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S P6 8 8 7 4 CHANNEL REGULATED CHARGE PUMP WLED DRIVER
FEATURES Peak Efficiency: 90% Individual Current Regulation 3-bit Digital Output Control 1x and 1.5x Modes of Operation Current Matching with a Max Tolerance of 3% Output Current up to 30mA per LED Fixed Frequency of 1MHz Open LED Protection Pb-Free Thin QFN-16 3mmx3mm Package APPLICATIONS Cellular Phones LED backlighting LCD Modules Handheld Devices Digital Cameras PDAs MP3 Players DESCRIPTION
The SP6887 is a 4-channel charge pump white LED driver, capable of driving up to 4 LEDs connected in parallel. The device operates in either 1x mode or 1.5x fractional mode, and it can switch from 1x mode to 1.5x mode automatically when the input voltage decreases. Its internal 4 current sink regulators ensure both LED current matching and brightness uniformity. The LED current can be programmed by an external resistor, RSET, connected between the RSET pin and ground. LED currents of up to 30mA are supported by the input supply voltage over a range of 2.7V to 5.5V, making the device optimized for Li-Ion battery applications. The SP6887 has a fixed switching frequency of 1MHz, allowing the use of very small value ceramic capacitors. The enable input pin allows the device to be placed in shutdown mode while reducing current consumption to less than 1A. LED dimming can be done by several methods including using a DC voltage to set the RSET pin current, adding a switched resistor in parallel with RSET or applying a PWM signal to CTRx pin or EN pin.
TYPICAL APPLICATION CIRCUIT
Rev. C - 10/31/2007
SP6887 4 Channel WLED Driver 1/10
(c) 2007 Exar Corporation
BLOCK DIAGRAM
Control Lines CTR2 0 0 0 0 1 1 1 1 CTR1 0 0 1 1 0 0 1 1 CTR0 0 1 0 1 0 1 0 1 LED4 ON ON ON ON ON -
LED Output LED3 LED2 ON ON ON ON LED1 ON ON ON ON -
1 = Logic high (or VIN), 0 = Logic low (or GND), - = LED output off
Table 1: LED Enable Logic
Rev. C - 10/31/2007
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(c) 2007 Exar Corporation
PIN DESCRIPTION
Thin QFN-16 3mmx3mm
Pin Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Name EN CTR0 CTR1 CTR2 RSET VOUT VIN C1+ C1C2C2+ GND LED4 LED3 LED2 LED1 Enable Input, Active High Digital Control Input 0 Digital Control Input 1 Digital Control Input 2 Set Resistance
Description
Charge pump output connected to the LED anodes Supply Voltage Bucket Capacitor 1 Terminal Bucket Capacitor 1 Terminal Bucket Capacitor 2 Terminal Bucket Capacitor 2 Terminal Ground Reference LED4 Cathode Terminal LED3 Cathode Terminal LED2 Cathode Terminal LED1 Cathode Terminal
ELECTRICAL SPECIFICATIONS
Absolute Maximum Ratings
These are stress ratings only and functional operation is not implied. Exposure to absolute maximum ratings for prolonged time periods may affect device reliability. All voltages are with respect to ground.
VIN, VOUT, LEDx, Pin Voltage............. -0.3V to 6V EN, CTRx Pin Voltage ..................... -0.3V to VIN RSET Pin Voltage ........................... -0.3V to VIN Ambient Temperature Range ..... -40C to +85C
Storage Temperature Range .... -65C to +160C Lead Temperature ................................. 300C ESD Ratings - Human Body Model ........... 2000V
Recommended Operating Conditions
Vin ........................................ +2.7V to +5.5V Input/Output/Bucket Capacitors...120%F(typ) Ambient Temperature ...............-40C to +85C ILED per LED ..............................0mA to 30mA
Thermal Information
Parameter Thermal Resistance (Junction to Ambient) Power Dissipation PD@TA=25C Symbol JA PD Package TQFN-16 TQFN-16 Max 34 2.9 Unit C W
Rev. C - 10/31/2007
SP6887 4 Channel WLED Driver 3/10
(c) 2007 Exar Corporation
ELECTRICAL SPECIFICATIONS
TA=25C, VIN=3.5V, CIN=COUT=1F, unless otherwise noted. Parameter Input Voltage Range Shutdown Current Quiescent Current RSET Regulated Voltage Programmed LED Current LED Current Accuracy LED Channel Matching Output Resistance (Open Loop) Charge Pump Frequency VIN at Mode Transition from 1x to 1.5x 1x to 1.5x Mode Transition Dropout Delay Input Leakage Current High Detect Threshold Low Detect Threshold Symbol VIN ISD IQ VRSET ILED ILED-ACC ILED-DEV RO fOSC VIN-Tran TDROP IEN-CTR IEN-CTRH IEN-CTRL On Inputs EN, CTR0, 1 & 2 On Inputs EN, CTR0, 1 & 2 On Inputs EN, CTR0, 1 & 2 1.5 0.4 ILED=15mA ILED=20mA 0.4 (ILED-ILEDAVG)/ILEDAVG 1x Mode, IO=100mA 1.5x Mode, IO=100mA RSET=90k RSET=29.3k RSET=14.7k VEN=0V Shutdown Mode 1X Mode, No Load 1.5x Mode, No Load 1.19 Conditions Min 2.7 0.05 0.6 2.5 1.23 5.0 15.0 30.0 5 3 1.7 4.3 1.0 3.45 3.60 0.6 0.9 1 Typ. Max 5.5 1 1.2 5 1.25 Unit V A mA mA V mA % % MHz V ms A V V
Typical Characteristics
VIN=3.6V, EN=VIN, RSET=24k, CIN=1F, TA=25C, unless otherwise noted.
1. Efficiency vs Input Voltage (4 LEDs)
2. Efficiency vs Output Current (4 LEDs)
Rev. C - 10/31/2007
SP6887 4 Channel WLED Driver 4/10
(c) 2007 Exar Corporation
ELECTRICAL SPECIFICATIONS
3. LED Current vs Input Voltage (4 LEDs)
4. Supply Current vs Input Voltage (4 LEDs)
5. Ground Current vs Input Voltage
6. Oscillator Frequency vs Input Voltage
7. Output Voltage vs Input Voltage
8. Output Resistance vs Input Voltage
Rev. C - 10/31/2007
SP6887 4 Channel WLED Driver 5/10
(c) 2007 Exar Corporation
ELECTRICAL SPECIFICATIONS
9. LED Current Setting Using RSET
10. LED Current Setting Using a DC Voltage to RSET Pin
11. LED Current Setting Using a PWM Signal to EN Pin
12. LED Current Setting Using a PWM Signal to CTR0 Pin
Rev. C - 10/31/2007
SP6887 4 Channel WLED Driver 6/10
(c) 2007 Exar Corporation
ELECTRICAL SPECIFICATIONS
13. Output Ripple
THEORY OF OPERATION
Detailed Description As shown in the block diagram on page 2, the main components within the SP6887 include a fractional charge pump, mode selection circuit, output selection logic, LED current setting detection circuit, and 4 current sense circuits. The fractional charge pump multiplies the input voltage a multiple of 1X and 1.5X times the input voltage. The charge pump switches at a fixed 1MHz when the mode is 1.5X. The charge pump does not switch during 1X mode, saving power and improving efficiency. The mode selection circuit automatically selects the mode as 1X or 1.5X based on circuit conditions such as LED voltage, input voltage and load current. 1X is the more efficient mode than 1.5X mode. Table 1 on page 2 shows the output selection logic control over the LED outputs for on and off functions with 8 different output states. The current set and detection circuit uses an external resistor and a 1.20V reference to program the LED current. 4 current regulating circuits sink matched currents from the LEDs. LEDs with matched forward voltage will produce the best possible matched currents. For best
Rev. C - 10/31/2007
matching performance it is recommended that the Vf between LEDs be under 250mV. The unused LED channels can be turned off by CTR0, CTR1 and CTR2, and connecting the respective LED pins to VOUT pin, in which case, the corresponding LED driver sink current is only about 20A. Methods for Setting LED Current There are 4 methods for setting and adjusting the LED current outlined here. The methods are: 1) RSET only 2) Analog Reference VSET 3) PWM Input at CTR0 4) PWM Input at EN Method 1: LED Current Setting Using the External Resistor RSET The most basic means of setting the LED current is with a resistor connected from RSET to GND, as shown in the application circuit on Page 1. The resistor RSET establishes the reference current needed for a constant LED current. Values of RSET for a fixed LED current are given in Table 2, "RSET
(c) 2007 Exar Corporation
SP6887 4 Channel WLED Driver 7/10
THEORY OF OPERATION
Resistor Selection". Method 2 is for setting the LED current while allowing for brightness control.
ILED (mA) RSET (k) 30 20 15 10 9 8 7 6 5 4 3 2 1 14.7 21.8 29.3 44.1 49.4 55.7 64.2 75.0 90.0 114.2 156.5 238.0 540.0
Standard Value % Value (k) Difference 14.7 0.0% 22.0 0.9% 29.4 0.3% 44.2 0.2% 49.9 1.0% 56.0 0.5% 63.4 0.3% 75.0 0.0% 88.7 -0.3% 115.0 0.7% 158.0 0.9% 237.0 -0.4% 536.0 -0.7%
Figure 1: Analog Voltage for LED Current Control
Method 3: LED Current Setting Using a PWM Signal to CTRx Pin The circuit in Figure 2 turns 4 LEDs on and off by a PWM signal to the CTR0. This circuit uses resistor RSET to set the on state current and the average LED current is, then proportional to the percentage of on-time when the CTR0 pin is logic low. Average LED current is approximately equal to: ILEDAVG = (tON*ILED_ON)/ (tON + tOFF) The recommended PWM frequency is between 100Hz and 1KHz. Due to start up delay and ramp up time, frequency >1KHz will result in error in the average value of ILED. Frequency <100Hz can naturally cause the LEDs to blink visibly.
Table 2: RSET Resistor Selection
Method 2: LED Current Setting Using a DC Voltage to RSET Pin The example circuit in Figure 1 uses a 14.7k resistor and an analog input DC voltage, VSET, which varies from 1.2V to 0V to control LED current from 1mA to 30mA. Table 3 shows the resulting output. If necessary, the analog VSET voltage can be sourced from a voltage higher than 1.20V, but the source must be divided down so that the V mode will not exceed 1.20V. For lower current applications and for higher resolution, a larger resistor may be substituted in this circuit. PWM applications are also possible with this circuit by application of RC filtering.
VSET (V) 0.0 0.1 0.2 0.3 0.4 0.5 0.6
ILED (mA) 30.0 27.7 25.3 22.9 20.5 18.1 15.7
VSET (V) 0.7 0.8 0.9 1.0 1.1 1.2
ILED (mA) 13.3 10.9 8.4 6.0 3.6 1.1
Table 3: Analog Voltage for LED Current Control
Figure 2: PWM Signal at CTR0 Pin for LED Current Control (c) 2007 Exar Corporation
Rev. C - 10/31/2007
SP6887 4 Channel WLED Driver 8/10
THEORY OF OPERATION
Method 4: LED Current Setting Using a PWM Signal to EN Pin The 4 LEDs turn on and off by applying a PWM signal to the EN pin as shown in figure 3. The circuit is the same as to the method 3, using a resistor RSET to set the on state current and the average LED current is then proportional to the percentage of on-time when the EN pin is logic low. Average LED current is approximately equal to: ILEDAVG = (tON*ILED_ON)/ (tON + tOFF) Similarly, the recommended PWM frequency is between 100Hz and 1KHz. Due to start up delay and ramp up time, frequency >1KHz will result in error in the average value of ILED. Frequency <100Hz can naturally cause the LEDs to blink visibly. PCB Layout When the driver is in the 1.5X charge pump mode, the 1MHz switching frequency operation require to minimize the trace length and impedance to ground on all 4 capacitors. A ground plane should cover the area on the bottom side of the PCB opposite to the IC and the bypass capacitors. Capacitors CIN and Co require to short connection to ground which can be done with multiple vias as shown on Figure4. Square copper area matches the QFN 16 exposed pad (GND) which is connected by a trace to the pin 12 pad (GND). A large via (metalized hole) centered in the square pad provides a low impedance connection to the ground plane on the opposite side of the PCB and allows the heat dissipated by the driver IC to spread out resulting in excellent thermal performance.
Figure 3: PWM Signal at EN Pin for LED Current Control
Figure 4: PCB Lay-out
Rev. C - 10/31/2007
SP6887 4 Channel WLED Driver 9/10
(c) 2007 Exar Corporation
PACKAGE
Thin QFN-16 3mm x 3mm
Units: mm (angles in degrees) Coplanarity applies to the exposed pads as well as terminals DAP is 1.90mm x 1.90mm
ORDERING INFORMATION
Operating Temperature Range -40C to +85C Packing Quantity -
Part Number SP6887ER4-L
Package Thin QFN-16
Marking 6887 ER4 YWWX 6887 ER4 YWWX
SP6887ER4-L/TR
-40C to +85C
Thin QFN-16
3,000/T&R
"Y" = Year, "WW" = Work Week, "X" = first alpha of lot number
Exar Corporation Headquarters and Sales Offices 48720 Kato Road Fremont, CA 94538 - USA Tel.: +1 (510) 668-7000 Fax: +1 (510) 668-7030 www.exar.com
Rev. C - 10/31/2007 SP6887 4 Channel WLED Driver 10/10 (c) 2007 Exar Corporation

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