DESIGN FEATURES L White LED Driver and OLED Driver with Integrated Schottkys and Output Disconnect in 3mm × 2mm DFN by Alan Wei Introduction Linear Technology Magazine • October 2007 VIN 3V TO 5V 4.7µF 15µH 0.47µF 15µH 16V 24mA 1µF CAP1 SW1 10Ω VIN SW2 CAP2 VOUT2 10µF LT3498 LED1 20mA CTRL1 GND1 GND2 OFF ON SHUTDOWN AND DIMMING CONTROL CTRL2 FB2 2.21MΩ OFF ON SHUTDOWN AND CONTROL Figure 1. Li-Ion to six white LEDs and an OLED display ing battery life in application modes where the LED driver is temporarily disabled. Figure 1 shows a typical application driving 6 LEDs and an OLED. Figures 2 and 3 show the efficiency of the LED driver and OLED driver respectively. Features LED Driver High Side Sense The LED driver of the LT3498 features a unique high side LED current sense that enables the part to function as a 1-wire current source. This allows the cathode side of the bottom LED in the string to be returned to ground anywhere, resulting in a simple 1wire LED connection. Traditional LED drivers use a grounded resistor to sense LED current, requiring a 2-wire connection to the LED string since the ground must return to the part ground. In addition, high side sense allows the LT3498 LED driver to operate in unique applications (buck mode or buck boost mode, where the LED string is returned to the input) where traditional LED drivers cannot be used. 80 80 75 75 EFFICIENCY (%) EFFICIENCY (%) 70 70 65 60 400 VIN = 3.6V VOUT2 = 16V 350 EFFICIENCY FOR VOUT2 65 250 60 200 55 150 50 55 50 45 0 5 10 20 40 0.1 100 POWER LOSS FROM VOUT2 LED CURRENT (mA) 1 10 OLED CURRENT (mA) Figure 2. Efficiency of the LED driver in Figure 1 Figure 3. Efficiency of the OLED Driver in Figure 1 15 300 POWER LOSS (mW) The LT3498 is a dual boost converter featuring both an LED driver and OLED driver in a single 3mm × 2mm DFN package. It provides an internal power switch and Schottky diode for each converter as well as an output disconnect PMOS for the OLED driver. Both converters can be independently shutdown and dimmed. This highly integrated power solution is ideal for dual display portable electronics with tight space constraints. The LED driver is designed to drive up to six white LEDs in series from a LiIon cell. It is capable of regulating the LED current in a series configuration, providing equal brightness throughout an LED string regardless of variations in forward voltage drop. The 2.3MHz switching frequency allows the use of small external components and keeps switching noise out of critical wireless and audio bands. It features a high side LED current sense, which allows the converter to be used in a wide variety of application configurations. The LED driver also contains internal compensation, open-LED protection, analog or PWM controlled dimming, a 32V power switch and a 32V Schottky diode. The OLED driver of the LT3498 features a novel control technique resulting in low output voltage ripple as well as high efficiency over a wide load range. During operation, the converter controls power delivery by varying both the peak inductor current and switch off time. The off time is not allowed to exceed a fixed level, guaranteeing that the switching frequency stays above the audio band. This unique control scheme makes it ideal for noise sensitive applications such as MP3 players and mobile phones. When operated by itself, the OLED driver consumes a low 230µA quiescent current, extend- 50 0 100 23 L DESIGN FEATURES 240 16 14 VOUT2 VOLTAGE (V) 200 SENSE VOLTAGE (mV) 18 T = 25°C T = –50°C T = 125°C 160 120 80 12 10 6 4 40 0 8 2 0 500 1000 VCTRL1 (mV) 1500 0 2000 Figure 4. LED sense voltage vs CTRL1 pin voltage 0 500 1500 1000 CTRL2 VOLTAGE (V) Figure 5. VOUT2 voltage vs CTRL2 pin voltage VIN 3V TO 5V RSENSE1 10Ω PWM 10kHz TYP CAP1 SW1 CTRL1 LED1 CTRL1 GND1 Q1 Si2304BDS Figure 6. Filtered PWM dimming 24 GND2 CTRL2 FB2 5V 100k PWM FREQ 0V Figure 7. Li-Ion to four white LEDs with direct PWM dimming sets the LED current (see Figure 4). The CTRL2 pin regulates the VOUT2 voltage in a similar fashion as shown in Figure 5. Filtered PWM dimming works similarly to DC voltage dimming, except that the DC voltage input to the CTRL pins comes from an RC-filtered PWM signal. The corner frequency of the R1 and C1 should be much lower than the frequency of the PWM signal for proper filtering. Filtered PWM dimming is shown in Figure 6. 10000 PULSING MAY BE VISIBLE 1000 PWM DIMMING RANGE Dimming & Shutdown Control The LT3498 features a single pin shutdown and dimming control for each converter. To shutdown the LT3498, simply pull both control pins below 75mV. To enable each individual converter, increase the control pin (CTRL1 for the LED Driver and CTRL2 for the OLED Driver) voltage to 125mV or higher. On the LED side, the LED current can be set by modulating the CTRL1 pin. On the OLED side, the VOUT2 voltage can be set by modulating the CTRL2 pin. There are three types of dimming methods available in the LT3498: DC voltage dimming, filtered PWM signal dimming and direct PWM dimming. The LED current and VOUT2 voltage are proportional to the DC voltages at the CTRL1 and CTRL2 pins, respectively. To dim the LEDs or lower the VOUT2 voltage, reduce the voltage on the CTRL1 and CTRL2 pins. The dimming range of the LED driver extends from 1.5V at the CTRL1 pin for full LED current down to 125mV. The CTRL1 pin directly controls the regulated sense voltage across the sense resistor that VIN SW2 CAP2 VOUT2 LT3498 COUT1 1µF C1 0.1µF CIN 1µH L1 15µH LT3498 R1 100kΩ 2000 100 10 1 10 100 1000 PWM FREQUENCY (Hz) Figure 8. LED dimming range vs PWM dimming frequency 10000 Direct PWM dimming is typically used because it achieves a much wider dimming range compared to using a filtered PWM or a DC voltage. Direct PWM dimming uses a MOSFET in series with the LED string to quickly connect and disconnect the LED string. Figure 7 displays direct PWM dimming of the LEDs in a Li-Ion to 4 white LED application. A PWM signal is applied to the CTRL pin and MOSFET where the PWM signal controls both the turn-on and turn-off of the part. Figure 8 shows the linearity of PWM dimming across a range of frequencies. The available dimming range depends on the settling time of the application and the PWM frequency used. The application in Figure 7 achieves a dimming range of 250:1 using a 100Hz PWM frequency. OLED Driver PMOS Output Disconnect The low-noise boost converter of the LT3498 features a PMOS output disconnect switch. This PMOS switch is continued on page 38 Linear Technology Magazine • October 2007 L DESIGN IDEAS three different current levels using a single programming resistor. The current ratios are selected using the ENT and ENF pins. Table 1 shows the three different current ratios, and the ENT/ENF settings required to select them. RSETT refers to the resistor connected between the ISETT pin and GND, and RSETF refers to the resistor connected between the ISETF pin and GND. In the case where single-resistor programming is desired, the ISETT and ISETF pins can be shorted together and connected to a resistor to GND. Figure 2 shows an example of this configuration, along with the resulting output current levels. Dimming and Brightness Control LT3498, continued from page 24 pable of driving four LEDs in series, with 20mA of constant current as well as an OLED display. The efficiency for the LED driver in Figure 9 is shown in Figure 10. As shown above in Figure 1, the circuit can operate from a single Li-Ion battery (down to 3V) or 5V wall adapter and drive up to six LEDs in series at 20mA and an OLED display at 16V, 24mA out. turned on when the part is enabled. When the part is in shutdown, the PMOS switch turns off, allowing the VOUT2 node to go to ground. This type of disconnect function is often required for OLED applications. Li-Ion Powered Driver for Four White LEDs and OLED display Figure 9 highlights the LT3498’s simplicity and versatility. From a single 3mm × 2mm DFN, this circuit is caVIN 3V TO 5V CAP1 SW1 ILED(MAX ) = 850 • 1.21V R SETT where RSETT = R1 + R2 and the onresistance of M1 is small compared to RSETT. Resistor R1 should be greater than 1kΩ to provide adequate isola- Conclusion The LT3498 is a dual output boost converter that is capable of driving CIN 4.7µF L1 15µH C1 1µF Figure 3 shows how the LTC3218 can be configured to control LED brightness with just a few external components. By pulse-width modulating the gate of M1, the reference current in resistor R1 can be varied. The maximum LED current is determined by: Conclusion Due to its small size and low external parts count, the LTC3218 is ideally suited for compact, camera LED applications. Features such as its single resistor programmability, multiple current ratios and 2-second flash timeout make the part simple to use, without the need for complicated control algorithms. Its low shutdown current and high efficiency make it perfect for situations where battery power is at a premium. L up to 6 white LEDs and an OLED display from a single-cell Li-Ion input. The device features 32V internal power switches, 32V internal Schottky diodes, independent DC or PWM dimming control, open LED protection, OLED output disconnect and internal compensation. The LT3498 offers a highly integrated, space-saving solution for a wide range of applications including space-constrained and noise-sensitive portable applications such as cellular phones, MP3 players and digital cameras. L C2 0.47µF L2 10µH VIN tion between the 1µF capacitor and the internal servo-amplifier. 16V 24mA SW2 CAP2 VOUT2 80 C3 10µF 75 RSENSE1 10Ω 20mA LED1 CTRL1 GND1 GND2 OFF ON SHUTDOWN AND DIMMING CONTROL CTRL2 FB2 RFB2 2.21MΩ OFF ON SHUTDOWN AND CONTROL EFFICIENCY (%) LT3498 70 65 60 55 50 0 5 10 15 20 LED CURRENT (mA) CIN, C2: X5R OR X7R WITH SUFFICIENT VOLTAGE RATING C1: TAIYO YUDEN GMK212BJ105KG C3: TAIYO YUDEN TMK316BJ106ML L1: MURATA LQH32CN150K53 L2: MURATA LQH32CN100K53 Figure 10. Efficiency of the LED driver in Figure 9 Figure 9. Li-Ion to four white LEDs and an OLED display 38 Linear Technology Magazine • October 2007