MAX8901B [MAXIM]
Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package; 效率最高的供应2至6只串联WLED采用2mm x 2mm TDFN封装
| 型号： | MAX8901B |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Highest Efficiency Supply for 2 to 6 Series WLEDs in a 2mm x 2mm TDFN Package |
| 文件： | 总15页 (文件大小：299K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-0865; Rev 0; 8/07
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
/MAX8901B
General Description
Features
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 hand-
held devices. The MAX8901_ operate at a fixed 750kHz
(typ) switching frequency, allowing for tiny external com-
ponents, and are optimized for the highest possible effi-
ciency over the full 1-cell Li+/Li-Poly battery range.
♦ 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)
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).
♦ Input Undervoltage Lockout
♦ Input Overvoltage Lockout
♦ WLED Overvoltage Protection (25V typ)
♦ 0.01µA (typ) Shutdown Current
♦ No WLED Current in Shutdown
♦ Internal Soft-Start and Thermal Shutdown
The MAX8901_ features an internal soft-start to eliminate
inrush currents during startup, input overvoltage
protection, WLED overvoltage protection, and a shut-
down mode with 0.01µA (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 lock-
out (UVLO) and thermal shutdown.
Ordering Information
PKG
CODE
TOP
MARK
PART
PIN-PACKAGE
8 TDFN-EP*
2mm x 2mm
The MAX8901_ are available in tiny 8-pin, 2mm x 2mm
TDFN-EP packages (0.8mm max height).
MAX8901AETA+T
MAX8901BETA+T
T822-1
T822-1
+ABA
+ABB
Applications
8 TDFN-EP*
2mm x 2mm
Display Backlight (from 2 to 6 WLEDs)
Note: All devices are specified over the -40°C to +85°C operating
temperature range.
Cellular Phones
PDAs and Smartphones
MP3 and Portable Media Players
Portable Navigation Devices
Digital Cameras
+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
INPUT
2.6V TO 5.5V
OUTPUT
UP TO 25V
OUTPUT
UP TO 25V
3
7
4
1
5
LX
IN
3
4
1
5
LX
IN
MAX8901A
MAX8901B
COMP
GND
OVP
7
6
OVP
COMP
GND
6
2
PGND
PWM DIMMING
CONTROL
SERIAL-PULSE
DIMMING
CONTROL
PGND
CS
8
ON
CS
2
EP
8
ON
EP
________________________________________________________________ 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.
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
ABSOLUTE MAXIMUM RATINGS
IN to GND.................................................................-0.3V to +7V
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
CS, COMP, ON to GND...............................-0.3V to (V + 0.3V)
OVP, LX to GND .....................................................-0.3V to +28V
PGND to GND .......................................................-0.3V to +0.3V
IN
LX Current ..................................................................770mA
Continuous Power Dissipation (multilayer board at +70°C)
8-pin, 2mm x 2mm TDFN
RMS
(derate above +70°C by 11.9mW/°C)..........................953mW
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
(V = V
IN
= V
= 3.6V, V
= V
= 0V, COMP, CS, and LX are unconnected, T = -40°C to +85°C, unless otherwise noted.
GND A
ON
OVP
PGND
Typical values are at T = +25°C.) (Note 1)
A
PARAMETER
Input Voltage
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
V
2.6
5.5
V
IN
/MAX8901B
V
rising
2.25
2.50
IN
Input Undervoltage Lockout
Threshold
V
V
V
IN_UVLO
V
V
V
falling
rising
falling
2.20
6.2
2.53
6.3
6.3
1
IN
IN
IN
Input Overvoltage Lockout
Threshold
V
IN_OVLO
6.0
T
T
= +25°C
= +85°C
0.01
0.1
70
A
Shutdown Input Current
Quiescent Current
I
V
= 0V
µA
SHDN
ON
A
MAX8901A
135
185
V
= 0.55V, no
CS
I
µA
load (not
switching)
Q
MAX8901B
115
V
-
IN
Output Voltage Range
V
V
= external boost diode voltage drop
V
OV
V
V
OUT
DIODE
V
DIODE
OVP Overvoltage Protection
Threshold
V
6 LEDs with 25V OVP option
= 20V
24
25
26
OV
OVP Input Current
I
V
20
µA
V
OV
OVP
No dimming
Time V is below low threshold until
CS Regulation Voltage
V
0.475
1.18
1.3
0.50
0.525
1.50
CS
ON
ON Shutdown Delay
t
1.33
ms
SHDN
shutdown (Figure 1)
ON High Voltage
ON Low Voltage
V
2.6V < V < 5.5V
V
V
ON_HI
IN
V
2.6V < V < 5.5V
IN
0.4
1
ON_LO
T
T
= +25°C
= +85°C
0.01
0.1
A
ON Input Current
I
ON = IN
µA
µs
ON
A
First ON high pulse to enable IC
(MAX8901B) (Figure 1)
Initial ON High Pulse Width
t
40
HI_INIT
ON High Pulse Width
ON Low Pulse Width
t
MAX8901B (Figure 1)
MAX8901B (Figure 1)
0.5
0.5
µs
µs
HI
t
500.0
LO
2
_______________________________________________________________________________________
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
/MAX8901B
ELECTRICAL CHARACTERISTICS (continued)
(V = V
IN
= V
= 3.6V, V
= V
= 0V, COMP, CS, and LX are unconnected, T = -40°C to +85°C, unless otherwise noted.
GND A
ON
OVP
PGND
Typical values are at T = +25°C.) (Note 1)
A
PARAMETER
Serial Dimming
SYMBOL
CONDITIONS
MIN
-1
TYP
MAX
+1
UNITS
%
T
A
T
A
= +25°C
I
= 24.75mA
LED
Full-Scale LED Current Accuracy
(MAX8901B)
= -40°C to +85°C
-3
+3
%
PWM Frequency for PWM
Dimming Control
f
MAX8901A
100
60
kHz
µS
PWM
CS to COMP Transconductance
40
80
t
SOFT-
START
Soft-Start Interval
C
= 0.022µF (Figure 1)
10
ms
°C
COMP
Thermal Shutdown
OSCILLATOR
160
Operating Frequency
Maximum Duty Cycle
n-CHANNEL SWITCH
f
700
90
750
92
800
5
kHz
%
SW
D
V
V
= 0.4V
MAX
CS
LX
T
T
= +25°C
= +85°C
0.1
0.1
A
LX Leakage Current
I
= 27V
µA
LXLKG
A
n-Channel Switch On-Resistance
n-Channel Current Limit
R
0.7
1.4
Ω
LX
I
0.63
0.70
0.77
A
LIM
Note 1: Specifications to -40°C are guaranteed by design and characterization and are not production tested.
1
2
3
4
5
28
t
29
30
31
32
33
0
STEP
t
HI_INIT
ON
> 40μs
HI
t
SHDN
t
LO
t
500ns TO 500μs
FULL
> 500ns
SOFT-START
FULL
1.33ms (typ)
32/33
32/33
31/33
30/33
29/33
28/33
I
LED_
6/33
5/33
4/33
3/33
2/33
1/33
SHUTDOWN
SHUTDOWN
Figure 1. MAX8901B Timing Diagram
_______________________________________________________________________________________
3
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
Typical Operating Characteristics
(Circuit of Figure 5, I
= 24.75mA, V = V
= 3.6V, C1 = 1µF, C2 = 1µF, T = +25°C, unless otherwise noted.)
ON A
LED
IN
LED EFFICIENCY vs. INPUT VOLTAGE
LED EFFICIENCY vs. INPUT VOLTAGE
90
85
80
75
70
90
85
80
75
70
4 WLEDs
3 WLEDs
4 WLEDs
3 WLEDs
6 WLEDs
6 WLEDs
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
2
3
4
5
6
2
3
4
5
6
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
LED EFFICIENCY vs. LED CURRENT
(6 WLEDs)
LED EFFICIENCY vs. LED CURRENT
(6 WLEDs)
/MAX8901B
90
85
80
75
70
65
60
55
50
90
85
80
75
70
65
60
55
50
V
= 5.5V
V = 5V
V = 5.5V
IN
IN
V
= 5V
IN
IN
V
= 4.2V
IN
V
= 4.2V
IN
V
= 3.6V
IN
V
= 3V
V
= 3.6V
IN
IN
V
= 3V
IN
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
0
5
10
15
20
25
0
5
10
15
20
25
LED CURRENT (mA)
LED CURRENT (mA)
LED EFFICIENCY vs. LED CURRENT
(4 WLEDs)
LED EFFICIENCY vs. LED CURRENT
(4 WLEDs)
90
85
80
75
70
65
60
55
50
90
85
80
75
70
65
60
55
50
V
= 5.5V
V
= 5V
IN
IN
V
= 5V
V
= 5.5V
IN
IN
V
= 4.2V
IN
V
= 4.2V
IN
V
= 3.6V
IN
V
= 3.6V
IN
V
= 3V
IN
V
= 3V
IN
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
0
5
10
15
20
25
0
5
10
15
20
25
LED CURRENT (mA)
LED CURRENT (mA)
4
_______________________________________________________________________________________
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
/MAX8901B
Typical Operating Characteristics (continued)
(Circuit of Figure 5, I
= 24.75mA, V = V
= 3.6V, C1 = 1µF, C2 = 1µF, T = +25°C, unless otherwise noted.)
ON A
LED
IN
LED AND BOOST EFFICIENCY
vs. INPUT VOLTAGE
IN CURRENT (I )
IN
vs. SUPPLY VOLTAGE
800
700
600
500
400
300
200
100
0
95
90
85
80
75
70
BOOST
LED
MAX8901B
MAX8901A
6WLEDs,
INDUCTOR: TOKO A915AY-220M,
DIODE: CENTRAL SEMI CMDSH05-4
2
3
4
5
6
7
2
3
4
5
6
SUPPLY VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT OVERVOLTAGE
PROTECTION RESPONSE
SWITCHING WAVEFORMS
MAX8901A/B toc9
MAX8901A/B toc10
6.4V
1V/div
(AC-COUPLED)
V
IN
50mV/div
AC-COUPLED
V
I
5.4V
V
OUT
10V/div
0V
V
LX
10V/div
0V
100mV/div
AC-COUPLED
V
OUT
I
20mA/div
0mA
LEDs
100mA/div
0mA
I
LX
20mA/div
0V
V
LX
1μs/div
20ms/div
STARTUP AND SHUTDOWN
RESPONSE (MAX8901A)
STARTUP AND SHUTDOWN
RESPONSE (MAX8901B)
MAX8901A/B toc11
MAX8901A/B toc12
C2 = 0.1μF
2V/div
0V
2V/div
0V
V
V
ON
ON
10V/div
10V/div
V
OUT
V
OUT
0V
0V
100mA/div
100mA/div
0mA
I
SUPPLY
I
SUPPLY
0mA
I
LEDs
20mA/div
0mA
20mA/div
0mA
I
LEDs
10ms/div
10ms/div
_______________________________________________________________________________________
5
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
Typical Operating Characteristics (continued)
(Circuit of Figure 5, I
= 24.75mA, V = V = 3.6V, C1 = 1µF, C2 = 1µF, T = +25°C, unless otherwise noted.)
ON A
LED
IN
PWM DIMMING RESPONSE
(MAX8901A)
SERIAL PULSE DIMMING
RESPONSE (MAX8901B)
MAX8901A/B toc13
MAX8901A/B toc14
2V/div
0V
V
2V/div
0V
ON
V
V
ON
D = 90%
D = 10%
V
50mV/div
(AC-COUPLED)
V
50mV/div
(AC-COUPLED)
IN
IN
500mV/div
0V
V
CS
1V/div
0V
CS
V
OUT
V
20mV/div
0V
20mV/div
0V
OUT
f
= 100kHz, DUTY
ON_PWM
CYCLE CHANGE: 90% TO
10%, C2 = 0.1μF
I
LEDs
20mA/div
0mA
20mA/div
0mA
I
LEDs
4ms/div
10ms/div
LED CURRENT vs. PWM
DIMMING (MAX8901A)
V
CS
vs. LED CURRENT STEP
(MAX8901B)
/MAX8901B
501.0
500.8
500.6
500.4
500.2
500.0
499.8
499.6
499.4
30
25
20
15
10
5
I
= 24.75mA
LED
R
= 20Ω,
= 100kHz
CS
I
= 0.75mA
25
LED
20
f
PWM
0
0
20
40
60
80 100
0
5
10
15
30
35
PWM DUTY CYCLE (%)
LED CURRENT STEP
MINIMUM LED CURRENT
vs. TEMPERATURE (MAX8901B)
MAXIMUM LED CURRENT
vs. TEMPERATURE (MAX8901B)
750
745
740
735
730
25.0
24.8
24.6
24.4
24.2
24.0
-40
-15
10
35
60
85
85
-40
-15
10
35
60
TEMPERATURE (°C)
TEMPERATURE (°C)
6
_______________________________________________________________________________________
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
/MAX8901B
Pin Description
PIN
NAME
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.
1
OVP
Cycle V or toggle V
to restart the IC.
ON
IN
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 V is above V
and ON is driven high, the MAX8901A enters soft-start
IN
UVLO
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.
2
ON
For the MAX8901B: After ON is driven high for 40µs (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 1µF ceramic capacitor placed as close as possible to
the IC. If V exceeds the input overvoltage lockout threshold (6.5V max, V rising), the IC stops
IN
IN
3
IN
switching and no WLED current flows (if the forward voltage of the WLED string is greater than V ).
IN
When V falls below the overvoltage lockout hysteresis level (6.0V min, V falling), soft-start is
IN
IN
initiated and normal operation resumes.
4
5
6
LX
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.
PGND
GND
WLED Boost Compensation Node. Connect a 0.022µF ceramic capacitor from COMP to GND.
7
COMP
C
stabilizes the converter and sets the soft-start time. COMP discharges to GND when in
COMP
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
8
CS
EP
program the WLED current. The MAX8901B regulates V to 0.5V (typ) for all I
The MAX8901A
LED.
CS
regulates V to 0.5V (typ) for maximum duty cycle only.
CS
Exposed Metal Paddle. Connect EP to GND. For good thermal dissipation, the exposed paddle must
be connected to a large ground plane.
—
_______________________________________________________________________________________
7
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
MAX8901A
GND
REF
THERMAL
SHUTDOWN
IN
IC
POWER
BOOST
CONTROL
LOGIC
INTERNAL
750kHz OSC
LX
SHUTDOWN
TIMER
PGND
OVP
/MAX8901B
CS
Gm
D*0.5V
PWM
COMP
ON
EP
Figure 2. MAX8901A Block Diagram
switching frequency, and use small external components
to achieve minimal input and output ripple while occupy-
ing the smallest possible footprint.
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
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.
8
_______________________________________________________________________________________
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
/MAX8901B
MAX8901B
GND
REF
THERMAL
SHUTDOWN
IN
IC
POWER
BOOST
CONTROL
LOGIC
INTERNAL
750kHz OSC
LX
SHUTDOWN
TIMER
PGND
OVP
CS
Gm
PULSE
DIMMING
0.5V
COMP
ON
EP
Figure 3. MAX8901B Block Diagram
The MAX8901_ features:
Input Overvoltage Lockout
When V exceeds 6.2V (min), input overvoltage
IN
•
•
•
•
Internal soft-start that gradually illuminates the
WLEDs to eliminate inrush current during startup.
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
Input overvoltage protection (6.5V max) that
prevents IC switching when exceeded.
WLED overvoltage protection (25V typ) that latches
off the IC.
WLED string is greater than V . When V falls below
IN
IN
6V (min), the input OVLO condition is cleared, and the
IC is enabled and enters soft-start.
Shutdown mode that reduces current to 0.01µA (typ).
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,
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.
cycle V or toggle ON to reenable the IC and enter
IN
soft-start.
_______________________________________________________________________________________
9
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
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.
PWM Dimming Control (MAX8901A)
After V is above UVLO, apply a PWM signal to ON for
IN
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
When the MAX8901_ is first turned on, C
is
COMP
500kHz for a maximum I
current accuracy. If dim-
LED
charged to 1.25V with a 60µA current source. When
rises above 1.25V, C is charged with a
ming 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 pro-
V
COMP
COMP
4µA current source, and the internal MOSFET begins
switching at a reduced duty cycle. When V rises
COMP
grams maximum I
. See the Current-Sense Resistor
LED
above 2.25V, the duty cycle is at its maximum. The
where the IC exits soft-start depends on the
(MAX8901A Only) section for more details.
V
COMP
final duty cycle required by the load. Typical shutdown
timing characteristics are shown in the Typical
Operating Characteristics section.
Serial-Pulse Dimming Control (MAX8901B)
After the MAX8901B is enabled by driving ON high for
the minimum initial ON high pulse width (40µs, min),
soft-start is engaged and brings WLED current to maxi-
mum brightness. After soft-start is completed, dim the
MAX8901B by pulsing ON low (500ns to 500µs 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.
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.
Shutdown
/MAX8901B
The MAX8901_ enter shutdown when V
is held low
ON
for more than 1.33ms (typ), or when an output overvolt-
age condition is engaged. In shutdown, the supply cur-
rent is reduced to 0.01µA (typ) by powering down the
entire IC except for the ON voltage-detection circuitry.
C
is discharged to GND during shutdown,
COMP
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 for-
ward voltage is large enough to keep leakage current
low, less than 1µA (typ). Shutdown timing characteris-
tics are shown in the Typical Operating Characteristics.
Applications Information
Inductor Selection
Recommended inductor values range from 10µH to
47µH. A 22µH 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:
Thermal Shutdown
The MAX8901 includes a thermal shutdown feature that
protects the IC by turning it off when the die temperature
reaches +160°C (typ). After thermal shutdown occurs,
the MAX8901_ must be restarted by toggling the ON pin
low, then high, or by cycling the input voltage.
V
×I
V
IN(MIN) x 1.2
OUT(MAX) LED(MAX)
I
=
+
PEAK
900 x V
2 x L
IN(MIN)
where I
is in mA and L is in µH.
LED(MAX)
10 ______________________________________________________________________________________
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
/MAX8901B
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
Current-Sense Resistor (MAX8901A Only)
The MAX8901A uses a sense resistor (R ) connected
CS
from CS to GND to program the maximum WLED cur-
rent for 100% PWM duty cycle. The MAX8901A regu-
lates V
CS
to 0.5V (typ) for 100% duty cycle. Calculate
CS
R
(in ohms) using the following equation:
500
R
=
CS
breakdown voltage must exceed V
. The RMS diode
I
OUT
LED(MAX)
current can be approximated from the following
equation:
where I
is the maximum WLED current in
LED(MAX)
milliamps. Maximum WLED current is programmed to
25mA using a 20Ω resistor.
I
=
I
×I
DIODE(RMS)
LED PEAK
PCB Layout
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
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 1µF, and the typical value for the output
capacitor is 0.1µF (MAX8901A) or 1µF (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 0.022µF for most applications.
trace length between the IC and R . Keep traces
CS
short, direct, and wide. Keep noisy traces, such as the
LX node trace, away from CS. The input bypass
capacitor (C ) should be placed as close as possible
IN
to the IC. The ground connections of C and C
IN
OUT
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.
______________________________________________________________________________________ 11
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
L1
22μH
INPUT
2.6V TO 5.5V
C1
1μF
OUTPUT
UP TO 25V
D1
3
4
1
5
LX
OVP
IN
C2
0.1μF
MAX8901A
7
6
COMP
GND
D2–D7
C3
0.022μF
PGND
CS
PWM DIMMING
CONTROL
2
8
ON
EP
R1 (R
)
CS
20Ω
/MAX8901B
Figure 4. MAX8901A Application Circuit
L1
22μH
INPUT
2.6V TO 5.5V
C1
1μF
OUTPUT
UP TO 25V
D1
3
7
4
1
5
LX
IN
C2
1μF
MAX8901B
COMP
R1
0Ω
OVP
C3
0.022μF
6
2
PGND
GND
ON
SERIAL-PULSE
DIMMING
CONTROL
8
CS
EP
Figure 5. MAX8901B Application Circuit
12 ______________________________________________________________________________________
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
/MAX8901B
Pin Configuration
Chip Information
PROCESS: BiCMOS
TOP VIEW
CS COMP GND PGND
8
7
6
5
MAX8901
*EP
+
1
2
3
4
OVP ON
IN
LX
TDFN-EP
(2mm × 2mm)
*EXPOSED PAD
______________________________________________________________________________________ 13
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
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.)
/MAX8901B
PACKAGE OUTLINE
6 & 8L TDFN EXPOSED PAD, 2x2x0.80mm
1
21-0168
D
2
14 ______________________________________________________________________________________
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
/MAX8901B
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.)
COMMON DIMENSIONS
MIN.
0.70
1.90
1.90
0.00
0.20
MAX.
0.80
2.10
2.10
0.05
0.40
SYMBOL
A
D
E
A1
L
k
0.25 MIN.
0.20 REF.
A2
PACKAGE VARIATIONS
PKG. CODE
T622-1
N
6
D2
E2
e
b
r
[(N/2)-1] x e
1.30 REF
0.90±0.10
1.60±0.10
0.65 TYP.
0.30±0.05
0.150
T822-1
T822-2
8
8
0.70±0.10
0.80±0.10
1.30±0.10
1.20±0.10
0.50 TYP.
0.50 TYP.
0.25±0.05
0.25±0.05
0.125
0.125
1.50 REF
1.50 REF
PACKAGE OUTLINE
6 & 8L TDFN EXPOSED PAD, 2x2x0.80mm
2
21-0168
D
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
© 2007 Maxim Integrated Products
is a registered trademark of Maxim Integrated Products, Inc.
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