19-5164; Rev 0; 3/10
EVALUATION KIT
AVAILABLE
Six-String WLED Driver with Integrated
General Description
The MAX17127 is a high-efficiency driver for white lightemitting diodes (LEDs). It is designed for large liquidcrystal displays (LCDs) that employ an array of LEDs as
the light source. An internal switch current-mode step-up
converter drives the LED array, which can be configured
for up to six strings in parallel and 13 LEDs per string.
Each string is terminated with ballast that achieves Q2%
current-regulation accuracy, ensuring even LED brightness. The MAX17127 has a wide input voltage range
from 5V to 26V, and provides adjustable 10mA to 30mA
full-scale LED current.
The MAX17127 can implement brightness control through
the PWM signal input, and LED current is directly controlled by the external dimming signal’s frequency and
duty cycle.
The MAX17127 has multiple features to protect the controller from fault conditions. Once an open/short string is
detected, the fault string is disabled while other strings
can still operate normally. The controller features cycleby-cycle current limit to provide constant operation and
soft-start capability. If the MAX17127 is in current-limit
condition, the step-up converter is latched off after an
internal timer expires. A thermal-shutdown circuit provides another level of protection. When thermal shutdown happens, the MAX17127 is latched off.
The MAX17127’s step-up controller features an internal 0.12I (typ), 48V (max) power MOSFET with local
current-sense amplifier for accurate cycle-by-cycle current limit. This architecture greatly simplifies the external
circuitry and saves PCB space. Low-feedback voltage at each LED string helps reduce power loss and
improve efficiency. The MAX17127 features resistoradjustable switching frequency from 250kHz to 1MHz,
which enables a wide variety of applications that can
trade off component size for operating frequency.
The MAX17127 is available in a thermally enhanced,
lead-free, 20-pin, 4mm x 4mm thin QFN package.
Step-Up Converter
MAX17127
Features
S 5V to 26V Input Supply Voltage
S Up to Six Parallel Strings Multiple Series-
Connected LEDs
S 250kHz to 1MHz Adjustable Switching Frequency
S 0.12I Internal HV Power MOSFET (48V max)
S Low String Feedback Voltage: 480mV at 20mA
LED Current
S Full-Scale LED Current Adjustable from 10mA to
30mA
S Q2% Current-Regulation Accuracy Between
Strings
S 400ns Minimum String On-Time
S 100Hz to 25kHz PWM Input Range
S Open and Short LED Protection
S Output Overvoltage Protection
S Thermal Shutdown
S Small 20-Pin, 4mm x 4mm Thin QFN Package
Ordering Information
PART TEMP RANGE PIN-PACKAGE
MAX17127ETP+ -40°C to +85°C 20 TQFN-EP*
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
Applications
Notebook, Subnotebook, and Tablet Computer
Displays
Automotive Systems
Handy Terminals
Simplified Operating Circuit appears at end of data sheet.
_______________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
Six-String WLED Driver with Integrated
Step-Up Converter
ABSOLUTE MAXIMUM RATINGS
VIN to AGND ........................................................-0.3V to +30V
FB_, SW to PGND .................................................-0.3V to +52V
PGND to AGND ....................................................-0.3V to +0.3V
V
, PWM, EN, FPO, I.C. to AGND .....................-0.3V to +6V
DDIO
COMP, ISET, R_FPWM, OVP, FSLCT
to AGND ................................................-0.3V to V
SW Switch Maximum Continuous RMS Current ...................1.6A
Continuous Power Dissipation (TA = +70NC)
TQFN (derate 16.9mW/NC above +70NC) ..................1349mW
MAX17127
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
DDIO
+ 0.3V
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1. VIN = 12V, C
TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN Input Voltage Range 5 26 V
VIN Quiescent Current
V
Output Voltage
DDIO
V
Current Limit V
DDIO
V
UVLO Threshold Rising edge, typical hysteresis = 250mV 3.90 4.00 4.10 V
DDIO
VIN UVLO Threshold
BOOST CONVERTER
SW On-Resistance 20mA from SW to PGND 0.12 0.25
SW Leakage Current 40V on SW, TA = +25NC 1 FA
Operating Frequency
R
Maximum Duty Cycle At f
Minimum On-Time (Note 1) 50 80 ns
SW Current Limit Duty cycle = 75% 3.12 3.9 4.7 A
CONTROL INPUT
PWM, EN Logic-Input
High Level
PWM, EN Logic-Input Low
Level
EN Pulldown Resistor 120 200 280 kI
Range Operating range 90 500 kI
FSLCT
MAX17127 is enabled, V
MAX17127 is disabled, EN = AGND 5 10 FA
MAX17127 is enabled, V
5.4V < V
MAX17127 is enabled, VEN = 3.3V, VIN = 5V, I
dropout condition
MAX17127 is disabled, EN = AGND, 0A < I
DDIO
Falling edge 4.3 4.5 4.7
Rising edge 4.55 4.75 4.95
R
FSLCT
R
FSLCT
= 0.51nF, C
COMP
< 26V, 0A < I
IN
is forced to 4.2V 25 45 70 mA
= 100kI 0.95 1.0 1.05
= 400kI 0.225 0.25 0.275
= 1MHz 91 95 %
SW
= 4.7µF, R
COUT
= 3.3V, VIN = 26V 2.7 3.2 mA
EN
= 3.3V,
EN
< 10mA
VDDIO
Operating Temperature Range .......................... -40NC to +85NC
Junction Temperature .....................................................+150NC
Storage Temperature Range ............................ -60NC to +150NC
ESD
HBM ...................................................................................2kV
MM ...................................................................................200V
Lead Temperature (soldering, 10s) ................................+300NC
Soldering Temperature (reflow) ......................................+260NC
COMP
VDDIO
= 82.5kΩ, R
= 10mA,
VDDIO
< 50FA 3.1 3.7 4.1 V
= 180kΩ, R
ISET
= 100kΩ, L = 10µH,
FSLCT
4.85 5 5.15
4.6 4.75
2.1 V
0.8 V
V
V
I
MHz
2 ______________________________________________________________________________________
Six-String WLED Driver with Integrated
Step-Up Converter
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VIN = 12V, C
TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
FPO OUTPUT
FPO Off-Leakage Current Fault inactive, TA = +25NC 100 nA
FPO On Output-Voltage
Low
INPUT LEAKAGE
PWM Leakage Current TA = +25NC, V
OVP Leakage Current TA = +25NC, V
LED CURRENT
Full-Scale FB_ Output
Current
R
Range
ISET
Current Regulation
Between Strings
Minimum FB_ Regulation
Voltage
FB_ On-Resistance V
FB_ Bias Current V
FB_ Minimum On-Time 400 580 700 ns
FAULT PROTECTION
OVP Threshold Voltage Rising edge, typical hysteresis = 90mV 1.23 1.25 1.27 V
FB_ Overvoltage
Threshold
FB_ Enable Threshold
Voltage
FB_ Open Threshold
Voltage
FB_ Check LED Source
Current
FB_ Check LED Time 0.7 1.0 1.3 ms
Thermal-Shutdown
Threshold
Overcurrent Fault Timer Latch-off timer 128 Fs
PWM CONTROL
PWM Input On-Time 400 ns
PWM Input Frequency
Range
I
SINK
R
ISET
R
ISET
R
ISET
V
ISET
Operating range 100 400
Accuracy = 3% 120 360
10mA < I
I
FB_
FB_
I
FB_
FB_
FB_
(Note 1) +150 NC
= 0.51nF, C
COMP
= 1mA, fault active 0.4 V
= 0V, V
PWM
= 0V, V
OVP
= 120kI 29.1 30 30.9
= 180kI 19.6 20 20.4
= 360kI 9.7 10 10.3
< 0.7V 0.2 0.3 0.4
< 30mA -2.0 +2.0 %
FB_
= 30mA 400 555 770
= 20mA 460 670
= 10mA 350 630
= 50mV (includes 10I sense resistor) 17.5 28.4
= 40V, TA = +25NC 0.1 1 FA
= 4.7µF, R
COUT
= 5V -1 +1 FA
PWM
= 5V -0.1 +0.1 FA
OVP
COMP
= 82.5kΩ, R
= 180kΩ, R
ISET
= 100kΩ, L = 10µH,
FSLCT
7 8 9 V
1.2 V
130 280 mV
0.4 1.3 mA
0.1 25 kHz
MAX17127
mA
kI
mVI
I
_______________________________________________________________________________________ 3
Six-String WLED Driver with Integrated
Step-Up Converter
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1. VIN = 12V, C
TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN Input Voltage Range 5 26 V
VIN Quiescent Current
MAX17127
V
Output Voltage
DDIO
V
Current Limit V
DDIO
V
UVLO Threshold Rising edge, typical hysteresis = 250mV 3.90 4.10 V
DDIO
VIN UVLO Threshold
BOOST CONVERTER
SW On-Resistance 20mA from SW to PGND 0.25 ω
SW Leakage Current 40V on SW, TA = +25NC 1 µA
Operating Frequency
R
Maximum Duty Cycle At fSW = 1MHz 92 %
Boost Output Voltage With suitable OVP network 45 V
Minimum On-Time (Note 1) 80 ns
CONTROL INPUT
PWM, EN Logic-Input High Level 2.1 V
PWM, EN Logic-Input Low Level 0.8 V
EN Pulldown Resistor 110 290 kω
FPO OUTPUT
FPO On Output-Voltage Low I
LED CURRENT
Full-Scale FB_ Output Current
R
Current Regulation Between
Strings
Minimum FB_ Regulation Voltage
FB_ On-Resistance V
FB_ Bias Current V
Operative Range 90 500 kω
FSLCT
Range
ISET
= 0.51nF, C
COMP
MAX17127 is enabled, V
MAX17127 is disabled, EN = AGND 15 µA
MAX17127 is enabled, VEN = 3.3V,
5.4V < V
MAX17127 is enabled , VEN = 3.3V, VIN = 5V,
I
VDDIO
EN = AGND, 0A < I
DDIO
Falling edge 4.3 4.7
Rising edge 4.55 4.95
R
FSLCT
R
FSLCT
SINK
R
ISET
R
ISET
R
ISET
V
ISET
Operating range 100 400
Accuracy = 3% 120 360
10mA < I
I
FB_
FB_
I
FB_
FB_
FB_
< 26V, 0A < I
IN
= 10mA, dropout condition
is forced to 4.2V 25 70 mA
= 100kI 0.95 1.05
= 400kI 0.225 0.28
= 1mA, fault active 0.4 V
= 120kI 29.1 30.9
= 180kI 19.4 20.6
= 360kI 9.7 10.3
< 0.7V 0.2 0.4
< 30mA -2.0 +2.0 %
FB_
= 30mA 400 770
= 20mA 670
= 10mA 630
= 50mV (includes 10I sense resistor) 28.4 ω
= 40V, TA = +25NC 1 µA
COUT
VDDIO
= 4.7µF, R
= 3.3V, V
EN
< 10mA
VDDIO
< 50FA 3.1 4.1
= 82.5kΩ, R
COMP
= 26V 3.2 mA
IN
= 180kΩ, R
ISET
FSLCT
4.85 5.15
4.6
= 100kΩ, L = 10µH,
V
V
MHz
mA
kω
mVI
4 ______________________________________________________________________________________
Six-String WLED Driver with Integrated
Step-Up Converter
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VIN = 12V, C
TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
FB_ Minimum On-Time 400 700 ns
FAULT PROTECTION
OVP Threshold Voltage Rising edge, typical hysteresis = 90mV 1.23 1.27 V
FB_ Overvoltage Threshold 7 9 V
FB_ Open Threshold Voltage 130 280 mV
FB_ Check LED Source Current 0.4 1.3 mA
FB_ Check LED Time 0.7 1.3 ms
Overcurrent Fault Timer Latch-off timer 88 168 µs
PWM CONTROL
PWM Input On-Time 400 ns
PWM Input Frequency Range 0.1 25 kHz
Note 1: Specifications are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1. VIN = 12V, TA = +25NC, unless otherwise noted.)
COMP
= 0.51nF, C
COUT
= 4.7µF, R
COMP
= 82.5kΩ, R
= 180kΩ, R
ISET
= 100kΩ, L = 10µH,
FSLCT
MAX17127
BOOST CONVERTER EFFICIENCY vs. INPUT VOLTAGE (VS)
= 32V, I
(V
OUT
92
90
88
86
84
EFFICIENCY (%)
82
80
78
5 26
= 120mA, BRIGHTNESS = 100%)
OUT
23201714118
INPUT VOLTAGE (V)
MAX17127 toc01
BOOST CONVERTER EFFICIENCY vs. BRIGHTNESS
= 2V, V
(V
S
90
80
70
EFFICIENCY (%)
60
50
0 100
= 32V, I
OUT
BRIGHTNESS (%)
= 120mA AT 100%)
OUT
80604020
MAX17127 toc02
_______________________________________________________________________________________ 5
Six-String WLED Driver with Integrated
Step-Up Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, TA = +25NC, unless otherwise noted.)
LED CURRENT vs. BRIGHTNESS SETTING
20
f
= 200Hz
PWM
15
MAX17127
10
LED CURRENT (mA)
5
0
0 100
LED CURRENT (I
2.04
2.02
2.00
LED CURRENT (mA)
1.98
PWM DUTY CYCLE (%)
= 20mA AT 10% BRIGHTNESS)
LED
vs. INPUT VOLTAGE (V
LED CURRENT (I
20.20
MAX17127 toc03
80604020
20.15
20.10
LED CURRENT (mA)
20.05
20.00
5 26
= 20mA AT 100% BRIGHTNESS)
LED
vs. INPUT VOLTAGE (V
INPUT VOLTAGE (V)
)
S
MAX17127 toc04
23201714118
IN QUIESCENT CURRENT
)
S
MAX17127 toc05
6
5
4
3
2
QUIESCENT CURRENT (mA)
1
vs. IN VOLTAGE
MAX17127 toc06
100% BRIGHTNESS
200Hz/1% BRIGHTNESS
1.96
5 26
INPUT VOLTAGE (V)
IN SHUTDOWN CURRENT
vs. IN VOLTAGE
10
EN = LOW
8
6
4
SHUTDOWN CURRENT (µA)
2
0
5 26
IN VOLTAGE (V)
23201714118
MAX17127 toc07
23201714118
0
5 26
IN VOLTAGE (V)
SWITCHING WAVEFORMS
= 5V, BRIGHTNESS = 100%)
(V
S
V
= 32V, I
OUT
= 120mA
1µs/div
OUT
MAX17127 toc08
23201714117
6 ______________________________________________________________________________________
V
LX
20V/div
0V
INDUCTOR
CURRENT
500mA/div
0mA
Six-String WLED Driver with Integrated
Step-Up Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, TA = +25NC, unless otherwise noted.)
MAX17127
SWITCHING WAVEFORMS
= 26V, BRIGHTNESS = 100%)
(V
S
V
OUT
= 32V, I
OUT
= 120mA
1µs/div
STARTUP WAVEFORMS
(BRIGHTNESS = 20%)
12V
2ms/div
MAX17127 toc09
MAX17127 toc11
V
LX
20V/div
0V
INDUCTOR
CURRENT
200mA/div
0mA
V
EN
5V/div
0V
INDUCTOR
CURRENT
500mA/div
0A
V
LX
20V/div
0V
V
OUT
20V/div
0V
STARTUP WAVEFORMS
(BRIGHTNESS = 100%)
12V
1ms/div
MAX17127 toc10
LED CURRENT WAVEFORMS
(BRIGHTNESS = 50%)
1ms/div
MAX17127 toc12
V
EN
5V/div
0V
INDUCTOR
CURRENT
500mA/div
0A
V
LX
20V/div
0V
V
OUT
20V/div
0V
V
FB1
10V/div
0V
I
LED
20mA/div
0mA
INDUCTOR
CURRENT
500mA/div
0mA
LED CURRENT WAVEFORMS
(BRIGHTNESS = 1%)
1ms/div
MAX17127 toc13
V
FB1
10V/div
0V
I
FB1
20mA/div
0mA
INDUCTOR
CURRENT
500mA/div
0mA
LED-OPEN FAULT PROTECTION
(BRIGHTNESS = 100%, LED OPEN ON FB1)
20mA
200ms/div
0mA
MAX17127 toc14
32V
V
FB1
1V/div
0V
V
FB2
10V/div
0V
V
OUT
10V/div
10V
I
FB2
10mA/div
_______________________________________________________________________________________ 7
Six-String WLED Driver with Integrated
Step-Up Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, TA = +25NC, unless otherwise noted.)
LED-SHORT FAULT PROTECTION
(BRIGHTNESS = 100%, 3 LEDs SHORT ON FB1)
MAX17127
LINE-TRANSIENT RESPONSE
= 21V 9V, BRIGHTNESS = 100%)
(V
S
21V
20mA
10µs/div
200µs/div
MAX17127 toc15
MAX17127 toc17
9V
V
FB1
10V/div
0V
I
FB1
50mA/div
0mA
0V
V
OUT
(AC-COUPLED)
1V/div
V
S
10V/div
0V
0A
INDUCTOR
CURRENT
1A/div
I
FB1
10mA/div
0mA
LINE-TRANSIENT RESPONSE
= 9V 21V, BRIGHTNESS = 100%)
(V
S
9V
20mA
200µs/div
MAX17127 toc16
21V
MAXIMUM UNBALANCE RATE BETWEEN STRING
vs. BRIGHTNESS (V
1.0
0.8
0.6
0.4
0.2
MAXIMUM UNBALANCE RATE (%)
0
10 100
MAXIMUM
UNBALANCE RATE (%)
= 12V, I
S
I I
FB_ FB(AVG) MAX
=
BRIGHTNESS (%)
−
I
FB(AVG)
LED
80 90706050403020
= 20mA)
V
OUT
(AC-COUPLED)
2V/div
0V
V
S
10V/div
0V
0A
INDUCTOR
CURRENT
1A/div
I
FB1
10mA/div
0mA
MAX17127 toc18
%
MAXIMUM UNBALANCE RATE BETWEEN STRINGS
= 20mA) vs. INPUT VOLTAGE (VS)
(I
LED
0.8
0.7
0.6
0.5
0.4
MAXIMUM UNBALANCE RATE (%)
0.3
0.2
MAXIMUM
UNBALANCE RATE (%)
5 26
INPUT VOLTAGE (V)
−
I I
FB_ FB(AVG) MAX
=
I
FB(AVG)
MAX17127 toc19
%
23201714118
8 ______________________________________________________________________________________
Six-String WLED Driver with Integrated
Step-Up Converter
Pin Configuration
MAX17127
TOP VIEW
SW
I.C.
COMP
V
PWM
OVP
PGND
15 14 12 11
16
17
+
1 2
DDIO
V
MAX17127
EN
18
19
IN
20
R_FPWM
13
3
FSLCT
FB1
EP
4 5
ISET
FB2
FPO
FB3
10
AGND
9
8
FB4
FB5
7
FB6
6
THIN QFN
(4mm × 4mm)
Pin Description
PIN NAME FUNCTION
1 V
DDIO
2 EN
3 FSLCT
4 ISET
5 FPO
6 FB6
5V Linear Regulator Output. V
a ceramic capacitor of 1FF or greater.
Enable Pin. EN = high enables the MAX17127. An internal 200kI (typ) pulldown resistor keeps the
MAX17127 in disabled mode if the EN pin is high impedance.
Oscillator Frequency-Adjustment Pin. The resistance from FSLCT to AGND sets the step-up
converter’s oscillator frequency:
The acceptable resistance range is 100kI < R
frequency of 1MHz > fSW > 250kHz.
Full-Scale LED Current-Adjustment Pin. The resistance from ISET to AGND controls the full-scale
current in each LED string:
The acceptable resistance range is 120kI < R
current of 30mA > I
LEDMAX
full-scale LED current.
Fault-Diagnostic Output. Open drain, active low. The FPO output is asserted low when the following
faults occur: overcurrent fault, thermal fault, output-voltage short condition, or output overvoltage.
LED String 6 Cathode Connection. FB6 is the open-drain output of an internal regulator, which
controls current through FB6. FB6 can sink up to 30mA. If unused, connect FB6 to AGND.
provides power to the MAX17127. Bypass V
DDIO
fSW = 1MHz O 100kI/R
FSLCT
I
= 20mA O 180kI/R
LEDMAX
ISET
FSLCT
< 400kI, which corresponds to the switching
ISET
< 360kI, which corresponds to a full-scale LED
to AGND with
DDIO
> 10mA. Connecting ISET to AGND sets the test mode for 0.3mA (typ)
_______________________________________________________________________________________ 9
Six-String WLED Driver with Integrated
Step-Up Converter
Pin Description (continued)
PIN NAME FUNCTION
7 FB5
8 FB4
9 AGND Analog Ground
MAX17127
10 FB3
11 FB2
12 FB1
13 R_FPWM Connect R_FPWM to AGND
14 OVP
15 PGND Boost Regulator Power Ground
16 SW Boost Regulator Power Switch Node
17 I.C. Internal Connection. Not connected externally.
18 COMP
19 V
20 PWM
— EP
IN
LED String 5 Cathode Connection. FB5 is the open-drain output of an internal regulator, which
controls current through FB5. FB5 can sink up to 30mA. If unused, connect FB5 to AGND.
LED String 4 Cathode Connection. FB4 is the open-drain output of an internal regulator, which
controls current through FB4. FB4 can sink up to 30mA. If unused, connect FB4 to AGND.
LED String 3 Cathode Connection. FB3 is the open-drain output of an internal regulator, which
controls current through FB3. FB3 can sink up to 30mA. If unused, connect FB3 to AGND.
LED String 2 Cathode Connection. FB2 is the open-drain output of an internal regulator, which
controls current through FB2. FB2 can sink up to 30mA. If unused, connect FB2 to AGND.
LED String 1 Cathode Connection. FB1 is the open-drain output of an internal regulator, which
controls current through FB1. FB1 can sink up to 30mA. If unused, connect FB1 to AGND.
Overvoltage Sense. Connect OVP to the boost converter output through a resistor:
V
= 1.25V O (1 + R1/R2 )
OVP
Step-Up Converter Compensation Pin. Connect a ceramic capacitor in series with a resistor from
COMP to AGND.
Supply Input. VIN biases the internal 5V linear regulator that powers the device. Bypass VIN to
AGND directly at the pin with a 0.1FF or greater ceramic capacitor.
PWM Signal Input. This signal is used for brightness control. The brightness is proportional to the
PWM duty cycle, and the PWM signal directly controls the LED turning on/off.
Exposed Backside Pad. Solder to the circuit board ground plane with sufficient copper connection
to ensure low thermal resistance. See the PCB Layout Guidelines section.
10 _____________________________________________________________________________________
Six-String WLED Driver with Integrated
Step-Up Converter
Typical Operating Circuit
MAX17127
The MAX17127 typical operating circuit is shown as
Figure 1. Table 1 lists some recommended components,
L1
10
µH D1
SWV
PGND0.1µF
OVP
COMP
AGND
MAX17127
FB1
FB2
FB3
FB4
5V TO 26V
V
S
180kI
C
IN
4.7µF
V
IN
1µF
R
ISET
R
FSLCT
100kI
3.3V
IN
V
DDIO
ISET
FSLCT
I.C.
EN
PWM
R_FPWM
and Table 2 lists the contact information for component
suppliers.
C
OUT
4.7µF
R
1
2.21MI
R
2
R
COMP
82.5kI
C
510pF
COMP
71.5kI
10kI
FAULT INDICATOR
Figure 1. Typical Operating Circuit
Table 1. Component List
DESIGNATION DESCRIPTION
4.7FF Q10%, 25V X5R ceramic capacitor
C
IN
C1, C2
D1
(1206)
Murata GRM319R61E475KA12D
2.2FF Q20%, 50V X7R ceramic capacitors
(1206)
Murata GRM31CR71H225K
2A, 40V Schottky diode (M-flat)
Toshiba CMS11
______________________________________________________________________________________ 11
FPO
FB5
FB6
EP
DESIGNATION DESCRIPTION
10FH, 1.2A power inductor
L1
Sumida CR6D09HPNP-100MC
TDK VLP6810T-100M1R2
White LED
3.2V (typ), 3.5V (max) at 20mA
Nichia NSSW008C
Six-String WLED Driver with Integrated
Step-Up Converter
Table 2. Component Suppliers
SUPPLIER PHONE WEBSITE
Murata Electronics North America, Inc. 770-436-1300 www.murata.com
Nichia Corp. 248-352-6575 www.nichia.com
Sumida Corp. 847-545-6700 www.sumida.com
Toshiba America Electronic Components, Inc. 949-455-2000 www.toshiba.com/taec
Vishay 203-268-6261 www.vishay.com
MAX17127
EN
FAULT
CONTROL
1.25V
OVP
V
DDIO
FSLCT
COMP
ISET
PWM
R_FPWM
V
IN
EN
5V LINEAR REGULATOR
OSCILLATOR
ERROR
AMPLIFIER
Gm
PWM CONTROL
V
ISET
DDIO
OVP
1.25V
CLAMP
ERROR
AMPLIFIER
MAX17127
SLOPE
COMPENSATION
TO FAULT
CONTROL
Gm
ERROR
COMPARATOR
OVERVOLTAGE
COMPARATOR
VSAT
CONTROL
AND
DRIVER
LOGIC
Σ
CURRENT
SENSE
8V
HVC
S&H
LVC
EN
N
FB6
FB5
FB4
FB3
FB2
N
SW
PGND
FB1
AGND
I.C. V
FPO
DDIO
FAULT
CONTROL
CURRENT SOURCE
CURRENT SOURCE FB3
CURRENT SOURCE FB4
CURRENT SOURCE FB5
CURRENT SOURCE FB6
Figure 2. Functional Diagram
12 _____________________________________________________________________________________
FB2
Six-String WLED Driver with Integrated
Detailed Description
The MAX17127 is a high-efficiency driver for arrays of
white LEDs. It contains a fixed-frequency current-mode
PWM step-up controller, a 5V linear regulator, a dimming
control circuit, an internal power MOSFET, and six regulated current sources. Figure 2 shows the MAX17127
functional diagram. When enabled, the step-up controller boosts the output voltage to provide sufficient headroom for the current sources to regulate their respective
string currents. The MAX17127 features resistor-adjustable switching frequency (250kHz to 1MHz), which
allows trade-offs between external component size and
operating efficiency.
The MAX17127 can implement brightness control
through the PWM signal input. The LED current is directly controlled by the external dimming signal's frequency
and duty cycle.
The MAX17127 has multiple features to protect the controller from fault conditions. Separate feedback loops limit
the output voltage in all circumstances. The MAX17127
checks each FB_ voltage during operation.
If one or more strings are open, the corresponding FB_
voltages are pulled below 180mV (max), and an opencircuit fault is detected. As a result, the respective current sources are disabled.
When one or more LEDs are shorted and the related
FB_ voltage exceeds 8V, short fault is detected and the
respective current source is disabled if at least one FB_
voltage is lower than the minimum FB_ regulation voltage
+460mV (typ).
When in LED open or short conditions, the fault string is
disabled while other strings can still operate normally.
The MAX17127 also includes other kinds of fault protections, which are overcurrent, thermal shutdown, and
output overvoltage. The MAX17127 features cycle-bycycle current limit to provide consistent operation and
soft-start protection. In an overcurrent condition, the
IC latches off if the fault still exists after a 128Fs overcurrent fault timer expires. The output overvoltage is a
nonlatched operation, and the step-up converter stops
switching during the fault. A thermal-shutdown circuit
provides another level of protection. The MAX17127 is
latched off once thermal shutdown occurs.
The MAX17127 includes a 5V linear regulator that provides the internal bias and gate driver for the step-up
controller.
Step-Up Converter
Fixed-Frequency Step-Up Controller
The MAX17127’s fixed-frequency, current-mode, stepup controller automatically chooses the lowest active FB_
voltage to regulate the feedback voltage. Specifically,
the difference between the lowest FB_ voltage and the
current source control signal plus an offset is integrated
at the COMP output. The resulting error signal is compared to the internal switch current plus slope compensation to determine the switch on-time. As the load
changes, the error amplifier sources or sinks current to
the COMP output to deliver the required peak inductor
current. The slope-compensation signal is added to the
current-sense signal in order to improve stability at high
duty cycles.
Internal 5V Linear Regulator and UVLO
The MAX17127 includes an internal low-dropout linear
regulator (V
ear regulator generates a 5V supply to power the internal
PWM controller, control logic, and MOSFET driver. The
V
voltage drops to 3.3V in shutdown. If 5V < VIN <
DDIO
5.5V, V
powered from an external 5V supply. There is a body
diode from V
V
The MAX17127 is disabled until V
threshold. The hysteresis on UVLO is approximately
250mV. In standby mode, the internal LDO is in low-power
mode with 10FA (max) input current and approximately
regulated at 3.3V (typ). When EN = high, the internal LDO
is enabled and regulated accurately at 5V (typ).
The V
minimum 1FF ceramic capacitor.
At startup, the MAX17127 performs a diagnostic test of
the LED array. In the test phase, all FB_ pins are pulled
up by a given current source (0.4mA min) during 1ms
(typ). If some FB_ voltage is lower than 1.2V (max), the
string is considered to be unused. Therefore, when a
string is not in use, it should be connected to AGND. All
other strings with FB_ higher than 1.2V (max) are detected as in use. After the LED string diagnostic phases are
finished, the boost converter starts. An additional 1ms
after boost soft-start end is used as minimum FB_ control. The total startup time is less than 10ms, including
2ms (typ) soft-start. Figure 3 shows the sequence.
DDIO
(see Figure 2).
DDIO
DDIO
). When VIN is higher than 5.0V, this lin-
DDIO
and VIN can be connected together and
to VIN, so VIN must be greater than
DDIO
exceeds the UVLO
DDIO
pin should be bypassed to AGND with a
Startup
MAX17127
______________________________________________________________________________________ 13
Six-String WLED Driver with Integrated
Step-Up Converter
Shutdown
The MAX17127 can be placed into shutdown by pulling
the EN pin low. When a critical failure is detected, the
IC also enters shutdown mode. In shutdown mode, all
functions of the IC are turned off, including the 5V linear regulator. Only a crude linear regulator remains on,
providing a 3.3V (typ) output voltage to V
DDIO
with 1FA
current-sourcing capability.
MAX17127
0V
0V
Frequency Selection
The boost converter switching frequency can be adjusted by the external resistor on the FSLCT pin. The
switching frequency adjustable range is 250kHz to
1MHz. High-frequency (1MHz) operation optimizes the
regulator for the smallest component size at the expense
of efficiency due to increased switching losses. Lowfrequency (250kHz) operation offers the best overall efficiency, but requires larger components and PCB area.
V
IN
V
OUT
I
LED
0V
0V
CHECK LED
Figure 3. Startup Sequence
14 _____________________________________________________________________________________
STEP-UP REGULATOR
SOFT-START
MIN FB_ CONTROL
(1ms)
V
EN
V
DDIO
Six-String WLED Driver with Integrated
Overvoltage Protection
To protect the step-up regulator when the load is open,
or if the output voltage becomes excessive for any reason, the MAX17127 features a dedicated overvoltagefeedback input (OVP). The OVP pin is connected to
the center tap of a resistive voltage-divider from the
high-voltage output. When the OVP pin voltage, V
exceeds 1.25V (typ), a comparator turns off the internal
power MOSFET. This step-up regulator switch is reenabled after the V
the protection threshold. This overvoltage-protection
feature ensures the step-up regulator fail-safe operation
when the LED strings are disconnected from the output.
drops 90mV (typ hysteresis) below
OVP
LED Current Sources
Maintaining uniform LED brightness and dimming capability is critical for backlight applications. The MAX17127
is equipped with a bank of six matched current sources.
These specialized current sources are accurate within
P 3% and match each other within 2%. They can be
switched on and off at PWM frequencies of up to 25kHz.
LED full-scale current is set through the ISET pin (10mA
< I
< 30mA).
LED
The minimum voltage drop across each current source
is 480mV (typ) when the LED current is 20mA. The lowvoltage drop helps reduce dissipation while maintaining
sufficient compliance to control the LED current within
the required tolerances.
The LED current sources can be disabled by connecting the respective FB_ pin to AGND at startup. When the
IC is enabled, the controller scans settings for all FB_
pins. If an FB_ pin is not connected to AGND, an internal circuit pulls this pin high, and the controller enables
the corresponding current source to regulate the string
current. If the FB_ pin is connected to AGND, the controller disables the corresponding current regulator. The
current regulator cannot be disabled by connecting the
respective FB_ pin to AGND after the IC is enabled.
All FB_ pins in use are combined to extract a lowest FB_
voltage (LVC) (see Figure 2). LVC is fed into the step-up
regulator’s error amplifier and is used to set the output
voltage.
OVP
Step-Up Converter
Current-Source Fault Protection
LED fault open/short is detected after startup. When one
or more strings fail after startup, the corresponding current source is disabled. The remaining LED strings are
still operated normally. The LED open/short detection is
not executed when LED on-time is less than 2Fs.
,
The MAX17127 can tolerate a slight mismatch between
LED strings. When severe mismatches or WLED shorts
occur, the FB_ voltages are uneven because of mismatched voltage drops across strings. At each LED
turn-on, the FB_ voltage is brought down to the regulation voltage quickly. When FB_ voltage is higher than
8V (typ) after LED turn-on, an LED short is detected if
at least one FB_ voltage is lower than the minimum FB_
regulation voltage +460mV (typ). The remaining LED
strings can still operate normally. The LED short protection is disabled during the soft-start phase of the step-up
regulator.
Open Current-Source Protection
The MAX17127 step-up regulator output voltage is
regulated according to the minimum FB_ voltages on all
the strings in use. If one or more strings are open, the
respective FB_ pins are pulled to ground. For any FB_
lower than 180mV, the corresponding current source is
disabled. The remaining LED strings can still operate
normally. If all strings in use are open, the MAX17127
shuts the step-up regulator down.
FPO Function
The fault conditions trigger FPO function and pull the
FPO pin low. Table 3 shows the state of the FPO pin with
different fault conditions.
Dimming Control
The MAX17127 performs brightness control with a PWM
input signal. Dimming duty cycle and frequency of current sources follow the signal at the PWM pin directly.
MAX17127
Table 3. FPO Function Table
THERMAL FAULT OUTPUT OVERVOLTAGE INPUT OVERCURRENT
LATCHED
FPO PIN STATE
______________________________________________________________________________________ 15
Yes No (stop switching) Yes (after time expires)
Low Low Low
FAULT CONDITION
Six-String WLED Driver with Integrated
( )
V
Step-Up Converter
Full-Scale and Low-Level LED Current
The full-scale LED current is set by:
20mA 180k
I
LED_MAX
The acceptable resistance range for ISET is 120kI <
R
< 360kI, which corresponds to full-scale LED
ISET
current of 30mA > I
=
LED_MAX
MAX17127
The MAX17127 includes a thermal-protection circuit.
When the local IC temperature exceeds +150NC (typ),
the controller and current sources shut down. When
the thermal shutdown happens, the FPO output pin is
asserted low. The controller and current sources do not
restart until the next enable signal is sent or input supply
is recycled.
× Ω
R
ISET
> 10mA.
Thermal Shutdown
Design Procedure
All MAX17127 designs should be prototyped and tested
prior to production.
External component value choice is primarily dictated
by the output voltage and the maximum load current, as
well as maximum and minimum input voltages. Begin by
selecting an inductor value. Once the inductor is known,
choose the diode and capacitors.
Step-Up Converter Current Calculation
To ensure the stable operation, the MAX17127 includes
slope compensation, which sets the minimum inductor
value. In continuous conduction mode (CCM), the minimum
inductor value is calculated with the following equation:
V V 2 V R
L
CCM(MIN)
where:
SF is a scale factor from the slope compensation
depending on input voltage (this allows a higher current
capability), the L
for stable operation in CCM, and RS = 15mI (typ) is
the equivalent sensing scale factor from the controller’s
internal current-sense circuit.
OUT(MAX) DIODE IN(MIN) S
=
SF 72mV, when V 12.5V
= <
SF , when V 12.5V
72mV
= >
V 12.5V
IN
+
1
CCM(MIN)
+ − × ×
2 SF f
× ×
SW(MIN)
IN
−
10.6V
is the minimum inductor value
IN
The controller can also operate in discontinuous conduction mode (DCM). In this mode, the inductor value
can be lower, but the peak inductor current is higher
than in CCM. In DCM, the maximum inductor value is
calculated with the following equation:
L 1
DCM(MAX)
×
2 f V I
× × ×
where the L
DCM, E is the nominal regulator efficiency (85%), and
I
OUT(MAX)
The output current capability of the step-up regulator
is a function of current limit, input voltage, operating
frequency, and inductor value. Because the slope compensation is used to stabilize the feedback loop, the
inductor current limit depends on the duty cycle, and is
determined with the following equation:
where SF is the scale factor from the slope compensation, 2.5A is the current limit specified at 75% duty cycle,
and D is the duty cycle.
The output current capability depends on the currentlimit value and operating mode. The maximum output
current in CCM is governed by the following equation:
I I
OUT_CCM(MAX) LIM
where I
nominal regulator efficiency (85%), and D is the duty
cycle. The corresponding duty cycle for this current is:
where V
diode and RON is the internal MOSFET’s on-resistance
(0.2I typ).
DCM(MAX)
is the maximum output current.
I 0.97, when D 30%
I (1.27-D), when D 30%
LIM
is the current limit calculated above, E is the
LIM
D
DIODE
= −
V V
V
IN(MIN)
SW(MAX) OUT(MAX) OUT(MAX)
is the maximum inductor value for
SF
= × <
LIM
R
S
SF
= × >
R
S
= − × × η
V V V
− +
OUT IN DIODE
=
V I R V
− × +
OUT LIM ON DIODE
is the forward voltage of the rectifier
IN(MIN)
OUT(MAX) DIODE
0.5 D V V
+
2
× η
× ×
IN IN
f L V
×
SW OUT
16 _____________________________________________________________________________________
Six-String WLED Driver with Integrated
( )
The maximum output current in DCM is governed by the
following equation:
2
L I f V V
× × × η × +
I
OUT_DCM(MAX)
The inductance, peak current rating, series resistance,
and physical size should all be considered when selecting an inductor. These factors affect the converter’s
operating mode, efficiency, maximum output load capability, transient response time, output voltage ripple, and
cost. The maximum output current, input voltage, output
voltage, and switching frequency determine the inductor value. Very high inductance minimizes the current
ripple, and therefore reduces the peak current, which
decreases core losses in the inductor and I2R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of wire,
which increase physical size and I2R copper losses. Low
inductor values decrease the physical size but increase
the current ripple and peak current. Finding the best
inductor involves compromises among circuit efficiency,
inductor size, and cost.
In choosing an inductor, the first step is to determine the
operating mode: continuous conduction mode (CCM) or
discontinuous conduction mode (DCM). The MAX17127
has a fixed internal slope compensation, which requires
a minimum inductor value. When CCM mode is chosen,
the ripple current and the peak current of the inductor can
be minimized. If a small-size inductor is required, DCM
mode can be chosen. In DCM mode, the inductor value
and size can be minimized, but the inductor ripple current
and peak current are higher than those in CCM. The controller can be stable, independent of the internal slopecompensation mode, but there is a maximum inductor
value requirement to ensure the DCM operating mode.
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current to
the average DC inductor current at the full-load current.
The controller operates in DCM mode when LIR is higher
than 2.0, and it works in CCM mode when LIR is lower
than 2.0. The best trade-off between inductor size and
converter efficiency for step-up regulators generally has
an LIR between 0.3 and 0.5. However, depending on the
AC characteristics of the inductor core material and ratio
LIM SW OUT DIODE
=
2 V V V V
× × + −
( )
OUT OUT DIODE IN
( )
Inductor Selection
Step-Up Converter
of inductor resistance to other power-path resistances,
the best LIR can shift up or down. If the inductor resistance is relatively high, more ripples can be accepted
to reduce the number of required turns and increase
the wire diameter. If the inductor resistance is relatively
low, increasing inductance to lower the peak current can
reduce losses throughout the power path. If extremely
thin high-resistance inductors are used, as is common
for LCD panel applications, LIR higher than 2.0 can be
chosen for DCM operating mode.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions. The detailed
design procedure for CCM can be described as follows.
Calculate the approximate inductor value using the
typical input voltage (VIN), the maximum output current (I
OUT(MAX)
from an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
L
The MAX17127 has a minimum inductor value limitation
for stable operation in CCM mode at low-input voltage
because of the internal fixed-slope compensation. The
minimum inductor value for stability is calculated with the
following equation:
L
CCM(MIN)
where SF is a scale factor from slope compensation,
and RS is the equivalent current-sensing scale factor
(15mI typ).
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current
at the minimum input voltage V
tion of energy and the expected efficiency at that operating point (E
Typical Operating Characteristics:
), the expected efficiency (E
2
V V V
IN(MIN) OUT IN(MIN)
=
V I f LIR
OUT OUT(MAX) SW
V V 2 V R
=
) taken from an appropriate curve in the
MIN
I
IN(DC,MAX)
OUT(MAX) DIODE IN(MIN) S
−
+ − × ×
2 SF f
× ×
IN(MIN),
I V
OUT(MAX) OUT
=
V
IN(MIN) MIN
×
SW(MIN)
using conserva-
×
× η
η
TYP
TYP
) taken
MAX17127
______________________________________________________________________________________ 17
Six-String WLED Driver with Integrated
( )
Step-Up Converter
Calculate the ripple current at that operating point and
the peak current required for the inductor:
V V V
I
RIPPLE
When DCM operating mode is chosen to minimize the
MAX17127
inductor value, the calculations are different from those
above in CCM mode. The maximum inductor value for
DCM mode is calculated with the following equation:
L 1
DCM(MAX)
×
2 f V I
The peak inductor current in DCM is calculated with the
following equation:
=
I
PEAK
The inductor’s saturation current rating should exceed
I
and the inductor’s DC current rating should
PEAK,
exceed I
inductor with less than 0.1I series resistance.
Considering the circuit with six 10-LED strings and
20mA LED full-scale current, the maximum load current
(I
OUT(MAX)
input voltage of 7V.
Choosing a CCM operating mode with LIR = 0.7 at 1MHz
and estimating efficiency of 85% at this operating point:
IN(DC,MAX)
IN(MIN) OUT(MAX) IN(MIN)
=
I I
PEAK IN(DC,MAX)
× × ×
SW(MAX) OUT(MAX) OUT(MAX)
I 2 V
OUT(MAX) OUT(MAX)
× + −
( )
× × η× +
L f V V
SW(MIN) OUT(MAX) DIODE
) is 120mA with a 32V output and a minimal
× −
( )
L V f
× ×
OUT(MAX) SW
I
V
2
× η
RIPPLE
2
IN(MIN)
+
= +
= −
V V
OUT(MAX) DIODE
V
IN(MIN)
× ×
V V V
OUT(MAX) DIODE IN(MIN)
( )
. For good efficiency, choose an
A 10FH inductor is chosen, which is higher than the minimum L that guarantees stability in CCM.
The peak inductor current at minimum input voltage is
calculated as follows:
7V 32V 7V
I 0.95A
PEAK
Alternatively, choose a DCM operating mode by using
lower inductance and estimating efficiency of 85% at this
operating point. Since DCM has higher peak inductor
current at lower input, it causes current limit when the
parameters are not chosen properly. Considering the
case with six 10-LED strings and 20mA LED full-scale
current to prevent excessive switch current from causing
current limit:
A 3.3FH inductor is chosen. The peak inductor current at
minimum input voltage is calculated as follows:
I 1.40A
120mA 32V
= + =
= =
PEAK
×
7V 0.85 2 10 H 32V 0.9MHz
× × µ × ×
L 1
DCM(MAX)
× = µ
2 1.1MHz 32V 120mA
× × ×
120mA 2 32V 32V 0.4V 7V
3.3 H 1.1MHz 0.85 32V 0.4V
µ × × × +
= −
2
(7V) 0.85
× × × + −
× −
7V
32V 0.4V
+
×
( )
( )
3.9 H
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharging on the output capacitor, and the ohmic ripple due to
the capacitor’s equivalent series resistance (ESR):
and:
V V V
RIPPLE RIPPLE(C) RIPPLE(ESR)
V
RIPPLE(C)
= +
I V V
OUT(MAX) OUT(MAX) IN(MIN)
≈
C V f
OUT OUT(MAX) SW
−
×
2
7V 32V 7V 0.85
L 12.1 H
= = µ
32V 120mA 1MHz 0.7
In CCM, the inductor has to be higher than L
32V 0.4V 2 7V 13.7m
L 5.5 H
CCM(MIN)
18 _____________________________________________________________________________________
( )
= = µ
−
×
+ − × × Ω
2 25.5mV 0.9MHz
× ×
CCM(MIN)
where I
Inductor Selection section).
:
The output voltage ripple should be low enough for the
FB_ current-source regulation. The ripple voltage should
be less than 200mV
put voltage ripple is typically dominated by V
V I R≈
RIPPLE(ESR) PEAK ESR(COUT)
is the peak inductor current (see the
PEAK
. For ceramic capacitors, the out-
P-P
RIPPLE(C)
.
Six-String WLED Driver with Integrated
R1
The voltage rating and temperature characteristics of the
output capacitor must also be considered.
Step-Up Converter
MAX17127
tion can be tolerated on CIN if IN is decoupled from CIN
using an RC lowpass filter.
Rectifier Diode Selection
The MAX17127’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. The diode should be rated to
handle the output voltage and the peak switch current.
Make sure that the diode’s peak current rating is at least
I
calculated in the Inductor Selection section and
PEAK
that its breakdown voltage exceeds the output voltage.
Overvoltage-Protection Determination
The overvoltage-protection circuit ensures the circuit
safe operation; therefore, the controller should limit the
output voltage within the ratings of all MOSFET, diode,
and output capacitor components, while providing sufficient output voltage for LED current regulation. The
OVP pin is connected to the center tap of a resistive
voltage-divider (R1 and R2 in Figure 1) from the highvoltage output. When the controller detects the OVP pin
voltage reaching the threshold V
overvoltage protection is activated. Hence, the step-up
converter output overvoltage-protection point is:
V V (1 )
OUT(OVP) OVP_TH
V
OUT(OVP)
each string and V
and where V
string.
In Figure 1, the output OVP voltage is set to:
depends on how many LEDs are used for
is the LED’s operating voltage for each
OUT
V 1.25V (1 ) 39.71V
OUT(OVP)
= × +
OUT(OVP)
= × + =
OVP_TH
= 1.25V x V
2.21M
71.5k
, typically 1.25V,
R2
, generally
OUT
Ω
Ω
LED Selection and Bias
The series/parallel configuration of the LED load and
the full-scale bias current have a significant effect on
regulator performance. LED characteristics vary significantly from manufacturer to manufacturer. Consult
the respective LED data sheets to determine the range
of output voltages for a given brightness and LED current. In general, brightness increases as a function of
bias current. This suggests that the number of LEDs
could be decreased if higher bias current is chosen;
however, high current increases LED temperature and
reduces operating life. Improvements in LED technology
are resulting in devices with lower forward voltage while
increasing the bias current and light output.
LED manufacturers specify LED color at a given LED
current. With lower LED current, the color of the emitted light tends to shift toward the blue range of the
spectrum. A blue bias is often acceptable for business
applications, but not for high-image-quality applications
such as DVD players. Direct-DPWM dimming is a viable
solution for reducing power dissipation while maintaining
LED color integrity. Careful attention should be paid to
switching noise to avoid other display-quality problems.
Using fewer LEDs in a string improves step-up converter
efficiency, and lowers breakdown voltage requirements
of the external MOSFET and diode. The minimum number of LEDs in series should always be greater than
maximum input voltage. If the diode voltage drop is
lower than maximum input voltage, the voltage drop
across the current-sense inputs (FB_) increases and
causes excess heating in the IC. Between 8 and 12
LEDs in series are ideal for input voltages up to 20V.
Input Capacitor Selection
The input capacitor (CIN) filters the current peaks drawn
from the input supply and reduces noise injection into
the IC. A 4.7FF ceramic capacitor is used in the typical
operating circuit (Figure 1) because of the high source
impedance seen in typical lab setups. Actual applications usually have much lower source impedance since
the step-up regulator often runs directly from the output
of another regulated supply. In some applications, C
can be reduced below the values used in the typical
operating circuit. Ensure a low-noise supply at IN by
using adequate CIN. Alternatively, greater voltage varia-
______________________________________________________________________________________ 19
IN
Applications Information
LED V
The forward voltage of each white LED may vary up
to 25% from part to part and the accumulated voltage
difference in each string equates to additional power
loss within the IC. For the best efficiency, the voltage
difference between strings should be minimized. The
difference between lowest voltage string and highest
voltage string should be less than 8V (typ). Otherwise,
the internal LED short-protection circuit disables the high
FB_ voltage string.
Variation
FB_
Six-String WLED Driver with Integrated
Step-Up Converter
FB Pin Maximum Voltage
The current through each FB_ pin is controlled only
during the step-up converter’s on-time. During the converter off-time, the current sources are turned off. The
output voltage does not discharge and stays high. The
MAX17127 disables the FB_ current source, which the
string is shorted. In this case, the step-up converter’s
output voltage is always applied to the disabled FB_ pin.
The FB_ pin can withstand 45V.
MAX17127
Careful PCB layout is important for proper operation.
Use the following guidelines for good PCB layout:
1) Minimize the area of high-current switching loop of
rectifier diode, internal MOSFET, and output capacitor to avoid excessive switching noise.
2) Connect high-current input and output components
with short and wide connections. The high-current
input loop goes from the positive terminal of the input
capacitor to the inductor, to the internal MOSFET,
and then to the input capacitor’s negative terminal.
The high-current output loop is from the positive terminal of the input capacitor to the inductor, to the rectifier diode, and to the positive terminal of the output
capacitors, reconnecting between the output capacitor and input capacitor ground terminals. Avoid using
vias in the high-current paths. If vias are unavoidable,
use multiple vias in parallel to reduce resistance and
inductance.
PCB Layout Guidelines
3) Create a ground island (PGND) consisting of the
input and output capacitor ground. Connect all
these together with short, wide traces or a small
ground plane. Maximizing the width of the power
ground traces improves efficiency and reduces output voltage ripple and noise spikes. Create an analog
ground island (AGND) consisting of the overvoltage
detection divider (R1 and R2) ground connection; the
ISET, FSLCT, COMP resistor connections; and the
device’s exposed backside pad. Connect the AGND
and PGND islands by connecting the AGND pins
directly to the exposed backside pad. Make no other
connections between these separate ground planes.
4) Place the overvoltage-detection divider resistors as
close to the OVP pin as possible. The divider’s center
trace should be kept short. Placing the resistors far
away causes the sensing trace to become antennae
that can pick up switching noise. Avoid running the
sensing traces near SW.
5) Place the VIN pin and V
tors as close to the device as possible. The ground
connection of the bypass capacitors should be connected directly to AGND pins with a wide trace.
6) Minimize the size of the SW node while keeping it
wide and short. Keep the SW node away from the
feedback node and ground. If possible, avoid running the SW node from one side of the PCB to the
other. Use DC traces as a shield if necessary.
Refer to the MAX17127 Evaluation Kit data sheet for an
example of proper board layout.
pin bypass capaci-
DDIO
20 _____________________________________________________________________________________
Six-String WLED Driver with Integrated
Step-Up Converter
Simplified Operating Circuit (Direct-PWM Mode)
V
IN
DDIO
ISET
FSLCT
MAX17127
I.C.
EN
PWM
R_FPWM
3.3V
SWV
PGND
OVP
COMP
AGND
FB1
FB2
FB3
FB4
MAX17127
FB5
FPO
FB6
EP
Chip Information
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns,
go to www.maxim-ic.com/packages. Note that a “+”, “#”, or
“-” in the package code indicates RoHS status only. Package
drawings may show a different suffix character, but the drawing
pertains to the package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
20 TQFN T2044+3
21-0139
______________________________________________________________________________________ 21
Six-String WLED Driver with Integrated
Step-Up Converter
Revision History
REVISION
NUMBER
0 3/10 Initial release —
REVISION
DATE
MAX17127
DESCRIPTION
PAGES
CHANGED
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied.
Maxim reserves the right to change the circuitry and specifications without notice at any time.
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©
2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
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