Maxim MAX8847ZETE+T High-performance negative charge pump for 6 white leds in 3mm x 3mm thin qfn Datasheet

19-5196; Rev 0; 4/10
TION KIT
EVALUA BLE
A
IL
A
V
A
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
The MAX8847Y/MAX8847Z negative charge pumps
drive up to 6 white LEDs with regulated constant current
for display backlight applications. By utilizing an inverting charge pump and extremely low-dropout adaptive
current regulators, these ICs achieve very high efficiency
over the full 1-cell Li+ battery voltage range even with
large LED forward voltage mismatch. The 1MHz fixedfrequency switching allows for tiny external components.
The regulation scheme is optimized to ensure low EMI
and low input ripple. The MAX8847Y/MAX8847Z include
thermal shutdown, open- and short-circuit protection.
The MAX8847Y/MAX8847Z support independent LED
on/off and dimming control. The MAX8847Z has PWM
dimming control for LED1–LED6. The MAX8847Y has
PWM dimming control for LED1–LED4 and serial-pulse
dimming control for LED5 and LED6. The serial-pulse
dimming ranges are pseudo-logarithmic from 24mA to
0.1mA and off in 32 steps. All devices include a temperature derating function to safely allow bright 24mA fullscale output current while automatically reducing current
gradually to protect LEDs at high ambient temperatures
above +60°C.
The MAX8847Y/MAX8847Z are available in 16-pin,
3mm x 3mm thin QFN packages.
Features
S Negative 1x/1.5x Charge Pump
S Adaptive Current Regulators
S Independent Voltage Supply for Each LED
S 24mA to 0.1mA Serial-Pulse Dimming (MAX8847Y)
S 24mA to 0mA PWM Dimming (MAX8847Z)
S 2% (max) LED Current Accuracy, 1% (typ) Matching
S Low 120µA Quiescent Current
S Low 0.4µA Shutdown Current
S Inrush Current Limit
S Temperature Derating Function
S 16-Pin, 3mm x 3mm Thin QFN Packages
Typical Operating Circuit
1µF
INPUT
2.7V TO 5.5V
C1P
1µF
C1N
C2P
1µF
IN
NEG
1µF
GND
EP
Applications
White LED Backlighting
Cellular Phones
LED ENABLE
AND
DIMMING
CONTROL
PDAs, Digital Cameras, and Camcorders
ENA
ENB
Ordering Information
PIN-PACKAGE
TOP
MARK
PART
DIMMING
MAX8847YETE+T
Serial pulse/
PWM
16 Thin QFN-EP*
AHQ
MAX8847ZETE+T
PWM
16 Thin QFN-EP*
AHP
Note: All devices are specified over the -40°C to +85°C
extended temperature range.
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
C2N
MAX8847Y
MAX8847Z
LED1
LED2
LED3
CPWM
0.22µF
LED4
LED5
LED6
D1
D2
D3
D4
D5
D6
________________________________________________________________ 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.
MAX8847Y/MAX8847Z
General Description
MAX8847Y/MAX8847Z
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
ABSOLUTE MAXIMUM RATINGS
IN to GND..............................................................-0.3V to +6.0V
IN to NEG..............................................................-0.3V to +6.0V
NEG, C2N to GND...................................................-6V to +0.3V
C1P, C2P, CPWM, ENA, ENB to GND........-0.3V to (VIN + 0.3V)
C2P to C1N.................................................-0.3V to (VIN + 0.3V)
LED_, C1N, C2N, ENA, ENB to NEG..........-0.3V to (VIN + 0.3V)
Continuous Power Dissipation (TA = +70°C)
16-Pin Thin QFN Multilayer PCB
(derate 20.8mW/°C above +70°C)..........................1666.7mW
Junction-to-Case Thermal Resistance (θJC) (Note 1)......10°C/W
Junction-to-Ambient Thermal Resistance (θJA) (Note 1)
Multilayer PCB...............................................................48°C/W
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
Soldering Temperature (reflow).......................................+260°C
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial.
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 = 3.6V, VGND = 0V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
PARAMETER
CONDITIONS
IN Operating Voltage
Undervoltage Lockout (UVLO)
Threshold
MIN
VIN rising
2.35
Undervoltage Lockout
Hysteresis
IN Shutdown Supply Current
TYP
2.7
2.45
MAX
UNITS
5.5
V
2.55
V
100
VEN_ = 0V, all outputs off
TA = +25NC
0.4
TA = +85NC
0.4
mV
2.5
Charge pump inactive, 2 LEDs enabled at 0.1mA setting
120
Charge pump active, 1MHz switching, all LEDs enabled
at 0.1mA setting
1.6
mA
Thermal Shutdown Threshold
+160
NC
Thermal Shutdown Hysteresis
20
NC
IN Operating Supply Current
150
FA
FA
PWM DIMMING CONTROL
PWM Low-Level Input
0.4
PWM High-Level Input
1.4
EN_ PWM Input Signal
Frequency Range
CCPWM = 0.22FF
PWM Dimming Filter Corner
Frequency
CCPWM = 0.22FF
Current Dimming Range
Duty cycle = 0 to 100%
PWM Dimming Resolution
1% P duty cycle P 100%
V
V
0.2
200
2
0
kHz
Hz
24
0.24
mA
mA/%
SERIAL-PULSE LOGIC (MAX8847Y LED5 and LED6 only)
EN_ Logic Input High Voltage
1.4
V
EN_ Logic Input Low Voltage
EN_ Logic-Input Current
0.4
VIL = 0V or VIH = 5.5V
TA = +25NC
TA = +85NC
-1
0.01
0.1
2 _______________________________________________________________________________________
+1
V
FA
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
(VIN = 3.6V, VGND = 0V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
PARAMETER
CONDITIONS
EN_ Low Shutdown Delay
tSHDN
EN_ tLO
See Figure 2
EN_ tHI
See Figure 2
Initial EN_ tINIT
See Figure 2, first EN_ high pulse
MIN
TYP
5
8
1
MAX
UNITS
ms
500
Fs
1
Fs
120
Fs
CHARGE PUMP
Switching Frequency
Soft-Start Time
4.3
1
MHz
0.5
ms
5
V
Output Regulation Voltage
VIN - VNEG
Open-Loop NEG Output
Resistance
(VNEG - 0.5 x VIN)/INEG
2
NEG Shutdown Discharge
Resistance
VEN_ = 0V, all outputs off
10
4
I
kI
LED1–LED6 CURRENT REGULATOR
Current Setting Range
LED_ Current Accuracy
Serial-pulse interface or PWM
VLED_= 0.5V for charge
pump inactive,
VLED_ = -0.9V,
VNEG_= -1.4V
0.1
24mA setting, TA = +25NC
-2
24mA setting, TA = -40NC
to derating function start
temperature (Note 3)
-5
1.6mA setting, TA = +25NC
LED_ Dropout Voltage
(Note 4)
LED_ Current Regulator
Switchover Threshold
(Inactive to Active)
From derating function start temperature
85
Charge pump inactive,
24mA setting
TA = +85NC
95
Charge pump active,
24mA setting
TA = +25NC
110
TA = +85NC
124
135
150
%/NC
125
mV
165
100
All LEDs off
%
NC
-2.5
TA = +25NC
LED_ Current Regulator
Switchover Hysteresis
LED_ Leakage in Shutdown
+5
+60
VLED_ falling
mA
+2
Q5
Derating Function Start
Temperature
Derating Function Slope
24.0
Q1
TA = +25NC
0.01
TA = +85NC
0.1
mV
mV
5
FA
Note 2: Limits are 100% production tested at TA = +25°C. Limits over the operating temperature range are guaranteed by design.
Note 3: Guaranteed by design. Not production tested.
Note 4: LED dropout voltage is defined as the LED_ to GND voltage at which current into LED_ drops 10% from the value at
VLED_ = 0.5V.
_______________________________________________________________________________________ 3
MAX8847Y/MAX8847Z
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.)
EFFICIENCY vs. Li+ BATTERY VOLTAGE
(6 MATCHED LEDs)
20.8mA/LED
80
70
6.4mA/LED
60
90
80
16mA/LED
6.4mA/LED
70
1.6mA/LED
60
100
80
70
16.0mA/LED
60
VIN FALLING
1.6mA/LED
50
50
3.0
2.7
3.3
20.8mA /LED
90
3.6
3.9
3.8
3.7
3.6
3.5 3.4 3.0
6.4mA/LED
1.6mA/LED
50
4.2 3.9
4.2
3.0
2.7
3.3
VIN FALLING
3.6
3.9
SUPPLY VOLTAGE (V)
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
SUPPLY VOLTAGE (V)
EFFICIENCY vs. Li+ BATTERY VOLTAGE
(6 MISMATCHED LEDs)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
(6 MATCHED LEDs)
SUPPLY CURRENT vs. Li+ BATTERY
VOLTAGE (6 MATCHED LEDs)
SUPPLY CURRENT (mA)
90
VIN FALLING
80
6.4mA/LED
70
1.6mA/LED
60
150
20.8mA/LED
16.0mA/LED
100
6.4mA/LED
50
200
150
20.8mA /LED
16mA /LED
100
6.4mA /LED
50
1.6mA /LED
1.6mA/LED
50
0
0
3.8
3.7
3.6
3.5 3.4 3.0
2.7
3.0
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
3.3
3.6
4.2 3.9
4.2
150
20.8mA/LED
16.0mA/LED
100
6.4mA/LED
50
200
SUPPLY CURRENT (mA)
VIN FALLING
MAX8847Y toc07
200
3.8
3.7
150
20.8mA /LED
16mA /LED
100
6.4mA /LED
50
1.6mA/LED
1.6mA /LED
0
0
2.7
3.0
3.3
3.6
SUPPLY VOLTAGE (V)
3.9
4.2
3.6
3.5 3.4 3.0
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
SUPPLY CURRENT vs. Li+ BATTERY
VOLTAGE (6 MISMATCHED LEDs)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
(6 MISMATCHED LEDs)
SUPPLY CURRENT (mA)
3.9
SUPPLY VOLTAGE (V)
MAX8847Y toc08
4.2 3.9
4.2
MAX8847Y toc06
200
SUPPLY CURRENT (mA)
16mA/LED
MAX8847Y toc05
20.8mA/LED
MAX8847Y toc04
100
MAX8847Y toc03
16.0mA/LED
MAX8847Y toc02
90
100
EFFICIENCY PLED /PBATT (%)
EFFICIENCY PLED /PBATT (%)
20.8mA/LED
MAX8847Y toc01
100
EFFICIENCY vs. SUPPLY VOLTAGE
(6 MISMATCHED LEDs)
EFFICIENCY PLED /PBATT (%)
EFFICIENCY vs. SUPPLY VOLTAGE
(6 MATCHED LEDs)
EFFICIENCY PLED /PBATT (%)
MAX8847Y/MAX8847Z
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
4.2 3.9
3.8
3.7
3.6
3.5 3.4 3.0
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
4 _______________________________________________________________________________________
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
20.8mA/LED
MAX8847Z
VIN FALLING
25
18.0
MAX8847Y toc09
17.2
16.0mA/LED
20
6.4mA/LED
15
10
5
LEDS HAVE
MISMATCHED VF
1.6mA/LED
16.8
16.4
16.0
15.6
15.2
14.8
14.4
0
14.0
2.7
3.0
3.3
3.6
3.9
2.7
4.2
3.1
3.5
3.9
4.3
4.7
5.1
INPUT VOLTAGE (V)
SUPPLY VOLTAGE (V)
LED CURRENT vs. TEMPERATURE
1x MODE OPERATING WAVEFORM
(VIN = 4V)
MAX8847Y toc11
30
25
LED CURRENT (mA)
MAX8847Z
16mA/LED, VIN FALLING
17.6
LED CURRENT (mA)
INPUT VOLTAGE RIPPLE (mV)
30
MAX8847Y toc10
LED CURRENT MATCHING
vs. SUPPLY VOLTAGE
INPUT VOLTAGE RIPPLE (PEAK TO PEAK)
vs. INPUT VOLTAGE
5.5
MAX8847Y toc12
50mV/div
AC-COUPLED
VIN
20
15
100mA /div
IIN
10
0A
5
ILED1
24mA/LED
ALL LEDS ON
24mA /LED
0
-40
-15
10
35
60
85
TEMPERATURE (°C)
1µs/div
1.5x MODE OPERATING WAVEFORM
(VIN = 3V)
STARTUP AND SHUTDOWN RESPONSE
(MAX8847Y)
MAX8847Y toc14
MAX8847Y toc13
100mV/div
AC-COUPLED
VIN
100mA/div
VENA/ENB
2V/div
ENA = ENB
24mA/LED
200mV/div
AC-COUPLED
VIN
IIN
0A
20mA /div
ALL LEDS ON
24mA/LED
ILED1
1µs/div
20mA /div
0A
0A
100mA/div
IIN
0A
20mA /div
ILED1
0A
20ms/div
_______________________________________________________________________________________ 5
MAX8847Y/MAX8847Z
Typical Operating Characteristics (continued)
(VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(VIN = 3.6V, VEN_ = VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.)
SINGLE-WIRE SERIAL-PULSE
DIMMING RESPONSE (MAX8847Y)
LINE TRANSIENT RESPONSE
(VIN = 4.3V TO 3.8V TO 4.3V)
MAX8847Y toc15
MAX8847Y toc16
5V/div
0V
VENB
4.3V
4.3V
3.8V
VIN
1V/div
96mA
TOTAL
ILED
96mA
50mA /div
MAX8847Y, 1x MODE
ALL LEDS OPERATING
24mA FOR
LED1–LED4
ILED5
100mA /div
IIN
0A
0A
20mA /div
20mA /div
ILED1
ALL LEDS ON
24mA/LED
0A
10ms/div
1ms/div
LINE TRANSIENT RESPONSE
(VIN = 3.8V TO 3.4V TO 3.8V)
LINE TRANSIENT RESPONSE
(VIN = 3.5V TO 3.0V TO 3.5V)
MAX8847Y toc17
VIN
3.8V
3.4V
0A
MAX8847Y toc18
3.8V
3.5V
1V/div
3.5V
3.0V
1V/div
VIN
200mA /div
IIN
200mA /div
IIN
0A
0A
24mA
20mA /div
ILED1
ALL LEDS ON
24mA/LED
20mA /div
ILED1
ALL LEDS ON
16mA/LED
0A
1ms/div
0A
1ms/div
PWM DIMMING LINEARITY
PWM DIMMING RESPONSE
MAX8847Y toc20
25
MAX8847Y toc19
20
5V/div
VENA
100mA /div
IIN
ILED1
0A
LED CURRENT (mA)
MAX8847Y/MAX8847Z
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
15
10
5
12mA
20mA /div
0
0
40µs/div
20
40
60
80
PWM DUTY CYCLE (%)
6 _______________________________________________________________________________________
100
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
LED5
LED4
LED3
LED2
TOP VIEW
12
11
10
9
LED6 13
8
LED1
ENB 14
7
NEG
6
C1N
5
C2N
MAX8847Y
MAX8847Z
CPWM 15
EP
2
3
4
C2P
IN
1
C1P
+
GND
ENA 16
THIN QFN
Pin Description
PIN
NAME
FUNCTION
1
IN
Supply Voltage Input. The input voltage range is 2.7V to 5.5V. Bypass IN to GND with a 1FF ceramic
capacitor as close as possible to the IC. IN is high impedance during shutdown. Connect IN to the
anodes of all the LEDs.
2
GND
Ground. Connect GND to system ground and the input bypass capacitor as close as possible to the IC.
3
C1P
Transfer Capacitor 1 Positive Connection. Connect a 1FF ceramic capacitor from C1P to C1N.
4
C2P
Transfer Capacitor 2 Positive Connection. Connect a 1FF ceramic capacitor from C2P to C2N.
5
C2N
Transfer Capacitor 2 Negative Connection. Connect a 1FF ceramic capacitor from C2P to C2N. An
internal 10kI resistor pulls C2N to GND during shutdown.
6
C1N
Transfer Capacitor 1 Negative Connection. Connect a 1FF ceramic capacitor from C1P to C1N.
7
NEG
Charge-Pump Negative Output. Connect a 1FF ceramic capacitor from NEG to GND. In shutdown, an
internal 10kI resistor pulls NEG to GND. Connect the exposed pad to NEG directly under the IC.
8–13
LED1–LED6
LED Current Regulators. Current flowing into LED_ is based on the ENA/ENB input. Connect LED_ to
the cathodes of the external LEDs. LED_ is high impedance during shutdown. Short any unused LED_
to IN prior to power-up to disable the corresponding current regulator.
ENB
Enable or Serial-Pulse Dimming Control Input B. ENB controls LED5 and LED6. For the MAX8847Z,
ENB functions as on/off control for LED5 and LED6. For the MAX8847Y, except on/off control function, ENB can also be used to control the LED5 and LED6 serial-pulse dimming. Drive ENB high to
turn on the LED5 and LED6 current regulators at 24mA. Drive ENB low for greater than 8ms to turn off
the LED5 and LED6 current regulators or drive both ENA and ENB low to place the IC in shutdown.
For the MAX8847Y LED5 and LED6 serial-pulse diming control, see the Serial-Pulse Dimming Control
(MAX8847Y) section for details.
14
_______________________________________________________________________________________ 7
MAX8847Y/MAX8847Z
Pin Configuration
MAX8847Y/MAX8847Z
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
Pin Description (continued)
PIN
NAME
FUNCTION
CPWM
Filter Capacitor Connection for PWM Dimming. Connect a capacitor from CPWM to GND to form a filter
with the internal 360kI resistor. The recommended capacitor for a 2Hz corner frequency is 0.22FF.
16
ENA
Enable or PWM Dimming Control Input A. For the MAX8847Y, ENA functions as on/off control for
LED1–LED4 and PWM dimming control for LED1–LED4. For the MAX8847Z, ENA functions as on/
off control for LED1–LED4 and PWM dimming control for LED1–LED6. Drive ENA high to turn on the
LED1–LED4 current regulators at 24mA each. Drive ENA low for greater than 8ms to turn off the LED1–
LED4 current regulators or drive both ENA and ENB low to place the IC in shutdown. Drive ENA with a
PWM signal from 200Hz to 200kHz to dim the LEDs. See the PWM Dimming Control section.
—
EP
15
Exposed Paddle. Connect EP to NEG directly under the IC.
Detailed Description
The MAX8847Y/MAX8847Z have an inverting charge
pump and six current regulators capable of 24mA each
to drive up to 6 white LEDs. The current regulators are
matched to within 1% (typ) providing uniform white LED
brightness for LCD backlight applications. To maximize
efficiency, the current regulators operate with as little as
0.15V voltage drop.
Individual white LED current regulators conduct current
to GND or NEG to extend usable battery life. In the case
of mismatched forward voltage of white LEDs, only the
white LEDs requiring higher voltage are switched to pull
current to NEG instead of GND, further raising efficiency
and reducing battery current drain.
Current Regulator Switchover
When VIN is higher than the LED forward voltage plus the
150mV dropout voltage of the current regulator, the LED
current returns through GND. If this condition is satisfied for all active white LEDs, the charge pump remains
inactive. When the input voltage drops so that the current regulator voltage (VLED_) cannot be maintained for
any of the individual white LEDs, the inverting charge
pump activates and generates a voltage on NEG that is
no greater than 5V below VIN. For any current regulator
that is detected at the switchover threshold voltage of
150mV (typ, VIN falling), internal circuitry switches that
current regulator’s return path from GND to NEG to provide enough voltage across that regulator to overcome
dropout. When VLED_ rises to 250mV (typ), the return
of that current regulator is switched back from NEG to
GND. Each current regulator is independently monitored
to detect when switchover is required. Since the LED
current is switched for only the individual LED current
regulators requiring higher voltage, power consumption
is minimized.
Enable and Dimming Control Input
(ENA, ENB)
ENA and ENB inputs have dual functions: LED on/off
control and PWM or serial-pulse dimming control. See
Table 1 for details. For the MAX8847Y, ENA functions as
an on/off control and PWM dimming control for LED1–
LED4. ENB functions as on/off control and serial-pulse
dimming control for LED5 and LED6. For the MAX8847Z,
ENA functions as an on/off control for LED1–LED4 as
well as PWM dimming control for LED1–LED6. ENB is
used for on/off control for LED5 and LED6.
Table 1. ENA and ENB Enable and Dimming Control
PART
ENA
ENB
MAX8847Y
LED1–LED4 enable and PWM dimming control
LED5 and LED6 enable and serial-pulse
dimming control
MAX8847Z
LED1–LED4 enable and LED1–LED6 PWM dimming control
LED5 and LED6 enable control
8 _______________________________________________________________________________________
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
C1P
VIN
2.7V TO 5.5V
C3
1µ F
C1N
IN
C2P
C2N
NEG
INVERTING CHARGE PUMP
C1
1 µF
FB
GND
1MHz
OSCILLATOR
C4
1µF
EP
SEL
MIN
VIN
CURRENT
REGULATOR
ENABLE
AND
DIMMING
CONTROL
INPUTS
ENA
LED
ENABLE
LED1
ENB
CURRENT
REGULATOR
PWM
DIMMING
LED2
CPWM
C25
0.22µF
360kI
CURRENT
REGULATOR
LED3
CURRENT
SOURCE
CONTROL
CURRENT
REGULATOR
LED4
THERMAL
DERATING
AND
PROTECTION
CURRENT
REGULATOR
LED5
CURRENT
REGULATOR
MAX8847Y
MAX8847Z
LED6
Figure 1. Functional Diagram and Application Circuit
_______________________________________________________________________________________ 9
MAX8847Y/MAX8847Z
C2
1µ F
MAX8847Y/MAX8847Z
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
PWM Dimming Control
When VIN is above its undervoltage lockout threshold,
UVLO, apply a PWM signal to ENA to set the corresponding WLED current (see Table 1) that is proportional
to the signal duty cycle (0% duty cycle corresponds to
zero LED current and 100% duty cycle corresponds to
full LED current). The allowed PWM frequency range is
from 200Hz to 200kHz. If PWM dimming control is not
required, ENA works as a simple on/off control.
Serial-Pulse Dimming Control (MAX8847Y)
The MAX8847Y uses ENB as a serial-pulse control interface to program the intensity of LED5 and LED6. When
LED5 and LED6 are enabled by driving ENB high, the
0
1
2
3
4
MAX8847Y ramps LED5 and LED6 current to 24mA.
Subsequent pulses on ENB reduces the LED5 and LED6
current from 24mA to 0.1mA in 31 steps. After the current reaches 0.1mA, the next pulse restores the current
to 24mA. See Table 2 for the LED current values and the
corresponding ENB pulse count. Figure 2 shows a timing
diagram for ENB.
If dimming control is not required, ENB works as a simple
on/off logic control. Drive ENB high for at least 120µs to
enable the LED5 and LED6 current regulators, or drive
ENB low for greater than 8ms (typ) to place the LED5
and LED6 current regulators in shutdown. The LED current regulators operate at 100% brightness and off under
these conditions.
5
26
27
28
29
30
31
INITIAL tHI
tINIT > 120µs
ENB
24mA
tHI
> 1µs
tLO
1µs TO 500µs
22.4mA 20.8mA
19.2mA 17.6mA
16.0mA
ILED5/ILED6
24mA
22.4mA
0.6mA 0.5mA
0.4mA 0.3mA
0.2mA 0.1mA
SHDN
tSHDN
8ms (TYP)
SHDN
Figure 2. Timing Charateristics for LED Serial-Pulse Dimming Control
Table 2. ENB Serial-Pulse Dimming Count and Programmed LED_ Currents
ENB PULSE
COUNT
PROGRAMMED LED_ CURRENT
(mA)
ENB PULSE
COUNT
PROGRAMMED LED_ CURRENT
(mA)
Startup or ENB high
1
2
3
4
5
6
7
8
9
10
24.0
22.4
20.8
19.2
17.6
16.0
14.4
12.8
11.2
9.6
8.0
16
17
18
19
20
21
22
23
24
25
26
2.8
2.4
2.0
1.6
1.4
1.2
1.0
0.8
0.7
0.6
0.5
11
12
13
14
15
6.4
5.6
4.8
4.0
3.2
27
28
29
30
31
0.4
0.3
0.2
0.1
24.0
10 �������������������������������������������������������������������������������������
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
The MAX8847Y/MAX8847Z contain special circuitry to
detect short-circuit conditions at power-up and disable
the corresponding current regulator to avoid wasting
battery current. Connect any unused LED_ to IN to disable the corresponding current regulator. If an LED fails
short-circuit detection after startup, the current regulator
continues the current regulated operation until IC power
is cycled and the short circuit is detected during the
subsequent startup.
Shutdown Mode
An open-circuit LED failure drives the voltage on the corresponding LED current regulator output below the switchover threshold, enabling the negative charge pump.
Temperature Derating Function
The MAX8847Y/MAX8847Z include a thermal-limit circuit
that shuts down the IC above approximately +160°C.
The IC turns on after it cools by approximately 20°C.
The MAX8847Y/MAX8847Z are in shutdown mode when
both ENA and ENB are held low for 8ms or longer. In shutdown, NEG is pulled to GND with a 10kΩ internal resistor.
The MAX8847Y/MAX8847Z contain a derating function that automatically limits the LED current at high
temperatures in accordance with the recommended
derating curve of popular white LEDs. The derating
function enables the safe usage of higher LED current
at room temperature, thus reducing the number of LEDs
required to backlight the display. The derating circuit
lowers the LED current at approximately 2.5%/°C once
the die temperature is above +60°C. The typical derating
function characteristic is shown in the Typical Operating
Characteristics.
Table 3. Internal PWM Duty Cycle vs. LED
Set Current
ILED
(mA)
Power-Up LED Short Detection and
Open-Fault Protection
DUTY
MAXIMUM
CYCLE
ILED (mA)*
(n/8)
ILED
(mA)
MAXIMUM
ILED (mA)*
DUTY
CYCLE
(n/8)
6.4
6.4
8
1.2
1.6
6
5.6
6.4
7
1.0
1.6
5
4.8
6.4
6
0.8
0.8
8
4.0
6.4
5
0.7
0.8
7
3.2
3.2
8
0.6
0.8
6
2.8
3.2
7
0.5
0.8
5
2.4
3.2
6
0.4
0.8
4
2.0
3.2
5
0.3
0.8
3
1.6
1.6
8
0.2
0.8
2
1.4
1.6
7
0.1
0.8
1
Thermal Shutdown
Applications Information
Input Ripple
For LED drivers, input ripple is more important than
output ripple. The amount of input ripple depends on
the source supply’s output impedance. Add a lowpass
filter to the input of the MAX8847Y/MAX8847Z to further
reduce input ripple. Alternatively, increasing CIN from
1.0µF to 2.2µF (or 4.7µF) cuts input ripple in half (or in
fourth) with only a small increase in footprint.
Capacitor Selection
Ceramic capacitors are recommended due to their small
size, low cost, and low ESR. Select ceramic capacitors
that maintain their capacitance over temperature and DC
bias. Capacitors with X5R or X7R temperature characteristics generally perform well. Recommended values are
shown in the Typical Operating Circuit. Using a larger
value input capacitor helps to reduce input ripple (see
the Input Ripple section).
PCB Layout and Routing
The MAX8847Y/MAX8847Z are high-frequency switchedcapacitor voltage inverters. For best circuit performance,
use a solid ground plane and place all capacitors as
close as possible to the IC. Use large traces for the
power-supply inputs to minimize losses due to parasitic
trace resistance and to route heat away from the device.
Refer to the MAX8848Z evaluation kit data sheet for an
example PCB layout.
*Maximum ILED is the full reference current when the internal
PWM signal has 100% duty cycle at the lower level currents.
______________________________________________________________________________________ 11
MAX8847Y/MAX8847Z
Low LED Current Levels
The MAX8847Y internally generates a PWM signal to
obtain higher resolution at lower currents. See the SingleWire Serial-Pulse Dimming Response (MAX8847Y) graph
in the Typical Operating Characteristics section. When
the LED current is set below 6.4mA, the IC adjusts not
only LED DC current, but the duty cycle that is controlled
by the PWM signal. The frequency of the PWM dimming
signal is set at 16kHz with a minimum duty cycle of 1/8
to avoid the LED flickering effect to human eyes and
also to avoid interference in the audio frequency range.
Table 3 shows the current level and the corresponding
duty cycle.
MAX8847Y/MAX8847Z
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
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.
16 Thin QFN-EP
T1633+5
21-0136
12 �������������������������������������������������������������������������������������
High-Performance Negative Charge Pump
for 6 White LEDs in 3mm x 3mm Thin QFN
REVISION
NUMBER
REVISION
DATE
0
4/10
DESCRIPTION
Initial release
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
©
2010 Maxim Integrated Products
13
Maxim is a registered trademark of Maxim Integrated Products, Inc.
MAX8847Y/MAX8847Z
Revision History
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