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19-0865; Rev 0; 8/07
Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package
General Description
The MAX8901A/MAX8901B step-up converters drive from two to six series-connected white LEDs (WLEDs) with constant current to provide uniform WLED intensity for LCD backlighting in cell phones, PDAs, and other handheld devices. The MAX8901_ operate at a fixed 750kHz (typ) switching frequency, allowing for tiny external components, and are optimized for the highest possible efficiency over the full 1-cell Li+/Li-Poly battery range. These converters use a single input (ON) to enable the IC and to control WLED intensity. The MAX8901A requires a direct PWM input to regulate WLED intensity with the WLED current proportional to the PWM duty cycle. The MAX8901B uses single-wire, serial-pulse dimming that reduces the WLED intensity in 32 linear steps. Full-scale WLED current for serial-pulse dimming is 24.75mA (MAX8901B, 0.75mA/step). The MAX8901_ features an internal soft-start to eliminate inrush currents during startup, input overvoltage protection, WLED overvoltage protection, and a shutdown mode with 0.01A (typ) shutdown current. No WLED current is present in shutdown provided the WLED forward voltage is greater than the input supply voltage. Additional features include undervoltage lockout (UVLO) and thermal shutdown. The MAX8901_ are available in tiny 8-pin, 2mm x 2mm TDFN-EP packages (0.8mm max height). High Efficiency, Up to 91% 2.6V to 5.5V Input Voltage Range Fixed-Frequency Operation Supplies from 2 to 6 WLEDs with 1% LED Current Accuracy Flexible Dimming Control Direct PWM Dimming (MAX8901A) 32-Step, 1-Wire Serial Dimming (MAX8901B) Input Undervoltage Lockout Input Overvoltage Lockout WLED Overvoltage Protection (25V typ) 0.01A (typ) Shutdown Current No WLED Current in Shutdown Internal Soft-Start and Thermal Shutdown
KIT ATION EVALU LE B AVAILA
Features
MAX8901A/MAX8901B
Ordering Information
PART MAX8901AETA+T MAX8901BETA+T PIN-PACKAGE 8 TDFN-EP* 2mm x 2mm 8 TDFN-EP* 2mm x 2mm PKG CODE T822-1 T822-1 TOP MARK +ABA +ABB
Applications
Display Backlight (from 2 to 6 WLEDs) Cellular Phones PDAs and Smartphones MP3 and Portable Media Players Portable Navigation Devices Digital Cameras
Note: All devices are specified over the -40C to +85C operating temperature range. +Denotes a lead-free package. *EP = Exposed paddle. T = Tape and reel. Pin Configuration appears at end of data sheet.
Typical Operating Circuits
INPUT 2.6V TO 5.5V OUTPUT UP TO 25V
INPUT 2.6V TO 5.5V
3 4
IN MAX8901A COMP
LX
3
IN MAX8901B
LX
4
OUTPUT UP TO 25V
7
OVP
1
7
COMP
OVP
1
6 PWM DIMMING CONTROL 2
GND PGND
5
SERIAL-PULSE DIMMING CONTROL
6
GND
PGND
5
ON
EP
CS
8
2
ON EP
CS
8
________________________________________________________________ Maxim Integrated Products
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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.
Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B
ABSOLUTE MAXIMUM RATINGS
IN to GND .................................................................-0.3V to +7V CS, COMP, ON to GND ...............................-0.3V to (VIN + 0.3V) OVP, LX to GND .....................................................-0.3V to +28V PGND to GND .......................................................-0.3V to +0.3V LX Current ..................................................................770mARMS Continuous Power Dissipation (multilayer board at +70C) 8-pin, 2mm x 2mm TDFN (derate above +70C by 11.9mW/C) ..........................953mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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.
ELECTRICAL CHARACTERISTICS
(VIN = VON = VOVP = 3.6V, VPGND = VGND = 0V, COMP, CS, and LX are unconnected, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER Input Voltage Input Undervoltage Lockout Threshold Input Overvoltage Lockout Threshold Shutdown Input Current SYMBOL VIN VIN_UVLO VIN rising VIN falling VIN_OVLO ISHDN VIN rising VIN falling VON = 0V VCS = 0.55V, no load (not switching) TA = +25C TA = +85C MAX8901A MAX8901B VIN VDIODE 24 25 20 0.475 1.18 1.3 0.4 TA = +25C TA = +85C 40 0.5 0.5 500.0 0.01 0.1 1 0.50 1.33 0.525 1.50 CONDITIONS MIN 2.6 2.25 2.20 6.2 6.0 0.01 0.1 70 115 135 185 VOV 26 A TYP MAX 5.5 2.50 2.53 6.3 6.3 1 V A UNITS V V
Quiescent Current
IQ
Output Voltage Range OVP Overvoltage Protection Threshold OVP Input Current CS Regulation Voltage ON Shutdown Delay ON High Voltage ON Low Voltage ON Input Current Initial ON High Pulse Width ON High Pulse Width ON Low Pulse Width
VOUT VOV IOV VCS tSHDN VON_HI VON_LO ION tHI_INIT tHI tLO
VDIODE = external boost diode voltage drop 6 LEDs with 25V OVP option VOVP = 20V No dimming Time VON is below low threshold until shutdown (Figure 1) 2.6V < VIN < 5.5V 2.6V < VIN < 5.5V ON = IN
V V A V ms V V A s s s
First ON high pulse to enable IC (MAX8901B) (Figure 1) MAX8901B (Figure 1) MAX8901B (Figure 1)
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VON = VOVP = 3.6V, VPGND = VGND = 0V, COMP, CS, and LX are unconnected, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER Serial Dimming Full-Scale LED Current Accuracy PWM Frequency for PWM Dimming Control CS to COMP Transconductance Soft-Start Interval Thermal Shutdown OSCILLATOR Operating Frequency Maximum Duty Cycle n-CHANNEL SWITCH LX Leakage Current n-Channel Switch On-Resistance n-Channel Current Limit ILXLKG RLX ILIM 0.63 VLX = 27V TA = +25C TA = +85C 0.1 0.1 0.7 0.70 1.4 0.77 5 A A fSW DMAX VCS = 0.4V 700 90 750 92 800 kHz % tSOFTSTART
MAX8901A/MAX8901B
SYMBOL
CONDITIONS ILED = 24.75mA (MAX8901B) TA = +25C TA = -40C to +85C
MIN -1 -3
TYP
MAX +1 +3
UNITS % % kHz
fPWM
MAX8901A 40 CCOMP = 0.022F (Figure 1)
100 60 10 160 80
S ms C
Note 1: Specifications to -40C are guaranteed by design and characterization and are not production tested.
STEP tHI_INIT > 40s
0 ON
1
2
3
4
5
28
29
30
31
32
33
tHI tSOFT-START ILED_ SHUTDOWN FULL 32/33 31/33 30/33 29/33 tLO 500ns TO 500s 28/33 6/33 5/33 4/33 3/33 2/33 tSHDN > 500ns FULL 32/33 1.33ms (typ)
1/33
SHUTDOWN
Figure 1. MAX8901B Timing Diagram
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B
Typical Operating Characteristics
(Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1F, C2 = 1F, TA = +25C, unless otherwise noted.)
LED EFFICIENCY vs. INPUT VOLTAGE
MAX8901A/B toc01
LED EFFICIENCY vs. INPUT VOLTAGE
4 WLEDs 85 EFFICIENCY (%) 3 WLEDs 80 6 WLEDs
MAX8901A/B toc02
90
90
85 EFFICIENCY (%) 3 WLEDs 80 4 WLEDs 6 WLEDs 75 INDUCTOR: TOKO 1069AS-220M DIODE: CENTRAL SEMI CMDSH05-4 70 2 3 4 INPUT VOLTAGE (V) 5
75 INDUCTOR: TOKO 1098AS-100M DIODE: NXP PMEG3005EB 70 6 2 3 4 INPUT VOLTAGE (V) 5 6
LED EFFICIENCY vs. LED CURRENT (6 WLEDs)
VIN = 5V VIN = 5.5V
MAX8901A/B toc03
LED EFFICIENCY vs. LED CURRENT (6 WLEDs)
VIN = 5V VIN = 5.5V 85 80 EFFICIENCY (%) 75 70 65 60 VIN = 4.2V VIN = 3.6V VIN = 3V
MAX8901A/B toc04
90 85 80 EFFICIENCY (%) 75 70 65 60 55 50 0 5
90
VIN = 3.6V
VIN = 4.2V
VIN = 3V
INDUCTOR: TOKO 1069AS-220M DIODE: CENTRAL SEMI CMDSH05-4 10 15 20 25
55 50 0 5
INDUCTOR: TOKO 1098AS-100M DIODE: NXP PMEG3005EB 10 15 20 25
LED CURRENT (mA)
LED CURRENT (mA)
LED EFFICIENCY vs. LED CURRENT (4 WLEDs)
VIN = 5V VIN = 5.5V
MAX8901A/B toc05
LED EFFICIENCY vs. LED CURRENT (4 WLEDs)
VIN = 5V 85 80 EFFICIENCY (%) 75 70 65 60 VIN = 4.2V VIN = 5.5V
MAX8901A/B toc06
90 85 80 EFFICIENCY (%) 75 70 65 60 55 50 0 5
90
VIN = 4.2V VIN = 3.6V VIN = 3V
VIN = 3.6V VIN = 3V
INDUCTOR: TOKO 1069AS-220M DIODE: CENTRAL SEMI CMDSH05-4 10 15 20 25
55 50 0 5
INDUCTOR: TOKO 1098AS-100M DIODE: NXP PMEG3005EB 10 15 20 25
LED CURRENT (mA)
LED CURRENT (mA)
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package
Typical Operating Characteristics (continued)
(Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1F, C2 = 1F, TA = +25C, unless otherwise noted.)
LED AND BOOST EFFICIENCY vs. INPUT VOLTAGE
BOOST 90 EFFICIENCY (%) LED IIN (A) 85 500 400 300 200 6WLEDs, INDUCTOR: TOKO A915AY-220M, DIODE: CENTRAL SEMI CMDSH05-4 2 3 4 INPUT VOLTAGE (V) 5 6 100 0 2 3 4 5 6 7 SUPPLY VOLTAGE (V)
MAX8901A/B toc07
MAX8901A/MAX8901B
IN CURRENT (IIN) vs. SUPPLY VOLTAGE
700 600 MAX8901A MAX8901B
MAX8901A/B toc08 MAX8901A/B toc10
95
800
80
75
70
SWITCHING WAVEFORMS
VI
INPUT OVERVOLTAGE PROTECTION RESPONSE
50mV/div AC-COUPLED VIN 5.4V VOUT 6.4V 1V/div (AC-COUPLED)
MAX8901A/B toc9
VLX
10V/div 0V
10V/div 0V
VOUT ILX
100mV/div AC-COUPLED 100mA/div 0mA 1s/div
ILEDs
20mA/div 0mA
VLX 20ms/div
20mA/div 0V
STARTUP AND SHUTDOWN RESPONSE (MAX8901A)
MAX8901A/B toc11
STARTUP AND SHUTDOWN RESPONSE (MAX8901B)
MAX8901A/B toc12
C2 = 0.1F VON
2V/div 0V
VON
2V/div 0V
VOUT
10V/div 0V VOUT
10V/div 0V 100mA/div ISUPPLY 0mA 20mA/div 0mA 10ms/div
ISUPPLY ILEDs
100mA/div 0mA 20mA/div 0mA 10ms/div
ILEDs
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B
Typical Operating Characteristics (continued)
(Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1F, C2 = 1F, TA = +25C, unless otherwise noted.)
PWM DIMMING RESPONSE (MAX8901A)
MAX8901A/B toc13
SERIAL PULSE DIMMING RESPONSE (MAX8901B)
MAX8901A/B toc14
VON D = 90% VIN VCS D = 10%
2V/div 0V 50mV/div (AC-COUPLED) 500mV/div 0V VON VIN VCS
2V/div 0V 50mV/div (AC-COUPLED) 1V/div 0V 20mV/div 0V ILEDs 20mA/div 0mA 10ms/div
VOUT fON_PWM = 100kHz, DUTY CYCLE CHANGE: 90% TO 10%, C2 = 0.1F ILEDs 4ms/div
20mV/div 0V 20mA/div 0mA
VOUT
LED CURRENT vs. PWM DIMMING (MAX8901A)
MAX8901A/B toc15
VCS vs. LED CURRENT STEP (MAX8901B)
ILED = 24.75mA 500.8 500.6 VCS (mV) 500.4 500.2 500.0 499.8
MAX8901A/B toc16
30 25 20 ILED (mA) 15 10 5 0 0 20 40 60
501.0
RCS = 20, fPWM = 100kHz 80 100
499.6 499.4 0 5 10 15
ILED = 0.75mA 20 25 30 35
PWM DUTY CYCLE (%)
LED CURRENT STEP
MAXIMUM LED CURRENT vs. TEMPERATURE (MAX8901B)
MAX8901A/B toc17
MINIMUM LED CURRENT vs. TEMPERATURE (MAX8901B)
MAX8901A/B toc18
25.0
750
24.8
745 ILED (A)
ILED (mA)
24.6
740
24.4 735 24.2
24.0 -40 -15 10 35 60 85 TEMPERATURE (C)
730 -40 -15 10 35 60 85 TEMPERATURE (C)
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package
Pin Description
PIN 1 NAME OVP FUNCTION WLED Overvoltage Protection Input. OVP monitors voltage at the WLEDs. Connect OVP to the positive terminal of the output capacitor. If an OVP condition is detected, the MAX8901_ latches off. Cycle VIN or toggle VON to restart the IC. Enable and Dimming Control Input. Drive ON high to enable the IC. Drive ON low for greater than 1.33ms (typ) to shutdown the WLED current regulator. For the MAX8901A: After VIN is above VUVLO and ON is driven high, the MAX8901A enters soft-start and increases the WLED current to full brightness. Apply a minimum 30kHz (500kHz max) PWM signal to ON to adjust the WLED brightness from 100% to off, proportional to the duty cycle of the PWM signal. See the PWM Dimming Control (MAX8901A) section for more details. For the MAX8901B: After ON is driven high for 40s (min), the MAX8901B enters soft-start and increases the WLED current to full brightness. Subsequent pulses on ON cause the MAX8901B WLED current to decrease in 32 equal steps. See the Serial-Pulse Dimming Control (MAX8901B) section for more details. Power Supply Input. Bypass IN to GND with a 1F ceramic capacitor placed as close as possible to the IC. If VIN exceeds the input overvoltage lockout threshold (6.5V max, VIN rising), the IC stops switching and no WLED current flows (if the forward voltage of the WLED string is greater than VIN). When VIN falls below the overvoltage lockout hysteresis level (6.0V min, VIN falling), soft-start is initiated and normal operation resumes. Boost Inductor Node. Connect an inductor between IN and LX. LX is high impedance in shutdown. Power Ground. Connect to GND and the exposed paddle (EP) with a short, wide trace. Analog Ground. Connect GND to the exposed paddle with a short, wide trace. WLED Boost Compensation Node. Connect a 0.022F ceramic capacitor from COMP to GND. CCOMP stabilizes the converter and sets the soft-start time. COMP discharges to GND when in shutdown. WLED Current Sense Input. For the MAX8901A, connect a current-sense resistor from CS to GND. Voltage sensed at CS regulates the WLED current. For the MAX8901B, do not connect a sense resistor from CS to GND. The MAX8901B provides an internal current source from CS to GND to program the WLED current. The MAX8901B regulates VCS to 0.5V (typ) for all ILED. The MAX8901A regulates VCS to 0.5V (typ) for maximum duty cycle only. Exposed Metal Paddle. Connect EP to GND. For good thermal dissipation, the exposed paddle must be connected to a large ground plane.
MAX8901A/MAX8901B
2
ON
3
IN
4 5 6 7
LX PGND GND COMP
8
CS
--
EP
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B
MAX8901A
REF THERMAL SHUTDOWN BOOST CONTROL LOGIC GND
IC POWER
IN
INTERNAL 750kHz OSC
LX
SHUTDOWN TIMER
PGND OVP
CS Gm D*0.5V PWM COMP
ON EP
Figure 2. MAX8901A Block Diagram
Detailed Description
The high efficiency and tiny size of the MAX8901A/ MAX8901B WLED step-up converters make them ideally suited for driving LCD backlighting in cell phones, PDAs, and other portable and handheld devices. The MAX8901_ drive from 2 to 6 series-connected WLEDs with constant current for uniform WLED intensity and are optimized for the highest possible efficiency (up to 91%) over the full 1-cell Li+/Li-Poly battery range. These tiny 2mm x 2mm devices operate at a constant 750kHz
8
switching frequency, and use small external components to achieve minimal input and output ripple while occupying the smallest possible footprint. These converters use a single input (ON) to enable the IC and to control WLED intensity. The MAX8901A requires a direct PWM input to modulate WLED intensity, with WLED current proportional to the PWM duty cycle. The MAX8901B uses single-wire, serial-pulse dimming that reduces the WLED intensity in 32 equal steps. Full-scale WLED current for serial-pulse dimming is 24.75mA.
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B
MAX8901B
REF THERMAL SHUTDOWN BOOST CONTROL LOGIC GND
IC POWER
IN
INTERNAL 750kHz OSC
LX
SHUTDOWN TIMER
PGND OVP
CS Gm 0.5V PULSE DIMMING COMP
ON EP
Figure 3. MAX8901B Block Diagram
The MAX8901_ features: * Internal soft-start that gradually illuminates the WLEDs to eliminate inrush current during startup. * * * Input overvoltage protection (6.5V max) that prevents IC switching when exceeded. WLED overvoltage protection (25V typ) that latches off the IC. Shutdown mode that reduces current to 0.01A (typ).
Input Overvoltage Lockout
When V IN exceeds 6.2V (min), input overvoltage lockout (OVLO) is engaged to protect the MAX8901_ from high-input voltage conditions. Once input OVLO occurs, the MAX8901_ stops switching and no WLED current flows, provided the forward voltage of the WLED string is greater than VIN. When VIN falls below 6V (min), the input OVLO condition is cleared, and the IC is enabled and enters soft-start.
No WLED current is present in shutdown or during an overvoltage condition if the WLED forward voltage is greater than the input supply voltage. Figures 2 and 3 show the block diagrams for the MAX8901A and MAX8901B, respectively.
WLED Overvoltage Protection
WLED overvoltage protection (OVP) occurs when the WLED output voltage rises above the WLED OVP threshold. The WLED OVP protection circuitry latches off the IC and the IC enters shutdown when the WLED OVP threshold is exceeded. After OVP is engaged, cycle VIN or toggle ON to reenable the IC and enter soft-start.
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B
Startup and Soft-Start
The MAX8901_ use a single input (ON) to enable the IC and to control WLED intensity. After ON is driven high, the MAX8901_ enter soft-start and gradually increase the WLED current to full brightness. When the MAX8901_ is first turned on, C COMP is charged to 1.25V with a 60A current source. When VCOMP rises above 1.25V, CCOMP is charged with a 4A current source, and the internal MOSFET begins switching at a reduced duty cycle. When VCOMP rises above 2.25V, the duty cycle is at its maximum. The VCOMP where the IC exits soft-start depends on the final duty cycle required by the load. Typical shutdown timing characteristics are shown in the Typical Operating Characteristics section. A PWM signal (MAX8901A) or serial pulses (MAX8901B) are applied to dim the WLEDs. See the PWM Dimming Control (MAX8901A) and Serial-Pulse Dimming Control (MAX8901B) sections for more details.
PWM Dimming Control (MAX8901A)
After VIN is above UVLO, apply a PWM signal to ON for a WLED current that is proportional to the PWM signal's duty cycle (0% duty cycle corresponds to zero LED current and 100% duty cycle corresponds to full LED current). Restrict PWM frequency to between 30kHz to 500kHz for a maximum ILED current accuracy. If dimming control is not required, ON works as a simple on/off control. Drive ON high to enable the IC, or drive ON low for shutdown. A resistor connected from CS to GND programs maximum ILED. See the Current-Sense Resistor (MAX8901A Only) section for more details.
Serial-Pulse Dimming Control (MAX8901B)
After the MAX8901B is enabled by driving ON high for the minimum initial ON high pulse width (40s, min), soft-start is engaged and brings WLED current to maximum brightness. After soft-start is completed, dim the MAX8901B by pulsing ON low (500ns to 500s pulse width). Each pulse reduces the WLED current by 0.75mA. The maximum WLED current is 24.75mA for the MAX8901B (0.75mA/step). The overall dimming range for serial-pulse dimming control is from maximum WLED current down to 1/33 of maximum WLED current. The WLEDs are turned off by holding ON low for at least 1.33ms (typ). See Figure 1 for the serial-pulse dimming control timing diagram.
Shutdown
The MAX8901_ enter shutdown when VON is held low for more than 1.33ms (typ), or when an output overvoltage condition is engaged. In shutdown, the supply current is reduced to 0.01A (typ) by powering down the entire IC except for the ON voltage-detection circuitry. C COMP is discharged to GND during shutdown, allowing the device to reinitiate soft-start when it is re-enabled. Although the internal n-channel MOSFET does not switch in shutdown, there is still a DC current path between the input and the WLEDs through the inductor and Schottky diode. In shutdown, the minimum forward voltage of the WLED string must exceed the maximum input voltage to ensure that the WLEDs remain off. Typically, with two or more WLEDs, the forward voltage is large enough to keep leakage current low, less than 1A (typ). Shutdown timing characteristics are shown in the Typical Operating Characteristics.
Applications Information
Inductor Selection
Recommended inductor values range from 10H to 47H. A 22H inductor optimizes the efficiency for most applications while maintaining low input voltage ripple. With input voltages near 5V, a larger value of inductance may be more efficient. To prevent core saturation, ensure that the inductor saturation current rating exceeds the peak inductor current for the application. Calculate the peak inductor current with the following formula: IPEAK = VOUT(MAX) x ILED(MAX) 900 x VIN(MIN) + VIN(MIN) x 1.2 2xL
Thermal Shutdown
The MAX8901 includes a thermal shutdown feature that protects the IC by turning it off when the die temperature reaches +160C (typ). After thermal shutdown occurs, the MAX8901_ must be restarted by toggling the ON pin low, then high, or by cycling the input voltage.
where ILED(MAX) is in mA and L is in H.
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package
Schottky Diode Selection
The high switching frequency of the MAX8901_ demands a high-speed rectification diode for optimum efficiency. A Schottky diode is recommended due to its fast recovery time and low forward voltage drop. Ensure that the diode's average and peak current rating exceeds the average output current and peak inductor current. In addition, the diode's reverse breakdown voltage must exceed VOUT. The RMS diode current can be approximated from the following equation: IDIODE (RMS) = ILED x IPEAK
Current-Sense Resistor (MAX8901A Only)
The MAX8901A uses a sense resistor (RCS) connected from CS to GND to program the maximum WLED current for 100% PWM duty cycle. The MAX8901A regulates VCS to 0.5V (typ) for 100% duty cycle. Calculate RCS (in ohms) using the following equation: RCS = 500 ILED(MAX)
MAX8901A/MAX8901B
where I LED(MAX) is the maximum WLED current in milliamps. Maximum WLED current is programmed to 25mA using a 20 resistor.
PCB Layout Capacitor Selection
Ceramic capacitors with X5R, X7R, or better dielectric are recommended for stable operation over the entire operating temperature range. The exact values of input (C1) and output (C2) capacitors are not critical (see Figures 4 and 5). The typical value for the input capacitor is 1F, and the typical value for the output capacitor is 0.1F (MAX8901A) or 1F (MAX8901B). Higher value capacitors can reduce input and output ripple, but at the expense of size and higher cost. The compensation capacitor (C3) stabilizes the converter and controls soft-start. The compensation capacitor is typically chosen to be 0.022F for most applications. Due to fast switching waveforms and high-current paths, careful attention is required when the PCB layout is designed. Minimize trace lengths between the IC and the inductor, the diode, the input capacitor, and the output capacitor. For the MAX8901A, minimize the trace length between the IC and R CS. Keep traces short, direct, and wide. Keep noisy traces, such as the LX node trace, away from CS. The input bypass capacitor (CIN) should be placed as close as possible to the IC. The ground connections of CIN and COUT should be as close together as possible. PGND and GND should be connected together at the input capacitor ground terminal. Refer to the MAX8901 EV Kit for an example layout.
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B
INPUT 2.6V TO 5.5V C1 1F L1 22H OUTPUT UP TO 25V C2 0.1F OVP 1 D2-D7
3
IN MAX8901A
LX
4
D1
7 C3 0.022F PWM DIMMING CONTROL 2 6
COMP
GND PGND
5
ON
EP
CS
8 R1 (RCS) 20
Figure 4. MAX8901A Application Circuit
INPUT 2.6V TO 5.5V C1 1F
L1 22H OUTPUT UP TO 25V C2 1F OVP 1
3 7 R1 0 C3 0.022F 6
IN COMP MAX8901B
LX
4
D1
GND
PGND
5
SERIAL-PULSE DIMMING CONTROL
2
ON
EP
CS
8
Figure 5. MAX8901B Application Circuit
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package
Pin Configuration
PROCESS: BiCMOS
TOP VIEW CS 8 COMP GND PGND 7 6 5
Chip Information
MAX8901A/MAX8901B
MAX8901
*EP 1 OVP 2 ON 3 IN 4 LX
+
TDFN-EP (2mm x 2mm)
*EXPOSED PAD
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package MAX8901A/MAX8901B
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
8L TDFN EXPOSED PADS.EPS
PACKAGE OUTLINE 6 & 8L TDFN EXPOSED PAD, 2x2x0.80mm
21-0168
D
1 2
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Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
MAX8901A/MAX8901B
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 1.90 1.90 0.00 0.20 MAX. 0.80 2.10 2.10 0.05 0.40
0.25 MIN. 0.20 REF.
PACKAGE VARIATIONS PKG. CODE T622-1 T822-1 T822-2 N 6 8 8 D2 0.900.10 0.700.10 0.800.10 E2 1.600.10 1.300.10 1.200.10 e 0.65 TYP. 0.50 TYP. 0.50 TYP. b 0.300.05 0.250.05 0.250.05 r 0.150 0.125 0.125 [(N/2)-1] x e 1.30 REF 1.50 REF 1.50 REF
PACKAGE OUTLINE 6 & 8L TDFN EXPOSED PAD, 2x2x0.80mm
21-0168
D
2 2
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15
(c) 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.

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