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LT3486

3486fe

TYPICAL APPLICATION

DESCRIPTION

Dual 1.3A White LED

Step-Up Converters with

Wide Dimming

The LT

3486 is a dual step-up DC/DC converter

specifically designed to drive up to 16 White LEDs (8 in

series per converter) at constant current from a single

Li-Ion cell. Series connection of the LEDs provides identi-

cal LED currents resulting in uniform brightness. The two

independent converters are capable of driving asymmetric

LED strings.

The dimming of the two LED strings can be controlled

independently via the respective CTRL pins. An internal

dimming system allows the dimming range to be extended

up to 1000:1 by feeding a PWM signal to the respective

PWM pins. The LT3486 operating frequency can be set with

an external resistor over a 200kHz to 2.5MHz range. A low

200mV feedback voltage (±3% accuracy) minimizes power

loss in the current setting resistor for better efficiency.

Additional features include output voltage limiting when

LEDs are disconnected and overtemperature protection.

The LT3486 is available in a space saving 16-pin DFN

(5mm × 3mm × 0.75mm) and 16-pin thermally enhanced

TSSOP packages.

FEATURES

APPLICATIONS

Wide (1000:1) PWM Dimming Range with No

ColorShift

Independent Dimming and Shutdown Control of the

LED Drivers

Drives Up to 16 White LEDs at 25mA (8 per Driver)

from a Single Li-Ion Cell

Drives Up to 16 White LEDs at 100mA (8 per Driver)

from 12V Supply

±3% LED Current Programming Accuracy

Open LED Protection: 36V Clamp Voltage

Fixed Frequency Operation: Up to 2.5MHz

Wide Input Voltage Range: 2.5V to 24V

Low Shutdown Current: ICC < 1µA

Overtemperature Protection

Available in (5mm × 3mm × 0.75mm) 16-Pin DFN

and 16-Pin Thermally Enhanced TSSOP Packages

Notebook PC Display

LED Camera Light for Cell Phones

Car Dashboard Lighting

Avionics Displays

Li-Ion Powered Driver for Camera Flash and LCD Backlighting

3V TO 4.2V

10µF

10µH

0.1µF

2.2µF

63.4k

2.8k

4.7nF

FB2

8.06Ω

FB1

0.62Ω

2.2µF

0.1µF

3486 TA01a

100k

LED1

AOT3218

25mA

8 LEDs

OFF ON

DIMMING 2

GND

SW1 SW2

LT3486

OVP2

CTRL2

REF

PWM2

FB2

OVP1

CTRL1

SHDN

PWM1

FB1

DIMMING 1

Efficiency vs V

(V)

EFFICIENCY (%)

3.2

3.4 3.6 3.8

3486 TA01b

4 4.2

MOVIE MODE

LED1

= 175mA

FLASH MODE

LED1

= 320mA

8 LEDS/25mA

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and

ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property

of their respective owners.

Summary of content (20 pages)

PAGE 1
LT3486 Dual 1.3A White LED Step-Up Converters with Wide Dimming DESCRIPTION FEATURES n n n n n n n n n n n Wide (1000:1) PWM Dimming Range with No ColorShift Independent Dimming and Shutdown Control of the LED Drivers Drives Up to 16 White LEDs at 25mA (8 per Driver) from a Single Li-Ion Cell Drives Up to 16 White LEDs at 100mA (8 per Driver) from 12V Supply ±3% LED Current Programming Accuracy Open LED Protection: 36V Clamp Voltage Fixed Frequency Operation: Up to 2.5MHz Wide Input Voltage Range: 2.
PAGE 2
LT3486 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Voltage (VIN).....................................................25V SHDN Voltage............................................................25V SW1, SW2 Voltages ..................................................40V OVP1, OVP2 Voltages................................................40V CTRL1, CTRL2 Voltages.............................................10V PWM1, PWM2 Voltages.............................................10V FB1, FB2 Voltages...................
PAGE 3
LT3486 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, VCTRL1 = 3V, VCTRL2 = 3V, VPWM1 = 3V, VPWM2 = 3V, VSHDN = 3V, unless otherwise noted. PARAMETER CONDITIONS MIN Minimum Operating Voltage TYP MAX UNITS 2.
PAGE 4
LT3486 TYPICAL PERFORMANCE CHARACTERISTICS Switching Waveforms PWM Dimming Wavforms ILED 200mA/DIV VSW2 50V/DIV IL2 500mA/DIV IL 500mA/DIV VSW1 10V/DIV IL1 1A/DIV PWM 5V/DIV LED Current vs PWM Duty Cycle Wide Dimming Range (1000:1) 250 1 0.1 1 10 0.1 PWM DUTY CYCLE (%) ± 5mV 200 150 100 50 0 100 250 VIN = 3.6V TA = 25°C FEEDBACK VOLTAGE (mV) FEEDBACK VOLTAGE (mV) 1 0.5 1.5 CONTROL VOLTAGE (V) 0 3486 G01 37 VIN = 3.
PAGE 5
LT3486 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C RT = 63.4k 1000 RT vs Oscillator Frequency Oscillator Frequency vs VIN 1100 15 RT (kΩ) INPUT CURRENT (mA) 20 OSCILLATOR FREQUENCY (kHz) Input Current with Output 1 and Output 2 Open Circuit TA = 25°C unless otherwise specified. 10 100 5 0 4 2 6 10 8 10 12 14 16 18 20 22 24 VIN (V) 0 500 1000 1500 2000 OSCILLATOR FREQUENCY (kHz) Oscillator Frequency vs Temperature 12 RT = 309k 100 –50 –25 100 6 4 VIN = 3.
PAGE 6
LT3486 PIN FUNCTIONS SW1, SW2 (Pins 1, 16): The SW Pins are the Collectors of the Internal Power Transistors. Connect the inductors and Schottky diodes to these pins. Minimize trace area at these pins to minimize EMI. VIN (Pin 2): Input Supply Pin. Must be locally bypassed with an X5R or X7R type ceramic capacitor. OVP1, OVP2 (Pins 3, 14): Output Overvoltage Protection Pins. Connect these pins to the output capacitors.
PAGE 7
LT3486 BLOCK DIAGRAM SW1 1 RT VIN 4 2 SW2 16 14 OVP2 OVP1 3 OVERVOLT DETECTION OV1 EN1 CONVERTER1 OVERVOLT DETECTION CONVERTER2 OSC Q1 OSC A3 – + + – VC1 5 0.2V + – + EN1 8 REF 1.25V 0.
PAGE 8
LT3486 OPERATION Main Control Loop The LT3486 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. It incorporates two identical, but fully independent PWM converters. Operation can be best understood by referring to the block diagram in Figure 1. The oscillator, start-up bias and the bandgap reference are shared between the two converters. The control circuitry, power switch, dimming control etc., are all identical for both converters.
PAGE 9
LT3486 OPERATION output open circuit are shown in the Typical Performance Characteristics graphs. Figure 3a shows the transient response of switcher 1 with the LEDs disconnected from the output. When the LED1 string is disconnected from the output, the voltage at the feedback pin (FB1) drops to 0V. As a result, the error amplifier charges up the VC node to the clamp voltage level of 1.5V (typ). The converter starts switching at peak current limit and ramps up the output voltage.
PAGE 10
LT3486 APPLICATIONS INFORMATION Duty Cycle Operating Frequency Selection The duty cycle for a step-up converter is given by: The choice of operating frequency is determined by several factors. There is a trade-off between efficiency and component size. Higher switching frequency allows the use of smaller inductors albeit at the cost of increased switching losses and decreased efficiency.
PAGE 11
LT3486 APPLICATIONS INFORMATION 90 Several inductors that work well with the LT3486 are listed in Table 1. Consult each manufacturer for more detailed information and for their entire selection of related parts. VIN = 5V 8/8 LEDs 80 EFFICIENCY (%) RT = 63.4k 70 Table 1. Recommended Inductors RT = 21.5k 40 0 5 10 15 LED CURRENT (mA) 25 20 3486 F06b Figure 6b.
PAGE 12
LT3486 APPLICATIONS INFORMATION The average forward current in normal operation is equal to the output current, and the peak current is equal to the peak inductor current. A Schottky diode rated at 1A is sufficient for most LT3486 applications. Some recommended Schottky diodes are listed in Table 3. Table 3. Recommended Schottky Diodes PART NUMBER 200mV ILED1 200mV RFB2 = ILED2 RFB1 = Table 4. RFB Value Selection VR (V) IAVG (A) MANUFACTURER ILED (mA) RFB (Ω) MBR0530 MBRM120E 30 20 0.
PAGE 13
LT3486 APPLICATIONS INFORMATION The LED current can be set by: Pulse-Width Modulation (PWM) ILED ≈ (200mV/RFB), when VCTRL > 1.8V Adjusting the forward current flowing in the LEDs changes the intensity of the LEDs, as explained in the previous section. However, a change in forward current also changes the color of the LEDs. The chromaticity of the LEDs changes with the change in forward current. Many applications cannot tolerate any shift in the color of the LEDs.
PAGE 14
LT3486 APPLICATIONS INFORMATION the figure. A PWM logic input is applied to the gate of the NMOS and the PWM pin of the LT3486. When the PWM input is taken high, the LEDs are connected to the RFB resistor and a current ILED = 200mV/RFB flows through the LEDs. When the PWM input is taken low, the LEDs are disconnected and turn off. The low PWM input applied to the LT3486 ensures that the respective converter turns off and its VC pin goes high impedance.
PAGE 15
LT3486 TYPICAL APPLICATIONS Li-Ion Cell Powered Driver for Camera Flash and LCD Backlighting VIN 3V TO 5V CIN 10µF D1 D2 L1 10µH COUT1 2.2µF LED1 AOT3218 SW1 320mA DIMMING 1 OFF ON L2 10µH SW2 VIN OVP1 OVP2 CTRL1 CTRL2 SHDN LT3486 OFF ON RT VC1 Q1 REF 2.8k 4.7nF 0.1µF RFB2 8.06Ω 3486 TA02a CIN: 6.
PAGE 16
LT3486 TYPICAL APPLICATIONS 1 Li-Ion Cell to 8/8 White LEDs 3V TO 5V CIN 10µF D1 D2 L1 10µH COUT1 2.2µF SW1 8 LEDs 25mA L2 10µH VIN OVP2 VIN CTRL1 CTRL2 OFF ON SHDN LT3486 2.8k 100k 4.7nF REF CREF 0.1µF 25mA VC2 2.8k 63.4k COUT1, COUT2: 35V, X5R OR X7R CIN: 10V, X5R OR X7R 8.06Ω VIN FB2 RT VC1 Q1 8 LEDs PWM2 FB1 PWM1 100Hz SW2 OVP1 PWM1 5V COUT2 2.2µF 4.7nF PWM2 100Hz Q2 D1, D2: ZETEX ZLLS400 L1, L2: TOKO D53LC (TYPE A) Q1, Q2: FAIRCHILD 2N7002 8.
PAGE 17
LT3486 TYPICAL APPLICATIONS 5V to 16/16 White LEDs 5V D5 C3 1µF CIN 1µF D3 16 LEDs L1 15µH C1 0.1µF L2 15µH SW2 VIN OVP1 OVP2 VIN CTRL1 CTRL2 OFF ON SHDN REF LT3486 100k CREF 0.1µF 63.4k 4.7nF 8.06Ω CIN: 6.3V, X5R OR X7R COUT1, COUT2: 35V, X5R OR X7R C1-C4: 50V, X5R OR X7R CREF: 6.3V, X5R OR X7R VC2 22pF 4.02k 4.
PAGE 18
LT3486 PACKAGE DESCRIPTION DHC Package 16-Lead Plastic DFN (5mm × 3mm) (Reference LTC DWG # 05-08-1706) 5.00 ±0.10 (2 SIDES) R = 0.20 TYP 0.65 ±0.05 3.50 ±0.05 1.65 ±0.05 (2 SIDES) 3.00 ±0.10 (2 SIDES) PACKAGE OUTLINE 2.20 ±0.05 R = 0.115 TYP 9 0.40 ± 0.10 16 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) PIN 1 NOTCH 0.75 ±0.05 0.200 REF 0.25 ± 0.05 0.50 BSC 4.40 ±0.05 (2 SIDES) 0.00 – 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1.
PAGE 19
LT3486 REVISION HISTORY (Revision history begins at Rev D) REV DATE DESCRIPTION D 03/10 Corrected the Part Number in Description Section and Order Information PAGE NUMBER E 01/11 1, 2 Updated Typical Value for Switching Frequency Parameter in Electrical Characteristics 3 Updated FE package drawing 18 3486fe Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 20
LT3486 TYPICAL APPLICATION 12V to 8/8 White LEDs 12V (TYP) 9V TO 15V L1 10µH COUT1 2.2µF CIN 10µF L2 10µH LED Current and Efficiency vs PWM Duty Cycle 5V D1 COUT2 2.2µF D2 C1 1µF 90 120 EFFICIENCY 100mA SW2 VIN OVP1 OVP2 VIN CTRL1 CTRL2 OFF ON SHDN REF LT3486 RT VC1 PWM FREQ 1kHz 100k 2.2nF 100mA 22pF 21.5k RFB1 COUT1, COUT2: 35V, X5R OR X7R 2Ω CIN: 25V, X5R OR X7R C1: 10V, X5R OR X7R CREF: 6.3V, X5R OR X7R DIMMING INPUT 2 PWM FREQ? 1kHz 100k 3.65k 2.