19-5233; Rev 0; 4/10
EVALUATION KIT
AVAILABLE
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
General Description
The MAX8848Y/MAX8848Z negative charge pumps
drive up to 7 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 MAX8848Y/MAX8848Z include
thermal shutdown, open- and short-circuit protection.
The MAX8848Y/MAX8848Z support independent LED
on/off and dimming control. The MAX8848Y 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 full-scale output
current while automatically reducing current gradually
to protect LEDs at high ambient temperatures above
+60NC.
The MAX8848Y/MAX8848Z are available in 16-pin,
3mm x 3mm thin QFN packages.
Applications
White LED Backlighting
Cellular Phones
PDAs, Digital Cameras, and Camcorders
Ordering Information
PART DIMMING PIN-PACKAGE
MAX8848YETE+T
MAX8848ZETE+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.
Serial pulse/
PWM
16 Thin QFN-EP* AHQ
TOP
MARK
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 (MAX8848Y)
S 24mA to 0mA PWM Dimming (MAX8848Z)
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
INPUT
2.7V TO 5.5V
1µF
LED ENABLE
AND
DIMMING
CONTROL
0.22µF
1µF
C1P C1N C2P C2N
IN
GND
MAX8848Y
ENA
(ENB)
<CPWM>
MAX8848Z
1µF
NEG
LED1
LED2
LED3
LED4
LED5
LED6
LED7
1µF
EP
D1
D2
D3
D4
D5
D6
D7
MAX8848Y/MAX8848Z
NOTE: ( ) FOR MAX8848Y, < > FOR MAX8848Z
_______________________________________________________________ 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.
High-Performance Negative Charge Pump
for 7 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
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-
layer 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, V
MAX8848Y/MAX8848Z
PARAMETER CONDITIONS MIN TYP MAX UNITS
IN Operating Voltage 2.7 5.5 V
Undervoltage Lockout (UVLO)
Threshold
Undervoltage Lockout
Hysteresis
IN Shutdown Supply Current VEN_ = 0V, all outputs off
IN Operating Supply Current
Thermal Shutdown Threshold +160
Thermal Shutdown Hysteresis 20
PWM DIMMING CONTROL (MAX8848Z)
PWM Low-Level Input 0.4 V
PWM High-Level Input 1.4 V
EN_ PWM Input Signal
Frequency Range
PWM Dimming Filter Corner
Frequency
Current Dimming Range Duty cycle = 0 to 100% 0 24 mA
PWM Dimming Resolution
SERIAL-PULSE LOGIC (MAX8848Y)
EN_ Logic Input High Voltage 1.4 V
EN_ Logic Input Low Voltage 0.4 V
EN_ Logic-Input Current VIL = 0V or VIH = 5.5V
= 0V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
GND
VIN rising 2.35 2.45 2.55 V
Charge pump inactive, 2 LEDs enabled at 0.1mA setting 120 150
Charge pump active, 1MHz switching, all LEDs enabled
at 0.1mA setting
C
= 0.22FF
CPWM
C
= 0.22FF
CPWM
1% P duty cycle P 100%
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
100 mV
TA = +25NC
TA = +85NC
TA = +25NC
TA = +85NC
0.2 200 kHz
-1 0.01 +1
0.4 2.5
0.4
1.6 mA
2 Hz
0.24 mA/%
0.1
FA
FA
NC
NC
FA
2 ______________________________________________________________________________________
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, V
PARAMETER CONDITIONS MIN TYP MAX UNITS
EN_ Low Shutdown Delay
t
SHDN
EN_ t
LO
EN_ t
HI
Initial EN_ t
CHARGE PUMP
Switching Frequency 1 MHz
Soft-Start Time 0.5 ms
Output Regulation Voltage VIN - V
Open-Loop NEG Output
Resistance
NEG Shutdown Discharge
Resistance
LED1–LED7 CURRENT REGULATOR
Current Setting Range Serial-pulse interface or PWM 0.1 24.0 mA
LED_ Current Accuracy
Derating Function Start
Temperature
Derating Function Slope From derating function start temperature -2.5
LED_ Dropout Voltage
(Note 4)
LED_ Current Regulator
Switchover Threshold
(Inactive to Active)
LED_ Current Regulator
Switchover Hysteresis
LED_ Leakage in Shutdown All LEDs off
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
V
LED_
= 0V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
GND
5 8 ms
See Figure 2 1 500
See Figure 2 1
INIT
= 0.5V.
See Figure 2, first EN_ high pulse 120
NEG
(V
- 0.5 x VIN)/I
NEG
VEN_ = 0V, all outputs off 10
V
= 0.5V for charge
LED_
pump inactive,
V
= -0.9V,
LED_
V
= -1.4V
NEG_
Charge pump inactive,
24mA setting
Charge pump active,
24mA setting
V
falling 135 150 165 mV
LED_
NEG
24mA setting, TA = +25NC
24mA setting, TA = -40NC
to derating function start
temperature (Note 3)
1.6mA setting, TA = +25NC Q5
TA = +25NC
TA = +85NC
TA = +25NC
TA = +85NC
TA = +25NC
TA = +85NC
4.3 5 V
-2
-5 +5
Fs
Fs
Fs
2 4
kI
Q1
+60
85 125
95
110
124
100 mV
0.01 5
0.1
+2
NC
%/NC
mV
FA
I
%
MAX8848Y/MAX8848Z
_______________________________________________________________________________________ 3
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
Typical Operating Characteristics
(VIN = 3.6V, V
= VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.)
EN_
EFFICIENCY vs. SUPPLY VOLTAGE
EFFICIENCY vs. Li+ BATTERY VOLTAGE
(7 MATCHED LEDs)
100
20.8mA/LED
90
(%)
BATT
/P
80
LED
70
EFFICIENCY P
60
50
2.7 4.2
MAX8848Y/MAX8848Z
EFFICIENCY vs. Li+ BATTERY VOLTAGE
16.0mA/LED
6.4mA/LED
1.6mA/LED
VIN FALLING
3.93.63.33.0
SUPPLY VOLTAGE (V)
MAX8848Y toc01
100
90
(%)
BATT
/P
80
LED
70
EFFICIENCY P
60
50
4.2 3.0
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
(7 MISMATCHED LEDs)
100
20.8mA/LED
90
(%)
BATT
/P
80
LED
70
EFFICIENCY P
60
50
4.2 3.0
6.4mA/LED
1.6mA/LED
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
16mA/LED
3.43.53.63.73.83.9
250
200
MAX8848Y toc04
150
100
SUPPLY CURRENT (mA)
50
0
2.7 4.2
(7 MATCHED LEDs)
20.8mA/LED
6.4mA/LED
1.6mA/LED
(7 MATCHED LEDs)
SUPPLY VOLTAGE (V)
16mA/LED
16.0mA/LED
1.6mA/LED
VIN FALLING
20.8mA/LED
6.4mA/LED
3.93.63.33.0
EFFICIENCY vs. SUPPLY VOLTAGE
(7 MISMATCHED LEDs)
100
VIN FALLING
90
MAX8848Y toc02
(%)
BATT
/P
80
LED
70
EFFICIENCY P
60
50
3.43.53.63.73.83.9
2.7 4.2
1.6mA/LED
SUPPLY VOLTAGE (V)
20.8mA/LED
MAX8848Y toc03
16.0mA/LED
6.4mA/LED
3.93.63.33.0
SUPPLY CURRENT vs. Li+ BATTERY
VOLTAGE (7 MATCHED LEDs)
250
200
MAX8848Y toc05
150
100
SUPPLY CURRENT (mA)
50
0
20.8mA/LED
16.0mA/LED
6.4mA/LED
1.6mA/LED
4.2 3.0
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
MAX8848Y toc06
3.43.53.63.73.83.9
SUPPLY CURRENT vs. SUPPLY VOLTAGE
(7 MISMATCHED LEDs)
250
200
150
100
SUPPLY CURRENT (mA)
50
0
2.7 4.2
SUPPLY VOLTAGE (V)
20.8mA/LED
16.0mA/LED
6.4mA/LED
1.6mA/LED
VIN FALLING
MAX8848Y toc07
SUPPLY CURRENT (mA)
3.93.63.33.0
INPUT CURRENT vs. Li+ BATTERY
VOLTAGE (7 MISMATCHED LEDs)
250
200
150
100
50
0
4.2 3.0
Li+ BATTERY VOLTAGE (V, TIME WEIGHTED)
20.8mA/LED
16.0mA/LED
6.4mA/LED
1.6mA/LED
INPUT VOLTAGE RIPPLE (PEAK-TO-PEAK)
vs. INPUT VOLTAGE
40
MAX8848Y toc08
3.43.53.63.73.83.9
20.8mA/LED
30
16.0mA/LED
20
6.4mA/LED
10
INPUT VOLTAGE RIPPLE (mV)
1.6mA/LED
0
2.7 4.2
LEDS HAVE MISMATCHED V
INPUT VOLTAGE (V)
4 ______________________________________________________________________________________
MAX8848Y
VIN FALLING
3.93.63.33.0
F
MAX8848Y toc09
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
Typical Operating Characteristics (continued)
(VIN = 3.6V, V
= VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.)
EN_
LED CURRENT MATCHING
vs. SUPPLY VOLTAGE
17.0
MAX8848Y
16.8
16mA/LED, VIN FALLING
16.6
16.4
16.2
16.0
15.8
LED CURRENT (mA)
15.6
15.4
15.2
15.0
2.7 5.5
SUPPLY VOLTAGE (V)
MAX8848Y toc10
5.14.73.9 4.33.53.1
LED CURRENT vs. TEMPERATURE
30
24mA/LED
25
20
15
LED CURRENT (mA)
10
5
0
-40 85
TEMPERATURE (°C)
MAX8848Y toc11
603510-15
MAX8848Y/MAX8848Z
V
ENA/ENB
1x MODE OPERATING WAVEFORM
= 4V)
(V
IN
V
IN
I
IN
I
ILED1
ALL LEDS ON
24mA/LED
1µs/div
STARTUP AND SHUTDOWN RESPONSE
(MAX8848Y)
V
IN
I
N
I
ILED1
ENA = ENB
24mA/LED
2ms/div
MAX8848Y toc12
MAX8848Y toc14
100mV/div
AC-COUPLED
100mA/div
0A
20mA/div
0A
2V/div
200mV/div
AC-COUPLED
200mA/div
0A
20mA/div
0A
1.5x MODE OPERATING WAVEFORM
= 3V)
(V
I
ILED1
V
I
IN
IN
IN
ALL LEDS ON
24mA/LED
1µs/div
MAX8848Y toc13
100mV/div
AC-COUPLED
100mA/div
0A
20mA/div
0A
SINGLE-WIRE SERIAL-PULSE DIMMING
RESPONSE (MAX8848Y)
= V
V
ENA
ENB
1x MODE
TOTAL
I
I
LED1
LED
ALL LEDS OPERATING
10ms/div
MAX8848Y toc15
5V/div
0V
50mA/div
0A
20mA/div
0A
_______________________________________________________________________________________ 5
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
Typical Operating Characteristics (continued)
(VIN = 3.6V, V
= VIN, circuit of Figure 1, TA = +25°C, unless otherwise noted.)
EN_
LINE TRANSIENT RESPONSE
= 4.3V TO 3.8V TO 4.3V)
(V
IN
V
IN
I
IN
I
LED1
ALL LEDS ON
24mA/LED
1ms/div
MAX8848Y/MAX8848Z
V
I
I
LED1
LINE TRANSIENT RESPONSE
= 3.5V TO 3.0V TO 3.5V)
(V
IN
3.5V
IN
IN
ALL LEDS ON
16mA/LED
16mA
1ms/div
3.8V
3.0V
MAX8848Y toc16
4.3V4.3V
MAX8848Y toc18
3.5V
1V/div
200mA/div
0A
20mA/div
0A
1V/div
100mA/div
0A
20mA/div
0A
I
LED1
V
I
V
I
ENA
LED1
LINE TRANSIENT RESPONSE
= 3.8V TO 3.4V TO 3.8V)
(V
IN
3.8V
IN
IN
ALL LEDS ON
24mA/LED
PWM DIMMING RESPONSE
I
IN
1ms/div
12mA
40µs/div
3.4V
MAX8848Y toc17
3.8V
1V/div
200mA/div
0A
20mA/div
0A
MAX8848Y toc19
5V/div
100mA/div
0A
20mA/div
MAX8848Z
25
20
15
10
LED CURRENT (mA)
5
0
0 100
PWM DUTY CYCLE (%)
MAX8848Z
80604020
MAX8848Y toc20
6 ______________________________________________________________________________________
PWM DIMMING LINEARITY
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
Pin Configuration
MAX8848Y/MAX8848Z
TOP VIEW
CPWM (ENB)
LED6
LED7
ENA
13
14
15
16
+
LED4
LED5
12 11 9
1 2
IN
10
MAX8848Y
MAX8848Z
3
GND
THIN QFN
C1P LED3
LED2
8
7
6
EP
4
5
C2P
( ) FOR MAX8848Y ONLY
LED1
NEG
C1N
C2N
Pin Description
PIN
MAX8848Y MAX8848Z
1 1 IN
2 2 GND
3 3 C1P
4 4 C2P
5 5 C2N
6 6 C1N
7 7 NEG
NAME FUNCTION
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.
Ground. Connect GND to system ground and the input bypass capacitor as
close as possible to the IC.
Transfer Capacitor 1 Positive Connection. Connect a 1FF ceramic capacitor from
C1P to C1N.
Transfer Capacitor 2 Positive Connection. Connect a 1FF ceramic capacitor from
C2P to 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.
Transfer Capacitor 1 Negative Connection. Connect a 1FF ceramic capacitor
from C1P to C1N.
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.
_______________________________________________________________________________________ 7
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
Pin Description (continued)
PIN
MAX8848Y MAX8848Z
8–14 8–14 LED1–LED7
15 — ENB
— 15 CPWM
MAX8848Y/MAX8848Z
16 16 ENA
— — EP Exposed Paddle. Connect EP to NEG directly under the IC.
NAME FUNCTION
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.
Enable and Serial-Pulse Dimming Control Input B for the MAX8848Y. ENB controls LED6 and LED7. Drive ENB high to turn on the LED6 and LED7 current
regulators at 24mA. Drive ENB low for greater than 8ms to turn off the current
regulators or drive both ENA and ENB low to place the IC in shutdown. In addition to the enable function, ENB can also be used to control the LED6 and LED7
serial-pulse dimming.
Filter Capacitor Connection for PWM Dimming for the MAX8848Z. 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.
Enable and PWM/Serial-Pulse Dimming Control Input A. ENA controls LED1–
LED5 for the MAX8848Y, and LED1–LED7 for MAX8848Z. Drive ENA high to turn
on all the controlled LED current regulators at 24mA. Drive ENA low for greater
than 8ms to turn off the 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 LED1–LED7 for the MAX8848Z. See the PWM Dimming Control (MAX8848Z)
section. For the MAX8848Y, ENA controls the LED1–LED5 serial-pulse dimming.
8 ______________________________________________________________________________________
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
C2
1
µF
C3
1
µF
MAX8848Y/MAX8848Z
2.7V TO 5.5V
ENABLE
AND
DIMMING
CONTROL
INPUTS
0.22
IN
1
C25
C1
µF
µF
IN
GND
ENA
(ENB)
<CPWM>
V
C1P
360kI
C1N
INVERTING CHARGE PUMP
LED
ENABLE
PWM
DIMMING
CURRENT
SOURCE
CONTROL
C2P C2N
1MHz
OSCILLATOR
NEG
C4
1
FB
SEL
MIN
CURRENT
REGULATOR
CURRENT
REGULATOR
CURRENT
REGULATOR
CURRENT
REGULATOR
EP
LED1
LED2
LED3
LED4
µF
V
IN
THERMAL
DERATING
AND
PROTECTION
MAX8848Y
MAX8848Z
NOTE: () FOR MAX8848Y ONLY, < > FOR MAX8848Z ONLY
Figure 1. Functional Diagram and Application Circuit
_______________________________________________________________________________________ 9
CURRENT
REGULATOR
CURRENT
REGULATOR
CURRENT
REGULATOR
LED5
LED6
LED7
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
Detailed Description
The MAX8848Y/MAX8848Z have an inverting charge
pump and seven current regulators capable of 24mA
each to drive up to 7 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 satis-
MAX8848Y/MAX8848Z
fied for all active white LEDs, the charge pump remains
inactive. When the input voltage drops so that the current regulator voltage (V
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 V
of that current regulator is switched back from NEG to
GND. Each current regulator is independently monitored
LED_
) cannot be maintained for
LED_
rises to 250mV (typ), the return
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 MAX8848Y, ENA functions as
on/off control and serial-pulse dimming control for LED1–
LED5. ENB functions as on/off control and serial-pulse
dimming control for LED6 and LED7. For the MAX8848Z,
only ENA functions as on/off control as well as PWM dimming control for LED1–LED7.
PWM Dimming Control (MAX8848Z)
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 (MAX8848Y)
The MAX8848Y uses ENA/ENB as a serial-pulse control
interface to program the intensity of LED1–LED7. When
the LEDs are enabled by driving ENA/ENB high, the
MAX8848Y ramps LED current to 24mA. Subsequent
pulses on ENA/ENB reduce the LED 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
Table 1. ENA and ENB Enable and Dimming Control
PART ENA ENB
MAX8848Y LED1–LED5 enable and serial-pulse dimming control
MAX8848Z LED1–LED7 enable and PWM dimming control —
10 _____________________________________________________________________________________
LED6 and LED7 enable and serial-pulse
dimming control
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
for the LED current values and the corresponding ENA/
ENB pulse count. Figure 2 shows a timing diagram for
ENA/ENB.
If dimming control is not required, ENA/ENB works as
a simple on/off logic control. Drive ENA/ENB high for
at least 120Fs to enable the LED current regulators, or
MAX8848Y/MAX8848Z
drive ENA/ENB low for greater than 8ms (typ) to place
the LED current regulators in shutdown. The LED current
regulators operate at 100% brightness and off under
these conditions.
EN_
I
LED_
SHDN
0
INITIAL t
t
> 120µs
INIT
24mA
1 2 3 4 5 26 27 28 29 30 31
HI
22.4mA
20.8mA
19.2mA
1µs
TO 500
17.6mA
µs
16.0mA
t
LO
0.6mA
> 1µs
0.5mA
t
HI
0.4mA
0.3mA
0.2mA
0.1mA
24mA
22.4mA
t
SHDN
8ms (TYP)
Figure 2. Timing Charateristics for LED Serial-Pulse Dimming Control
Table 2. ENA/ENB Serial-Pulse Dimming Count and Programmed LED_ Currents
EN_ PULSE
COUNT
Startup or EN_high 24.0 16 2.8
1 22.4 17 2.4
2 20.8 18 2.0
3 19.2 19 1.6
4 17.6 20 1.4
5 16.0 21 1.2
6 14.4 22 1.0
7 12.8 23 0.8
8 11.2 24 0.7
9 9.6 25 0.6
10 8.0 26 0.5
11 6.4 27 0.4
12 5.6 28 0.3
13 4.8 29 0.2
14 4.0 30 0.1
15 3.2 31 24.0
PROGRAMMED LED_ CURRENT
(mA)
EN_ PULSE
COUNT
PROGRAMMED LED_ CURRENT
(mA)
SHDN
______________________________________________________________________________________ 11
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
Low LED Current Levels
The MAX8848Y internally generates a PWM signal to
obtain higher resolution at lower currents. See the SingleWire Serial-Pulse Dimming Response (MAX8848Y) 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.
Shutdown Mode
The MAX8848Y/MAX8848Z 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.
Temperature Derating Function
The MAX8848Y/MAX8848Z contain a derating func-
MAX8848Y/MAX8848Z
tion 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
MAXIMUM
I
LED
I
(mA)*
6.4
5.6
4.8
4.0
3.2
2.8
2.4
2.0
1.6
1.4
LED
6.4 8 1.2 1.6 6
6.4 7 1.0 1.6 5
6.4 6 0.8 0.8 8
6.4 5 0.7 0.8 7
3.2 8 0.6 0.8 6
3.2 7 0.5 0.8 5
3.2 6 0.4 0.8 4
3.2 5 0.3 0.8 3
1.6 8 0.2 0.8 2
1.6 7 0.1 0.8 1
LED
(mA)
*Maximum I
PWM signal has 100% duty cycle at the lower level currents.
DUTY
CYCLE
(n/8)
is the full reference current when the internal
I
LED
(mA)
MAXIMUM
I
(mA)*
LED
CYCLE
DUTY
(n/8)
Power-Up LED Short Detection and
Open-Fault Protection
The MAX8848Y/MAX8848Z 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.
An open-circuit LED failure drives the voltage on the corresponding LED current regulator output below the switchover threshold, enabling the negative charge pump.
Thermal Shutdown
The MAX8848Y/MAX8848Z 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.
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 MAX8848Y/MAX8848Z 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 MAX8848Y/MAX8848Z 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.
12 _____________________________________________________________________________________
High-Performance Negative Charge Pump
for 7 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
MAX8848Y/MAX8848Z
______________________________________________________________________________________ 13
High-Performance Negative Charge Pump
for 7 White LEDs in 3mm x 3mm Thin QFN
Revision History
REVISION
NUMBER
0 4/10 Initial release —
REVISION
DATE
MAX8848Y/MAX8848Z
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.
14 Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
©
2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
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