V.
Schottky diode manufacturers include ON Semiconductor,
Diodes Incorporated, Central Semiconductor Corp., and Sanyo.
Rev. A | Page 13 of 16
ADD5207
Data Sheet
The grounds of the IC, input capacitors, output capacitors, and
output diode (if applicable), should be connected close together,
and directly to a ground plane. It is also a good idea to have a
ground plane on both sides of the PCB. This reduces noise by
reducing ground loop errors and by absorbing more of the EMI
radiated by the inductor.
Use the following general guidelines when designing PCBs:
•
•
Keep CIN close to the VIN and GND leads of the ADD5207.
Keep the high current path from CIN (through L1) to the
SW and GND leads as short as possible.
•
Keep the high current path from CIN (through L1), D1, and
C
OUT as short as possible.
For multilayer boards of more than two layers, a ground plane
can be used to separate the power plane (power traces and com-
ponents) and the signal plane (feedback, compensation, and
components) for improved performance. On multilayer boards,
the use of vias is required to connect traces and different planes.
If a trace needs to conduct a significant amount of current from
one plane to the other, it is good practice to use one standard
via per 200 mA of current. Arrange the components so that the
switching current loops curl in the same direction.
•
•
Keep high current traces as short and as wide as possible.
Keep nodes connected to SW away from sensitive traces,
such as COMP, to prevent coupling of the traces. If such
traces must be run near each other, place a ground trace
between the two as a shield.
Place the compensation components as close as possible to
the COMP pin.
Place the LED current setting resistors as close as possible
to each pin to prevent noise pickup.
Avoid routing noise-sensitive traces near high current
traces and components, especially the LED current setting
node (ISET).
Use a thermal pad size that is the same dimension as the
exposed pad on the bottom of the package.
•
•
•
Due to how switching regulators operate, there are two power
states: one state when the switch is on, and one when the switch
is off. During each state, there is a current loop made by the
power components currently conducting. Place the power
components so that the current loop is conducting in the same
direction during each of the two states. This prevents magnetic
field reversal caused by the traces between the two half cycles
and reduces radiated EMI.
•
Heat Sinking
When using a surface-mount power IC or external power
switches, the PCB can often be used as the heat sink. This is
done by using the copper area of the PCB to transfer heat from
the device. Users should maximize this area to optimize thermal
performance.
Layout Procedure
To achieve high efficiency, good regulation, and stability, a good
PCB layout is required. It is recommended that the reference
board layout be followed as closely as possible because it is
already optimized for high efficiency and low noise.
Rev. A | Page 14 of 16
Data Sheet
ADD5207
TYPICAL APPLICATION CIRCUITS
L1
10µH
D1
V
IN
C
14
13
IN
10µF
+
–
C
OUT
4µF
SW
OVP
1
VIN
C
IN2
0.1µF
ADD5207
9
10
8
PWM
OFF ON
SHDN
VDD
C
BYPASS
1µF
4
5
6
7
FB1
FB2
FB3
FB4
2
FSLCT
R
F
100kΩ
ISET
3
COMP
11
GND
12
R
R
6.8kΩ
SET
C
C2
OPEN
180kΩ
C
C
2.2nF
Figure 16. Typical Four-String Application Circuit
L1
10µH
D1
C
14
13
IN
10µF
+
–
C
OUT
4µF
SW
OVP
1
VIN
C
IN2
0.1µF
ADD5207
9
10
8
PWM
OFF ON
SHDN
VDD
C
BYPASS
1µF
4
5
6
7
FB1
FB2
FB3
FB4
2
FSLCT
R
F
100kΩ
ISET
3
COMP
11
GND
12
R
R
6.8kΩ
SET
180kΩ
C
C
2
OPEN
C
C
2.2nF
Figure 17. Typical Three-String Application Circuit
Rev. A | Page 15 of 16
ADD5207
Data Sheet
OUTLINE DIMENSIONS
3.40
3.30
3.15
4.00 BSC
0.20 MIN
8
7
14
1
1.80
1.70
1.55
EXPOSED
PAD
3.00 BSC
PIN 1
INDICATOR
0.50
0.40
0.30
PIN
1
INDICATOR
(R 0.20)
BOTTOM VIEW
TOP VIEW
0.80
0.75
0.70
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
0.05 MAX
0.02 NOM
COPLANARITY
0.08
SECTION OF THIS DATA SHEET.
SEATING
PLANE
0.30
0.25
0.20
0.15 REF
0.50 BSC
COMPLIANT TO JEDEC STANDARDS MO-220-WGED
Figure 18. 14-Lead Lead Frame Chip Scale Package [LFCSP_WD]
4 mm × 3 mm Body, Very Very Thin Dual
(CP-14-1)
Dimensions shown in millimeters
ORDERING GUIDE
Model1
ADD5207ACPZ-RL
Temperature Range
−25°C to +85°C
Package Description
Package Option
CP-14-1
14-Lead LFCSP_WD
1 Z = RoHS Compliant Part.
©2009–2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D08350-0-2/12(A)
Rev. A | Page 16 of 16
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