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Final Electrical Specifications
LTC3205 Multidisplay LED Controller
August 2003
FEATURES
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DESCRIPTIO
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Step-Up/Step-Down Fractional Charge Pump for Up to 92% Efficiency Independent Current and Dimming Control for 1-4 LED Main, 1-2 LED Sub and RGB LED Displays LED Currents Programmable Using 3-Wire Serial Interface 0.7% LED Current Matching Low Noise Constant Frequency Operation* Minimal Component Count Automatic Soft-Start Limits Inrush Current Four Programmable Dimming States for Main and Sub Displays Up to 4096 Color Combinations for RGB Display Low Shutdown Current: ICC < 1A Tiny 24-Lead (4mm x 4mm) QFN Package
The LTC(R)3205 is a highly integrated multidisplay LED controller. The part contains a high efficiency, low noise fractional step-up/step-down charge pump to provide power for both main and sub white LED displays plus an RGB color LED display. The LTC3205 requires only four small ceramic capacitors plus two resistors to form a complete 3-panel LED power supply and current controller. Maximum currents for the main/sub and RGB displays are set independently with a single resistor. Current for each LED is controlled with an internal current source. Dimming and ON/OFF control for all displays is achieved via a 3-wire serial interface. Four dimming states exist for the main and sub displays and 16 dimming states are available via internal PWM for the red, green and blue LEDs resulting in up to 4096 color combinations. The LTC3205 charge pump optimizes efficiency based on VIN and LED forward voltage conditions. The part powers up in step-down mode and automatically switches to stepup mode once any enabled LED current source begins to enter dropout. Internal circuitry prevents inrush current and excess input noise during start-up and mode switching. The LTC3205 is available in a 24-lead (4mm x 4mm) QFN package.
APPLICATIO S
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Cellular Phones Wireless PDAs Multidisplay Handheld Devices
, LTC and LT are registered trademarks of Linear Technology Corporation. * U.S. Patent 6,411,531
TYPICAL APPLICATIO
1F 1F
VIN 1F LTC3205
CPO 1F
EFFICIENCY (PLED/PIN) (%)
VIN 2.8V TO 4.5V
MAIN DISPLAY
SUB DISPLAY
RGB ILLUMINATOR
MAIN1-4 SUB1-2 SERIAL INTERFACE 3 RGB SERIAL PORT IMS IRGB
4 2 3
RED
GREEN BLUE
3205 TA01a
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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4-LED Main Panel Efficiency vs Input Voltage
100 90 80 70 60 50 40 30 20 10 FOUR LEDs AT 15mA/LED (TYP VF AT 15mA = 3.2V) TA = 25C 3.3 3.6 3.9 INPUT VOLTAGE (V) 4.2
3205 TA01b
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ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
GREEN MAIN3 MAIN4 SUB1 SUB2 BLUE
STEPUP
ENRGB
VIN, DVCC, CPO to GND............................. - 0.3V TO 6V DIN, SCLK, LD, STEPUP, ENRGB ...................................... - 0.3V to (DVCC + 0.3V) ICPO (Note 4)...................................................... 250mA IMAIN1-4, ISUB1,2 (Note 4) ..................................... 50mA IRED,GREEN,BLUE (Note 4) ..................................... 100mA IMS, IRGB (Note 4) .................................................. 1mA CPO Short-Circuit Duration ............................ Indefinite Operating Temperature Range (Note 2) .. - 40C to 85C Storage Temperature Range ................. - 65C to 125C
ORDER PART NUMBER
18 RED 17 SGND
24 23 22 21 20 19 MAIN2 1 MAIN1 2 C2- 3 C1- 4 C1+ 5 C2+ 6 7
DIN
LTC3205EUF
25
16 VIN 15 CPO 14 IMS 13 IRGB
8
SCLK
9 10 11 12
LD DVCC
UF PART MARKING 3205
UF PACKAGE 24-LEAD (4mm x 4mm) PLASTIC QFN
TJMAX = 150C, JA = 37C/W, JC = 2C/W EXPOSED PAD IS PGND (PIN 25) MUST BE SOLDERED TO PCB
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V, DVCC = 1.8V unless otherwise noted.
SYMBOL PARAMETER Input Power Supply VIN Operating Voltage DVCC Operating Voltage ICC Operating Current VIN Shutdown Current DVCC Shutdown Current White LED Current (MAIN1-MAIN4, SUB1, SUB2) IMS Servo Voltage Full-Scale LED Current Ratio (ILED/IMS) Half-Scale LED Current Ratio (ILED/IMS) LED Current Matching RGB LED Current (RED, GREEN, BLUE) IRGB Servo Voltage LED Current Ratio (ILED/IRGB) RGB PWM Frequency RGB LED Switching Frequency RGB PWM (Duty Factor) Range 0/15 3.5 15/15 kHz % 25A < IRGB < 75A
q q q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN 2.8 1.5
TYP
MAX 4.5 5.5
UNITS V V A mA
ICPO = 0, Step-Down Mode ICPO = 0, Step-Up Mode
50 4.2 1 1
25A < IMS < 75A
q
1.193 1.175 368 184 92
1.223 1.223 400 200 100 0.7
1.253 1.271 432 216 108
MAIN1-MAIN4, SUB1, SUB2 Voltage = 1V MAIN1-MAIN4, SUB1, SUB2 Voltage = 1V Any Two MAIN or SUB Outputs
q q q
mA/mA mA/mA mA/mA %
Quarter-Scale LED Current Ratio (ILED/IMS) MAIN1-MAIN4, SUB1, SUB2 Voltage = 1V
1.193 1.175 360
1.223 1.223 400
1.253 1.271 440
RED, GREEN, BLUE Voltage = 1V
mA/mA
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A A V V V V
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LTC3205
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V, DVCC = 1.8V unless otherwise noted.
SYMBOL PARAMETER Charge Pump (CPO) 1:1 Mode Output Impedance 2:3 Mode Output Impedance CPO Regulation Voltage CLK Frequency DIN, SCLK, LD, STEPUP, ENRGB VIL VIH IIH IIL tDS tDH tL tH tLW tCL tLC Low Level Input Voltage High Level Input Voltage Input Current Input Current DIN Valid to SCLK Setup DIN Valid to SCLK Hold SCLK Low Time SCLK High Time LD Pulse Width SCLK to LD LD to SCLK DIN, SCLK, LD, STEPUP, ENRGB = DVCC DIN, SCLK, LD, STEPUP, ENRGB = 0V
q q 0.85 * DVCC q q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
TYP 0.8
MAX
UNITS V
VIN = 3V, VCPO = 4.2V (Note 3) ICPO = 20mA, 2:3 Mode 0.6
2.5 4.7 0.8 1.1 0.15 * DVCC -1 -1 35 35 35 35 35 35 0 1 1
MHz V V A A ns ns ns ns ns ns ns
Serial Port Timing
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3205E is guaranteed to meet performance specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
Note 3: 2:3 mode output impedance is defined as (1.5VIN - VCPO)/IOUT. Note 4: Based on long term current density limitations.
TYPICAL PERFOR A CE CHARACTERISTICS
LED Pin Sink Current vs Voltage
VIN AC COUPLED (20mV/DIV) VOUT AC COUPLED (50mV/DIV)
ILED 3mA/DIV (100% SETTING)
EFFICIENCY (PLED/PIN) (%)
0mA 200mV/DIV
3205 G01
UW
Input and Output Charge Pump Noise
100 90 80 70 60 50 40 30 20 10
4-LED Main Panel Efficiency vs Input Voltage
ICPO = 150mA VIN = 3.6V CIN = CCPO = 1F
500ns/DIV
3205 G02
0 3.0
FOUR LEDs AT 15mA/LED (TYP VF AT 15mA = 3.2V) TA = 25C 3.3 3.6 3.9 INPUT VOLTAGE (V) 4.2
3205 TA01b
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LTC3205 TYPICAL PERFOR A CE CHARACTERISTICS
1:1 Mode Switch Resistance vs Temperature
1.0
3.2
ICPO = 100mA
OUTPUT RESISTANCE ()
VIN = 3.3V
SWITCH RESISTANCE ()
0.9 VIN = 3.6V 0.8 VIN = 3.9V
2.8 2.6 2.4 2.2
CPO VOLTAGE (V)
0.7
0.6 -40
-15
35 10 TEMPERATURE (C)
Oscillator Frequency vs Supply Voltage
1000 DVCC SHUTDOWN CURRENT (A) 0.5
900
FREQUENCY (kHz)
TA = 25C TA = -40C
0.4
VIN SHUTDOWN CURRENT (A)
800 TA = 85C 700
600 2.7 3.0 4.2 3.3 3.6 3.9 VIN SUPPLY VOLTAGE (V) 4.5
3205 G07
1:1 Mode No Load Supply Current vs VIN
90 TA = 25C IMS = IRGB = 0A
SUPPLY CURRENT (mA)
SUPPLY CURRENT (A)
80
70
60
50 2.7 3 3.3 3.6 3.9 INPUT VOLTAGE (V) 4.2 4.5
3205 G12
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UW
60
3205 G04
2:3 Mode Charge Pump OpenLoop Output Resistance vs Temperature (3/2VIN - VCPO)/ICPO
3.0 VIN = 3V VCPO = 4.2V CIN = CCPO = CFLY1 = CFLY2 = 1F
2:3 Mode CPO Voltage vs Load Current
4.8 4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 VIN = 3.1V VIN = 3.2V VIN = 3.3V VIN = 3.4V VIN = 3.0V 0 50 150 200 100 LOAD CURRENT (mA) 250
3205 G06
TA = 25C VIN = 3.6V VIN = 3.5V
85
2.0 -40
-15
10 35 TEMPERATURE (C)
60
85
3205 G05
3.8
DVCC Shutdown Current vs Input Voltage
1.0 VIN = DVCC 0.8
VIN Shutdown Current vs Input Voltage
DVCC = VIN
0.3
TA = 25C TA = -40C 0.6 TA = 85C 0.4
TA = 25C TA = -40C
0.2 TA = 85C 0.1
0.2
0
2.7
3.0
3.3 3.6 3.9 DVCC VOLTAGE (V)
4.2
4.5
3205 G08
0
2.7
3.0
4.2 3.3 3.6 3.9 VIN INPUT VOLTAGE (V)
4.5
3205 G09
2:3 Mode Supply Current vs ICPO (IIN - 3/2ICPO)
10 VIN = 3.6V 9 TA = 25C 8 7 6 5 4 3 2 1 0 0 100 50 150 LOAD CURRRENT (mA) 200
3205 G10
RGB LED Turn On and Off Characteristics
ILED 3mA/DIV
0mA 5s/DIV
3205 G011
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PI FU CTIO S
MAIN1-MAIN4 (Pins 2, 1, 24, 23): Current Source Outputs for the Main Display White LEDs. The current for the main display is controlled by the resistor on the IMS pin. The LEDs on the main display can be set to 100%, 50%, 25% or 0% of full-scale programmed current under software control. See Tables 1 and 2. C1+, C1-, C2+, C2- (Pins 5, 4, 6, 3): Charge Pump Flying Capacitor Pins. A 1F X7R or X5R ceramic capacitor should be connected from C1+ to C1- and from C2+ to C2-. DIN (Pin 7): Input Data for the 16-Bit Serial Port. Serial data is shifted in one bit per clock to control the LTC3205 (see Table 1). The logic level for DIN is referenced to DVCC. SCLK (Pin 8): Clock Input for the 16-Bit Serial Port (see Figure 3). The logic level for SCLK is referenced to DVCC. LD (Pin 9): Load Input for the 16-Bit Serial Port. Command data is loaded into the command latch on the falling edge of LD (see Figure 3). The logic level for LD is referenced to DVCC. ENRGB (Pin 10): This pin is used to enable and disable the red, green and blue current sources. Once ENRGB is brought high, the LTC3205 illuminates the RGB display with the color combination that was previously programmed via the serial port. When the main and sub displays are off and ENRGB is low, the LTC3205 will be in shutdown. The logic level for ENRGB is referenced to DVCC. STEPUP (Pin 11): A logic high on this pin forces the LTC3205's charge pump to operate in 2:3 step-up mode, thereby eliminating any possibility of the device switching from 1:1 mode to 2:3 mode during critical communication periods. The logic level for STEPUP is referenced to DVCC. DVCC (Pin 12): This pin sets the logic reference level of the LTC3205. IRGB (Pin 13): This pin controls the amount of LED current at the RED, GREEN and BLUE LED pins. The IRGB pin servos to 1.223V when there is a resistor to ground. The current in the RED, GREEN and BLUE LEDs will be 400 times the current at the IRGB pin when programmed to full scale (see Tables 1 and 3). IMS (Pin 14): This pin controls the maximum amount of LED current in both the main and sub LED displays. The IMS pin servos to 1.223V when there is a resistor to ground. The currents in the main and sub display LEDs will be 100, 200 or 400 times the current at the IMS pin depending on which setting is chosen from the serial port. CPO (Pin 15): Output of the Charge Pump. This output should be used to power white, blue and "true" green LEDs. Red LEDs can be powered from VIN or CPO. An X5R or X7R low impedance (ceramic) 1F charge storage capacitor is required on CPO. VIN (Pin 16): Supply Voltage for the Charge Pump. The VIN pin should be connected directly to the battery and bypassed with a 1F X5R or X7R ceramic capacitor. SGND (Pin 17): Ground for the Control Logic. This pin should be connected directly to a low impedance ground plane. RED, GREEN, BLUE (Pins 18, 19, 20): Current Source Outputs for the RGB Illuminator LEDs. The currents for the RGB LEDs are controlled by the resistor on the IRGB pin. The RGB LEDs can independently be set to any duty cycle from 0/15 through 15/15 under software control giving a total of 16 colors per LED and a total of 4096 colors for the illuminator. See Tables 1 and 3. The RGB LEDs are modulated at 1/240 the speed of the charge pump oscillator. SUB1, SUB2 (Pins 22, 21): Current Source Outputs for the Sub Display White LEDs. The current for the sub display is controlled by the resistor on the IMS pin. The LEDs on the sub display can be set to 100%, 50%, 25% or 0% of full scale under software control. See Tables 1 and 2. PGND (Pin 25, Exposed Pad): Power Ground for the Charge Pump. This pin should be connected directly to a low impedance ground plane.
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LTC3205
BLOCK DIAGRA
VIN 16 1:1 AND 2:3 CHARGE PUMP
IMS 14
IRGB 13 SGND 17
DVCC 12 STEPUP 11 ENRGB 10 LD 9 CONTROL LOGIC 2 2 COMMAND LATCH 16 DIN 7 SCLK 8 SHIFT REGISTER PWM 4 4 4
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C1+ 5 C1- 4 C2+ 6 C2- 3 800kHz OSCILLATOR 25 PGND 15 CPO
- +
ENABLECP
+
2 MAIN1
-
1 MAIN2 24 MAIN3 23 MAIN4
+ -
2 2 22 SUB1 21 SUB2
18 RED 19 GREEN 20 BLUE
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OPERATIO
Power Management To optimize efficiency, the power management section of the LTC3205 provides two methods of supplying power to the CPO pin: 1:1 direct connect mode or 2:3 boost mode. When either the main or sub displays of the LTC3205 are enabled, the power management system connects the CPO pin directly to VIN with a low impedance switch. If the voltage supplied at VIN is high enough to power all of the LEDs with the proper programmed current, the system will remain in this "direct connect" mode providing maximum efficiency. Internal circuits monitor all MAIN and SUB current sources for the onset of "dropout," the point at which the current sources can no longer supply programmed current. As the battery voltage falls, the LED with the largest forward voltage will reach the "drop out" threshold first. When any of the four main or two sub display LEDs reach the dropout threshold, the LTC3205 will switch to boost mode and automatically soft-start the 2:3 boost charge pump. The constant frequency charge pump is designed to minimize the amount of noise generated at the VIN supply. The 2:3 step-up charge pump uses a patented constant frequency architecture to combine the best efficiency with the maximum available power at the lowest noise level.
OUTPUT RESISTANCE ()
If the red, green or blue LEDs are programmed to be on at any duty cycle, the charge pump runs continuously. Soft-Start To prevent excessive inrush current and supply droop when switching into step-up mode, the LTC3205 employs a soft-start feature on its charge pump. The current available to the CPO pin is increased linearly over a period of 1.2ms. Charge Pump Strength When the LTC3205 operates in 2:3 boost mode, the charge pump can be modeled as a Thevenin-equivalent circuit to determine the amount of current available from the effective input voltage, 1.5VIN and the effective openloop output resistance, ROL (Figure 1). ROL is dependent on a number of factors including the switching term, 1/(2fOSC * CFLY), internal switch resistances and the nonoverlap period of the switching circuit.
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However, for a given ROL, the amount of current available will be directly proportional to the advantage voltage 1.5VIN - VCPO. Consider the example of driving white LEDs from a 3.1V supply. If the LED forward voltage is 3.8V and the current sources require 100mV, the advantage voltage is 3.1V * 1.5V - 3.8V - 0.1V or 750mV. Notice that if the input voltage is raised to 3.2V, the advantage voltage jumps to 900mV--a 20% improvement in available strength. From Figure 1, the available current is given by:
IOUT = 1.5VIN - VCPO ROL
Typical values of ROL as a function of temperature are shown in Figure 2.
ROL
+
CPO
+ -
1.5VIN
-
3205 F01
Figure 1. Equivalent Open-Loop Circuit
3.2 3.0 2.8 2.6 2.4 2.2 2.0 -40 VIN = 3V VCPO = 4.2V CIN = CCPO = CFLY1 = CFLY2 = 1F
-15
10 35 TEMPERATURE (C)
60
85
3205 F02
Figure 2. Typical ROL vs Temperature
Zero Shutdown Current Although the LTC3205 is designed to have very low shutdown current, it will draw about 400nA on VIN when in shutdown. For applications that require zero shutdown current, the DVCC pin can be grounded. This will reduce the VIN current to very near zero. Internal logic ensures that the
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LTC3205
OPERATIO
LTC3205 is in shutdown when DVCC is grounded. Note, however, that all of the logic signals that are referenced to DVCC (DIN, SCLK, LD, ENRGB and STEPUP) will need to be at DVCC or below (i.e., ground) to keep from violating the absolute maximum specifications on these pins. Serial Port The microcontroller compatible serial port provides all of the command and control inputs for the LTC3205. Data on the DIN input is loaded on the rising edge of SCLK. D15 is loaded first and D0 last. Once all bits have been clocked into the shift register, the command data is loaded into the command register by bringing LD low. At this time, the command register is latched and the LTC3205 will begin to act upon the new command set. The serial port uses static logic registers so there is no minimum speed at which it can be operated. Figure 3 shows the operation of the serial port. Table 1 shows the mapping of the serial port bits to the operation of the various displays. Bits D15 and D14 control the brightness of the four LEDs in the main display. Bits D13 and D12 control the brightness of the two LEDs in the sub display. The red, green and blue LEDs each have four bits assigned giving a linear range of 16 brightness levels to each of the LEDs. Programming the MAIN and SUB LED Currents Table 2 indicates the decoding of the Main and Sub display control bits.
tLC tDS tDH tH
Table 1. Serial Port Mapping
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 MAIN1MAIN4 Current Level (Table 2) SUB1SUB2 Current Level (Table 2) Blue LED Duty Cycle (Table 3) Green LED Duty Cycle (Table 3) Red LED Duty Cycle (Table 3)
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The current levels of both the main and sub displays are controlled by precisely mirroring a multiple of the current at the IMS pin. The main and sub display LED currents will follow the relationship:
IMAIN/SUB = N
1.223V RMAIN/SUB
where N is equal to 400, 200, 100 or 0 depending on which current setting is selected. RMAIN/SUB is the value of the resistor on the IMS pin. The scale factors are spaced pseudo logarithmically to match the vision perception of the human eye (zero is a special case needed for shutdown). The LTC3205 can power up to six white LEDs (four for the main display, two for the sub display), however, it is not necessary to have all six in each application. Any of the four main or two sub LED outputs can be disabled by connecting the unused output to CPO.
Table 2. Main and Sub Display Current Levels
D15 D13 0 0 1 1
tL tCL
D14 D12 0 1 0 1
tLW
FRACTION OF FULL-SCALE CURRENT (%) 0 25 50 100
SCLK
DIN
X
D15
D14
D2
D1
D0
X
LD
3205 F03
Figure 3. Serial Port Timing Diagram
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OPERATIO
Unused MAIN or SUB Display LED Pins Any of the six white LED pins (MAIN1-MAIN4, SUB1 and SUB2) can cause the LTC3205 to switch from 1:1 mode to 2:3 charge pump mode if they drop out. If an unused LED pin is left unconnected or grounded, it will automatically drop out and force the LTC3205 into charge pump mode. To avoid this problem, unused LED pins on the MAIN and SUB displays should be connected to CPO. However, power is not wasted in this configuration. When the LED pin voltage is within approximately 1V of CPO, its LED current is switched off and only a small 20A test current remains. Figure 4 shows a block diagram of each of the MAIN and SUB LED pins. The RED, GREEN and BLUE pins do not affect the state of the charge pump so they can be left floating or grounded if unused.
CPO 1V
Figure 4. Internal MAIN and SUB Panel LED Disable Circuit
RGB Illuminator Drivers The red, green and blue LEDs can be individually set to have a duty cycle ranging from 0/15 (off) to 15/15 (full on) with 1/15 increments in between. The combination of 16 possible brightness levels gives the RGB indicator LED a
APPLICATIO S I FOR ATIO
Interfacing to a Microcontroller
The serial port of the LTC3205 can be connected directly to an MC68HC11 style microcontroller's serial port. The microcontroller should be configured as the master device and its clock's idle state should be set to high (MSTR = 1, CPOL = 1 and CPHA = 1 for the MC68HC11 family). Figure 5 shows the recommended configuration and directon of data flow. Note that an additional I/O line is
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Table 3. RGB Duty Cycles
D11 D7 D3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 D10 D6 D2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 D9 D5 D1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D8 D4 D0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 HEX CODE 0 1 2 3 4 5 6 7 8 9 A B C D E F DUTY CYCLE (%) 0/15 1/15 2/15 3/15 4/15 5/15 6/15 7/15 8/15 9/15 10/15 11/15 12/15 13/15 14/15 15/15
+ -
ENABLE ILED
-+
MAIN1-MAIN4 SUB1, SUB2
total of 4096 colors. Table 3 indicates the decoding of the red, green and blue LEDs.
20A
3205 F04
The full-scale currents in the red, green and blue LEDs are controlled by the current at the IRGB pin in a similar manner to those in the main and sub panels. The IRGB pin is regulated at 1.223V and the LED current is a precise multiple of the IRGB current. The RGB panel LED currents will follow the relationship:
1.223V RRGB where RRGB is the value of the resistor on the IRGB pin. IRED,GREENBLUE = 400 ,
CONTROLLER MOSI SCK GPIO
LTC3205 DIN SCLK LD
3205 F05
Figure 5. Microcontroller Interface
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LTC3205
APPLICATIO S I FOR ATIO
necessary for LD to load the data once it has shifted into the device. Command data is latched into the command register on the falling edge of the LD signal. The LTC3205 will begin to act on new command data as soon as LD goes low. Any general purpose microcontroller I/O line can be configured to control the LD pin if the microcontroller doesn't provide this feature automatically. VIN, CPO Capacitor Selection The style and value of capacitors used with the LTC3205 determine several important parameters such as regulator control-loop stability, output ripple and charge pump strength. To reduce noise and ripple, it is recommended that low equivalent series resistance (ESR) multilayer ceramic capacitors be used on both VIN and CPO. Tantalum and aluminum capacitors are not recommended because of their high ESR. The value of the capacitor on CPO directly controls the amount of output ripple for a given load current. Increasing the size of this capacitor will reduce the output ripple. The peak-to-peak output ripple is approximately given by the expression:
VRIPPLEP-P
IOUT 3fOSC * COUT
where fOSC is the LTC3205's oscillator frequency (typically 1MHz) and COUT is the output charge storage capacitor on CPO. Both the style and value of the output capacitor can significantly affect the stability of the LTC3205. The LTC3205 uses a linear control loop to adjust the strength of the charge pump to match the current required at the output. The error signal of this loop is stored directly on the output charge storage capacitor. The charge storage capacitor also serves to form the dominant pole for the control loop. To prevent ringing or instability, it is important for the output capacitor to maintain at least 0.6F of capacitance over all conditions. Likewise, excessive ESR on the output capacitor will tend to degrade the loop stability of the LTC3205. The closed-loop output resistance of the LTC3205 is designed to be 0.6. For a 100mA load current change, the error signal will change by about 60mV. If the output capacitor has 0.6 or more of ESR, the closed-loop frequency response will cease to roll off in a simple one-pole fashion and poor load transient response or instability could result. Multilayer ceramic chip
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capacitors typically have exceptional ESR performance. MLCCs combined with a tight board layout will yield very good stability. As the value of COUT controls the amount of output ripple, the value of CIN controls the amount of ripple present at the input pin (VIN). The input current to the LTC3205 will be relatively constant while the charge pump is on either the input charging phase or the output charging phase but will drop to zero during the clock nonoverlap times. Since the nonoverlap time is small (~25ns), these missing "notches" will result in only a small perturbation on the input power supply line. Note that a higher ESR capacitor such as tantalum will have higher input noise due to the input current change times the ESR. Therefore, ceramic capacitors are again recommended for their exceptional ESR performance. Input noise can be further reduced by powering the LTC3205 through a very small series inductor as shown in Figure 6. A 10nH inductor will reject the fast current notches, thereby presenting a nearly constant current load to the input power supply. For economy, the 10nH inductor can be fabricated on the PC board with about 1cm (0.4") of PC board trace.
10nH VIN VIN 0.1F 1F LTC3205 GND
3205 F06
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Figure 6. 10nH Inductor Used for Input Noise Reduction (Approximately 1cm of Wire)
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or aluminum should never be used for the flying capacitors since their voltage can reverse upon start-up of the LTC3205. Ceramic capacitors should always be used for the flying capacitors.
The flying capacitor controls the strength of the charge pump. In order to achieve the rated output current it is necessary to have at least 0.7F of capacitance for each of the flying capacitors. Capacitors of different materials lose their capacitance with higher temperature and voltage at different rates. For example, a ceramic capacitor made of X7R material will retain most of its capacitance from -40C to 85C whereas a Z5U or Y5V style capacitor will
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APPLICATIO S I FOR ATIO
lose considerable capacitance over that range. Z5U and Y5V capacitors may also have a very poor voltage coefficient causing them to lose 60% or more of their capacitance when the rated voltage is applied. Therefore, when comparing different capacitors, it is often more appropriate to compare the amount of achievable capacitance for a given case size rather than comparing the specified capacitance value. For example, over rated voltage and temperature conditions, a 1F, 10V, Y5V ceramic capacitor in a 0603 case may not provide any more capacitance than a 0.22F, 10V, X7R available in the same 0603 case. The capacitor manufacturer's data sheet should be consulted to determine what value of capacitor is needed to ensure minimum capacitances at all temperatures and voltages. Table 4 shows a list of ceramic capacitor manufacturers and how to contact them:
Table 4. Recommended Capacitor Vendors
AVX Kemet Murata Taiyo Yuden Vishay www.avxcorp.com www.kemet.com www.murata.com www.t-yuden.com www.vishay.com
For very light load applications, the flying capacitors may be reduced to save space or cost. The theoretical minimum output resistance of a 2:3 fractional charge pump is given by:
ROL(MIN) 1.5VIN - VOUT 1 = IOUT 2fOSCCFLY
where fOSC is the switching frequency (800kHz typ) and CFLY is the value of the flying capacitors. Note that the
PIN 1
Figure 7. Optimum Single Layer PCB Layout
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charge pump will typically be weaker than the theoretical limit due to additional switch resistance, however for very light load applications, the above expression can be used as a guideline in determining a starting capacitor value. Layout Considerations and Noise Due to its high switching frequency and the transient currents produced by the LTC3205, careful board layout is necessary. A true ground plane and short connections to all capacitors will improve performance and ensure proper regulation under all conditions. Figure 7 shows the recommended layout configuration. The flying capacitor pins C1+, C2+, C1- and C2- will have very high edge rate waveforms. The large dv/dt on these pins can couple energy capacitively to adjacent printed circuit board runs. Magnetic fields can also be generated if the flying capacitors are not close to the LTC3205 (i.e., the loop area is large). To decouple capacitive energy transfer, a Faraday shield may be used. This is a grounded PC trace between the sensitive node and the LTC3205 pins. For a high quality AC ground, it should be returned to a solid ground plane that extends all the way to the LTC3205 Power Efficiency To calculate the power efficiency () of a white LED driver chip, the LED power should be compared to the input power. The difference between these two number represents lost power whether it is in the charge pump or the current sources. Stated mathematically, the power efficiency is given by:
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PLED PIN
GND VIN CPO
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LTC3205
APPLICATIO S I FOR ATIO
The efficiency of the LTC3205 depends upon the mode in which it is operating. Recall that the LTC3205 operates as a pass switch, connecting VIN to CPO until one of the LEDs on the main or sub displays drops out. This feature provides the optimum efficiency available for a given input voltage and LED forward voltage. When it is operating as a switch, the efficiency is approximated by:
PLED VLED * ILED VLED = PIN VIN * IIN VIN
since the input current will be very close to the LED current. At moderate to high output power, the quiescent current of the LTC3205 is negligible and the expression above is valid. For example, with VIN = 3.9V, IOUT = 20mA * 6 LEDs and VLED equal to 3.6V, the measured efficiency is 92.2%, which is very close to the theoretical 92.3% calculation. Once an LED drops out, the LTC3205 switches into stepup mode. Employing the fractional ratio 2:3 charge pump, the LTC3205 provides more efficiency than would be achieved with a voltage doubling charge pump. In 2:3 boost mode, the efficiency is similar to that of a linear regulator with an effective input voltage of 1.5 times the actual input voltage. This is because the input current for a 2:3 fractional charge pump is approximately 1.5 times the load current. In an ideal 2:3 charge pump, the power efficiency would be given by: P V *I V IDEAL LED = LED LED LED 3 PIN V * I 1.5VIN IN LED 2 Thermal Management For higher input voltages and maximum output current, there can be substantial power dissipation in the LTC3205. If the junction temperature increases above approximately 160C the thermal shutdown circuitry will automatically deactivate the output. To reduce the maximum junction temperature, a good thermal connection to the PC board is recommended. Connecting the PGND pin (exposed center pad) to a ground plane and maintaining a solid ground plane under the device can reduce the thermal resistance of the package and PC board considerably.
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Brightness Control Although the LTC3205 has three logarithmically spaced brightness settings for the main and sub displays, it is possible to control the brightness by alternative means. Figure 8 shows an example of how an external voltage source can be use to inject a current into the IMS or IRGB pins to control brightness. For example, if R1 and R2 are 50k, then the LED current would range from 20mA to 0mA as VCNTRL is swept from 0V to 2.5V.
V ILED = 400 1.223V - CNTRL R2 R1||R2 LTC3205 IMS IRGB 14 13 24.9k
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(
)
VCNTRL
R2 R1
Figure 8. Alternative Linear Brightness Control
Alternatively, if only digital outputs are available, the number of settings can be doubled from 3 to 6 by simply connecting VCNTRL to a digital signal. With a 1.8V logic supply, the circuit shown in Figure 9 has LED current settings of 2.5mA, 5mA, 7.5mA, 10mA, 15mA and 20mA. This topology can be extended to any number of bits and can also be applied to the RGB panel.
LTC3205 IMS IRGB
14 13 24.9k
71.5k 38.3k
VDIG 0V TO 1.3V OR HIGHER
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Figure 9. Alternative Digital Brightness Control
Finally, PWM brightness control can be achieved by applying a PWM signal to the IMS programming resistor as shown in Figure 10. The signal should range from 0V (full on) to any voltage above 1.3V (full off).
LTC3205 IMS IRGB 14 24.9k 13 24.9k PWM SIGNAL 0V TO 1.3V OR HIGHER BRIGHTNESS = 1 - D
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Figure 10. PWM Brightness Control of the MAIN and SUB Displays
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LTC3205
TYPICAL APPLICATIO S
Ultralow Brightness MAIN and SUB Displays
All Charge Pump Main, Sub, RGB and Camera Light Controller
SUB PANEL (DUTY CYCLE = 50%)
CPO LTC3205 MAIN1 MAIN2 MAIN3 MAIN4 SUB1 SUB2 RED GREEN BLUE IMS 14
15 1F 2 1 24 23 22 21 18 19 20 WHITE WHITE WHITE WHITE ILED = 40mA WHITE WHITE ILED = 20mA RED GREEN BLUE
IRGB 13 24.9k
12.4k 4 1F 10
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LTC3205 IMS IRGB
14 13 24.9k
487k
PWM SIGNAL 0V TO 1.3V OR HIGHER 50Hz TO 15kHz BRIGHTNESS = 1 - D
24.9k BRIGHT DIM
3205 TA08
CAMERA LIGHT
ILLUMINATOR
1F 8 VIN 79
1F 6 VOUT 3 1F 2 WHITE 30 WHITE 30 WHITE 30 WHITE 30
3205 TA02
C1+ C1- C2+ C2- LTC3202 D0 FB
MAIN PANEL
1
D1
GND
5, 11
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LTC3205
TYPICAL APPLICATIO S
Main, Sub and Keypad Illumination
MAIN PANEL CPO LTC3205 MAIN1 MAIN2 MAIN3 MAIN4 SUB1 SUB2 RED GREEN BLUE IMS 14 24.9k 2 1 24 23 22 21 18 19 20 WHITE WHITE WHITE WHITE WHITE WHITE BLUE 39 BLUE 39 BLUE 39 BLUE 39 BLUE 39 BLUE 39 15 1F SUB PANEL KEYPAD
IRGB 13 16.6k
High Efficiency 20mA x 6-LED Camera Light
CPO LTC3205 MAIN1 MAIN2 MAIN3 MAIN4 SUB1 SUB2 2 1 24 23 22 21 WHITE WHITE WHITE WHITE WHITE WHITE 15 1F
100 90
PLED/PVIN EFFICIENCY (%)
IMS 14 24.9k
3205 TA04a
CPO LTC3205 MAIN1 MAIN2 MAIN3 MAIN4 SUB1 SUB2 RED GREEN IRGB 13 24.9k IMS 14
15 1F 2 1 24 23 22 21 18 19 WHITE WHITE WHITE WHITE WHITE WHITE WHITE WHITE
24.9k
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3205 TA03
Efficiency vs Input Voltage
VLED = 3.3V 6 x 20mA
80 70 60 50 40 30 20 10 0 3.0 3.3 3.6 VIN (V) 3.9 4.2
3205 TA04b
8-LED Standalone Camera Light
3205 TA05
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LTC3205
TYPICAL APPLICATIO S
4-LED 160mA Standalone Camera Light
160mA CAMERA LIGHT CPO LTC3205 MAIN1 MAIN2 MAIN3 MAIN4 SUB1 SUB2 RED GREEN BLUE IMS 14 24.9k 2 1 24 23 22 21 18 19 20 WHITE WHITE WHITE WHITE 15 1F
IRGB 13 24.9k
PACKAGE DESCRIPTIO
4.50 0.05 2.45 0.05 3.10 0.05 (4 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05
PIN 1 TOP MARK (NOTE 5)
NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)--TO BE APPROVED 2. ALL DIMENSIONS ARE IN MILLIMETERS 3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT 4. EXPOSED PAD SHALL BE SOLDER PLATED 5. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 6. DRAWING NOT TO SCALE
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3205 TA06
UF Package 24-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1692)
0.70 0.05
PACKAGE OUTLINE 0.25 0.05 0.50 BSC BOTTOM VIEW--EXPOSED PAD 0.23 TYP (4 SIDES) 23 24 0.38 0.10 1 2 2.45 0.10 (4-SIDES)
4.00 0.10 (4 SIDES)
R = 0.115 TYP
(UF24) QFN 0603
0.200 REF 0.00 - 0.05
0.25 0.05 0.50 BSC
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LTC3205
TYPICAL APPLICATIO
Li-Ion 4 1F 1F 1F 2.2F
5
4
6
3
C1+ C1-
C2 + C2 -
10 12 FROM MICROCONTROLLER 7 8 9
ENRGB DVCC DIN SCLK LD LTC3205
SGND STEPUP IMS 17 11 14
24.9k
RELATED PARTS
PART NUMBER LT(R)1618 LTC1911-1.5 LT1932 LT1937 LTC3200-5 LTC3201 LTC3202 LTC3251 LTC3405/LTC3405A LTC3406/LTC3406B LTC3440 LT3465/LT3465A DESCRIPTION Constant Current, Constant Voltage, 1.4MHz High Efficiency Boost Regulator 250mA (IOUT), 1.5MHz High Efficiency Step-Down Charge Pump Constant Current, 1.2MHz High Efficiency White LED Boost Regulator Constant Current, 1.2MHz High Efficiency White LED Boost Regulator Low Noise, 2MHz Regulated Charge Pump White LED Driver Low Noise, 1.7MHz Regulated Charge Pump White LED Driver Low Noise, 1.5MHz Regulated Charge Pump White LED Driver 500mA (IOUT), 1MHz to 1.6MHz Spread Spectrum Step-Down Charge Pump 300mA (IOUT), 1.5MHz Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz Synchronous Step-Down DC/DC Converter 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 1.2MHz/2.7MHz with Internal Schottky COMMENTS Up to 16 White LEDs, VIN: 1.6V to 18V, VOUT(MAX): 34V, IQ: 1.8mA, ISD: 1A, 10-Lead MS 75% Efficiency, VIN: 2.7V to 5.5V, VOUT(MIN): 1.5V/1.8V, IQ: 180A, ISD: 10A, MS8 Up to 8 White LEDs, VIN: 1V to 10V, VOUT(MAX): 34V, IQ: 1.2mA, ISD: 1A, ThinSOTTM Up to 4 White LEDs, VIN: 2.5V to 10V, VOUT(MAX): 34V, IQ: 1.9mA, ISD: 1A, ThinSOT, SC70 Up to 6 White LEDs, VIN: 2.7V to 4.5V, VOUT(MAX): 5V, IQ: 8mA, ISD: 1A, ThinSOT Up to 6 White LEDs, VIN: 2.7V to 4.5V, VOUT(MAX): 5V, IQ: 6.5mA, ISD: 1A, 10-Lead MS Up to 8 White LEDs, VIN: 2.7V to 4.5V, VOUT(MAX): 5V, IQ: 5mA, ISD: 1A, 10-Lead MS 85% Efficiency, VIN: 3.1V to 5.5V, VOUT(MIN): 0.9V to 1.6V, IQ: 9A, ISD: 1A, 10-Lead MS 95% Efficiency, VIN: 2.7V to 6V, VOUT(MIN): 0.8V, IQ: 20A, ISD: 1A, ThinSOT 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN): 0.6V, IQ: 20A, ISD: 1A, ThinSOT 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN): 2.5V, IQ: 25A, ISD: 1A, 10-Lead MS Up to 6 White LEDs, VIN: 12.7V to 16V, VOUT(MAX): 34V, IQ: 1.9mA, ISD: <1A, ThinSOT
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ThinSOT is a trademark of Linear Technology Corporation.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
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Using the RGB Display as a Camera Light
L1 2.2H 5 VIN SHDN LT1930A GND 16 VIN CPO 15 1F MAIN1 MAIN2 MAIN3 MAIN4 SUB1 SUB2 RED GREEN BLUE IRGB 13 24.9k Si1406DH 6.2k IFLASH = 100mA PER R, G, B, LED FLASH 2 1 24 23 22 21 18 19 20
3205 TA07
D1 1 SW FB
VRGB = 6V IRGB = 300mA TOTAL 38.3k
3 10k
2
10F
WHITE
WHITE
WHITE
WHITE
WHITE
WHITE ILED = 20mA
RED
GREEN
BLUE
LT/TP 0803 1K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2003

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