PDF LTC3524 Data sheet ( Hoja de datos )

Número de pieza	LTC3524
Descripción	Adjustable TFT Bias Supply
Fabricantes	Linear Technology Corporation
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Hay una vista previa y un enlace de descarga de LTC3524 (archivo pdf) en la parte inferior de esta página. Total 16 Páginas
No Preview Available ! FEATURES ■ Generates Three Adjustable, Low Noise Rails for Small/Medium TFT Displays ■ Drives Up to Ten White LEDs ■ LED Dimming and Open-Circuit Protection ■ Controlled Power-Up/Power-Down Sequencing ■ 1.5MHz Fixed Frequency, Low Noise Operation www.DataSh■eetV4UIN.cRomange 2.5V to 6V, VOUT Range 3V to 6V ■ TFT Supply Efﬁciency Up to 90% ■ LED Supply Efﬁciency Up to 78% ■ Two Independantly Enabled LED Strings ■ 200 to 1 True Color PWMTM Dimming ■ Tiny External Solution ■ 24-Lead QFN Package (4mm × 4mm × 0.75mm) APPLICATIONS ■ PDAs, Palmtop Computers ■ Digital Still and Video Cameras ■ Handheld GPS ■ Portable Instrument Displays ■ Portable Media Players LTC3524 Adjustable TFT Bias Supply with WLED Driver DESCRIPTION The LTC®3524 is an integrated BIAS and white LED power converter solution for small/medium-sized polysilicon thin ﬁlm transistor (TFT) liquid crystal (LCD) display panels. The device operates from a single Lithium-Ion/polymer battery or any voltage source between 2.5V and 6V. A 1.5MHz synchronous boost converter generates a pro- grammable low noise, high efﬁciency 25mA TFT supply of up to 6.0V. Regulated, low ripple charge pumps are used to generate up to +20V and –20V at 2mA. Output sequencing is internally controlled to insure proper initialization and rapid discharge of the LCD panel in shutdown. A second 1.5MHz boost converter powers one or two LED strings with up to ﬁve series elements each. LED current and display brightness can be controlled over a wide range using analog or digital means up to 25mA. The LTC3524 is offered in the 4mm × 4mm 24-pin QFN package, minimizing the total solution footprint. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. True Color PWM is a registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION +5V, –7.5V, +12.5V, 8 LED Power Supply + – +5V 25mA +12.5V 2mA Li-Ion 2.2μF 10μH 3.3μH SW1 VIN SW2 VLED 10μF 324k +10V 0.47μF 0.47μF 220k VOUT 1M LED2 0.1μF FBVO VNIN V2x C2+ C2– VH LTC3524 LED1 PROG ELED2 ELED1 ELCD FBN 2M FBH CH+ CH– GND CN+ VN 0.1μF 0.1μF 10μF 100k for 20mA 470k 1M VIN VOUT –7.5V 2mA 0.47μF 3524 TA01a LCD Bias and LED Efﬁciency VIN = 3.6V, VOUT = 5V, 8 LEDs 100 VIN = 3.6V 90 LCD 80 LED 70 60 5 10 15 20 25 VOUT OR LED STRING CURRENT (mA) 3524 TA01b 3524f 1 1 page LTC3524 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. LCD Bias Sequencing VH 5V/DIV www.DataSheet4U.com V2X VN VOUT 5ms/DIV LCD Bias Sequencing SW1 Voltage and 10μH Inductor Current at 25mA Load 5V/DIV 200mA/DIV 3524 G11 VH VOUT SW1 2V/DIV VN ILCD BOOST INDUCTOR CURRENT ILCD BOOST INDUCTOR CURRENT 50mA/DIV 5ms/DIV 3524 G12 200ns/DIV 3524 G13 SW1 Voltage and 10μH Inductor Current at 5mA Load SW2 Voltage and 4.7μH Inductor Current at 20mA SW1 2V/DIV ILCD BOOST INDUCTOR CURRENT 50mA/DIV 200ns/DIV SW2 200mV/DIV ILCD BOOST INDUCTOR CURRENT 200mA/DIV SW2 5V/DIV 3524 G14 200ns/DIV 3524 G15 LED Initial Start-Up Waveforms 500mV/DIV ILED BOOST INDUCTOR CURRENT VLED 5V/DIV LED1 LED2 50μs/DIV 3524 G16 LED Burst Dimming Waveforms LED1 and SW2 ELED1 AND ELED2 ILED BOOST INDUCTOR LED1 SW2 12V 500μs/DIV 5V/DIV 200mA/DIV 10V/DIV 10V/DIV 3524 G17 LED Burst Dimming Waveforms LED2 and VLED ELED1 AND ELED2 ILED BOOST INDUCTOR VLED LED2 12.5V 12V 500μs/DIV 5V/DIV 200mA/DIV 10V/DIV 10V/DIV 3524 G18 3524f 5 5 Page LTC3524 OPERATION output voltage is not set to a ﬁxed voltage, but rather controlled to produce the programmed current in the LED strings. The output (VLED) is rated for a maximum of 21V which will support two strings of up to ﬁve series LED in most cases. The boost output is used to power one or two white LED strings with a common ground. If only one string is en- abled (ELED1 or ELED2) the voltage on that string (LED1 www.DataShoeret4LUE.Dco2m) will be controlled to regulate the LED current set at the PROG pin. The voltage on VLED will be slightly greater due to the overhead needed for the internal sense element and share circuitry. For example, a single string application with four white LEDs programmed at 20mA would require 14.4V on LED1 if the forward drop on each LED is 3.6V. The voltage on VLED may need to be 15V to support the drops on the internal share circuitry. For ap- plications with ﬁve or fewer LED elements, a single-string operation will provide better efﬁciency. If both strings are enabled, the boost output (VLED) will generate the voltage required to regulate current in the higher voltage string. Voltage on the lower string is controlled by the internal share circuit to provide the pro- grammed current. The LTC3524 achieves current matching between the strings while minimizing the voltage drop between VLED and the higher voltage string (to maintain high efﬁciency). For example, an application with four LEDs on LED1 and ﬁve LEDs on LED2 is programmed for 20mA (RPROG = 100k). In this instance, assuming a 3.6V forward drop, LED1 is 14.4V, LED2 is 18V, and VLED is 18.6V. The drop between VLED and LED1 is 4V at 20mA, resulting in lower efﬁciency. For this reason, it is recom- mended when possible to keep the number of LEDs in each string matched. Analog Dimming: The LTC3524’s white LED driver allows both analog and PWM dimming to be implemented. Analog dimming provides a lower noise solution but a reduced dynamic range. Analog dimming can be implemented by resis- tively summing a current into the PROG pin. The LED string currents with RPROG, VSUM, and RSUM will be: ILED = 1625 • ⎛ ⎝⎜ 1.225V RPROG + 1.225V − VSUM RSUM ⎞ ⎠⎟ A 0V to 3V VSUM with RSUM = 300k and RPROG = 150k will produce LED currents between 3mA and 20mA. VSUM 0V – 3V RSUM 300k PROG RPROG 150k 3524 F03 Figure 3. Analog Dimming Circuit Using VSUM True Color PWM Dimming: PWM dimming can be implemented by enabling and dis- abling the LED strings with ELED1 and ELED2. A PWM frequency between 100Hz and 500Hz is generally recom- mended to get wide dimming range while operating at a frequency faster than the eye can detect. For best results, the LCD bias portion of the device should be enabled (to keep the device out of shutdown) and ELED1 and ELED2 should be driven with a common low frequency PWM signal. PWM dimming waveforms are shown in the Typical Performance Characteristics section of this datasheet. The achievable dimming range is dependant on the PWM dimming frequency (FPWM) and the settling time of the LED strings when enabled (TSETTLE). The minimum duty cycle (or light output) that the strings can be controlled to is given by: MinDuty = FPWM • TSETTLE For example, if the settling time is 50μS and the PWM frequency is 100Hz, the minimum duty cycle is 0.5% which corresponds to a 200:1 dimming range. Open LED: The LTC3524 has internal over voltage protection in the event that one of the white LED strings becomes open circuited. If VLED reaches 24V (nominal) due to an open circuit on either string, the boost converter will regulate at 24V while current in the remaining string (if enabled) is controlled to the programmed value. 3524f 11 11 Page

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