CAT4237 High Voltage CMOS Boost White LED Driver Features Product Description • • • • • • • • • • • The CAT4237 is a DC/DC step-up converter that delivers an accurate constant current ideal for driving LEDs. Operation at a constant switching frequency of 1MHz allows the device to be used with small value external ceramic capacitors and inductor. LEDs connected in series are driven with a regulated current set by the external resistor R1. LED currents up to 40mA can be supported over a wide range of input supply voltages from 2.8V to 5.5V, making the device ideal for battery-powered applications. The CAT4237 high-voltage output stage is perfect for driving six, seven or eight white LEDs in series with inherent current matching in LCD backlight applications. Drives 6 to 8 White LEDs in series from 3V Up to 87% Efficiency Low Quiescent Ground Current 0.6mA Adjustable Output Current (up to 40mA) High Frequency 1MHz Operation High Voltage Power Switch Shutdown current less than 1µA Open LED low power mode Automatic Shutdown at 1.9V (UVLO) Thermal shutdown protection Thin SOT23 5-Lead (1mm max height) Applications • • • • • • Color LCD and keypad Backlighting Cellular Phones Handheld Devices Digital Cameras PDAs Portable Game Machine LED dimming can be done by using a DC voltage, a logic signal, or a pulse width modulation (PWM) signal. The shutdown input pin allows the device to be placed in power-down mode with “zero” quiescent current. Ordering Information Part Number CAT4237TS CAT4237TD Package TSOT23-5 TSOT23-5 1 Lead Free Note 1: NiPdAu Lead Plating Quantity per Reel 3000 3000 Pin Configuration Package Marking LS LT In addition to thermal protection and overload current limiting, the device also enters a very low power operating mode during “Open LED” fault conditions. The device is housed in a low profile (1mm max height) 5-lead thin SOT23 package for space critical applications. Typical Application Circuit Top View 5-Lead Thin SOT23 1mm max height L: Sumida CDRH3D16-330 D: Central CMDSH05-4 (rated 40V) C2: Taiyo Yuden UMK212BJ224 (rated 50V) © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 1 of 13 Doc. No. 5013, Rev. A CAT4237 ABSOLUTE MAXIMUM RATINGS Parameter VIN, FB voltage SHDN* voltage SW voltage Storage Temperature Range Junction Temperature Range Lead Temperature Rating -0.3 to +7 -0.3 to +7 -0.3 to 60 -65 to +160 -40 to +125 300 Unit V V V °C °C °C RECOMMENDED OPERATING CONDITIONS Typical application circuit with external components are shown on page 1. Parameter Range Unit VIN 2.8 to 5.5 V SW pin voltage 0 to 30 V Ambient Temperature Range -40 to +85 °C 6, 7 or 8 LEDs 1 to 40 mA ELECTRICAL OPERATING CHARACTERISTICS VIN = 3.6V, ambient temperature of 25ºC (over recommended operating conditions unless specified otherwise) Symbol Parameter IQ Operating Current ISD VFB IFB Shutdown Current FB Pin Voltage FB pin input leakage ILED Programmed LED Current VIH VIL SHDN* Logic High SHDN* Logic Low FSW ILIM RSW ILEAK VUVLO VOV-SW Switching Frequency Switch Current Limit Switch “On” Resistance Switch Leakage Current Thermal Shutdown Thermal Hysteresis Undervoltage Lockout (UVLO) Threshold Overvoltage Threshold © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice Conditions VFB = 0.2V VFB = 0.4V (not switching) VSHDN* = 0V 8 LEDs with ILED=20mA Min R1 = 10Ω R1 = 15Ω R1 = 20Ω Enable Threshold Level Shutdown Threshold Level 28.5 19 14.25 285 0.4 0.8 350 ISW = 100mA Switch Off, VSW = 5V Typ 0.6 0.1 0.1 300 30 20 15 0.8 0.7 1.0 450 1.0 1 150 20 1.9 35 2 of 13 Max 1.5 0.6 1 315 1 31.5 21 15.75 1.5 Unit mA 1.3 600 2.0 5 MHz mA µA mV µA mA V V Ω µA ºC ºC V V Doc. No. 5013, Rev. A CAT4237 PIN DESCRIPTION VIN is the supply input for the internal logic SW pin is connected to the drain of the internal The device is compatible with supply voltages down to 2.8V and up to 5.5V. It is recommended that a small bypass ceramic capacitor (4.7uF) be placed between the VIN and GND pins near the device. If the supply voltage drops below 1.9V, the device stops switching. CMOS power switch of the boost converter. The inductor and the Schottky diode anode should be connected to the SW pin. Traces going to the SW pin should be as short as possible with minimum loop area. An over-voltage detection circuit is connected to the SW pin. When the voltage reaches 35V, the device enters a low power operating mode preventing the SW voltage from exceeding the maximum rating. SHDN* is the shutdown logic input. When the pin is tied to a voltage lower than 0.4V, the device is in shutdown mode, drawing nearly zero current. When the pin is connected to a voltage higher than 1.5V, the device is enabled. FB feedback pin is regulated at 0.3V. A resistor connected between the FB pin and ground sets the LED current according to the formula: ILED = 0.3V/R1 GND is the ground reference pin. This pin should be connected directly to the ground place on the PCB. Pin # 1 2 3 4 5 Name SW GND FB SHDN* VIN The lower LED cathode is connected to the FB pin. Function Switch pin. This is the drain of the internal power switch. Ground pin. Connect the pin to the ground plane. Feedback pin. Connect to the last LED cathode. Shutdown pin (Logic Low). Set high to enable the driver. Power Supply input. BLOCK DIAGRAM © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 3 of 13 Doc. No. 5013, Rev. A CAT4237 DEVICE OPERATION The CAT4237 is a fixed frequency (1MHz), low noise, inductive boost converter that provides a constant current with excellent line and load regulation. The device uses a high-voltage CMOS power switch between the SW pin and ground to energize the inductor. When the switch is turned off, the stored energy in the inductor is released into the load via the Schottky diode. every 60µs and keep it on for about 1µs only. Thermal overload protection circuitry has been included to prevent the device from operating at unsafe junction temperatures above 150ºC. In the event of a thermal overload condition the device will automatically shutdown and wait till the junction temperatures cools to 130ºC before normal operation is resumed. The on/off duty cycle of the power switch is internally adjusted and controlled to maintain a constant regulated voltage of 0.3V across the feedback resistor connected to the feedback pin (FB). The value of the resistor sets the LED current accordingly (0.3V/R1). Light load operation Under light load condition (under 4mA) and with input voltage above 4.2V, the CAT4237 driving 6 LEDs, the driver starts pulse skipping. Although the LED current remains well regulated, some lower frequency ripple may appear. During the initial power-up stage, the duty cycle of the internal power switch is limited to prevent excessive in-rush currents and thereby provide a “soft-start” mode of operation. While in normal operation, the device can deliver up to 40mA of load current into a string of up to 8 white LEDs. In the event of a “Open LED” fault condition, where the feedback control loop becomes open, the output voltage will continue to increase. Once this voltage exceeds 35V, an internal protection circuit will become active and place the device into a very low power safe operating mode where only a small amount of power is transferred to the output. This is achieved by pulsing the switch once © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice Switching Waveform VIN = 4.2V, ILED = 4mA 4 of 13 Doc. No. 5013, Rev. A CAT4237 Typical Characteristics VIN = 3.6V, CIN = 4.7uF, COUT = 0.22µF, L = 33µH with 8 LEDs at 20mA, TAMB=25ºC, unless otherwise specified. Quiescent Current vs. VIN (Not Switching) Quiescent Current vs. VIN (Switching) 140 SUPPLY CURRENT [mA] 2.0 INPUT CURRENT [uA] 120 100 80 60 40 VFB =0.4V (not switching) 20 1.5 1.0 0.5 0.0 0 2.7 3.0 3.3 3.6 3.9 4.2 4.5 2.5 4.8 3.5 4.0 4.5 5.0 INPUT VOLTAGE [V] INPUT VOLTAGE [V] FB Pin Voltage vs. Supply Voltage FB Pin Voltage vs. Output Current 315 315 FB PIN VOLTAGE [mV] 8LEDs at 20mA VOUT= 26V 310 FEEDBACK [mV] 3.0 305 300 295 290 310 8 LEDs 305 300 295 290 285 285 2.7 3.0 3.3 3.6 3.9 4.2 4.5 INPUT VOLTAGE [V] 0 4.8 Switching Frequency vs. Supply Voltage 5 10 15 20 25 OUTPUT CURRENT [mA] 30 Switching Waveforms 1040 FREQUENCY [kHz] 1020 1000 980 960 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8 INPUT VOLTAGE [V] © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 5 of 13 Doc. No. 5013, Rev. A CAT4237 Typical Characteristics VIN = 3.6V, CIN = 4.7uF, COUT = 0.22µF, L = 33µH, with 8 LEDs at 20mA, TAMB=25ºC, unless otherwise specified. LED Current Regulation (20mA) LED Current vs. Input Voltage (8 LEDS) 35 1.0% CURRENT VARIATION [%] . LED CURRENT [mA] 30 RFB = 10Ω 25 RFB = 15Ω 20 15 RFB = 20Ω 10 5 0 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE [V] 0.5% 0.0% -0.5% -1.0% 3.0 5.0 3.3 3.6 3.9 4.2 4.5 INPUT VOLTAGE [V] 8 LED Efficiency vs. Input Voltage 8 LED Efficiency vs. Load Current 90 90 20mA 85 80 EFFICIENCY [%] EFFICIENCY [%] VIN = 4.2V VIN = 3.6V 75 8 LEDs VOUT ~ 27V at 20mA L = 33uH 70 85 15mA 80 75 8 LEDs VOUT ~ 27V at 20mA L = 33uH 70 65 65 5 10 15 20 25 3.0 30 7 LED Efficiency vs. Load Current 90 4.0 4.5 5.0 6 LED Efficiency vs. Load Current 90 VIN = 4.2V VIN = 4.2V EFFICIENCY [%] 85 VIN = 3.6V 80 75 7 LEDs VOUT ~ 23V at 20mA L = 33uH 70 3.5 INPUT VOLTAGE [V] LED CURRENT [mA] EFFICIENCY [%] 4.8 65 85 VIN = 3.6V 80 75 6 LEDs VOUT ~ 20V at 20mA L = 33uH 70 65 5 10 15 20 25 30 5 LED CURRENT [mA] © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 10 15 20 25 30 LED CURRENT [mA] 6 of 13 Doc. No. 5013, Rev. A CAT4237 Typical Characteristics VIN = 3.6V, CIN = 4.7uF, COUT = 0.22µF, L = 33µH, with 8 LEDs at 20mA, TAMB=25ºC, unless otherwise specified. Switch ON Resistance vs. Input Voltage Power-up with 8 LEDs at 20mA SWITCH RESISTANCE [ohm] 2.0 1.5 1.0 0.5 0.0 2.5 FB pin voltage vs. Temperature 4.5 Shutdown Voltage vs. Input Voltage 1.0 303 302 SHUTDOWN VOLTAGE [V] FEEDBACK VOLTAGE [mV] 3.0 3.5 4.0 INPUT VOLTAGE [V] 301 300 299 V IN =3.6V, 8LEDs 298 ILED =20mA 297 25°C -40°C 0.8 0.6 125° C 85°C 0.4 0.2 -50 0 50 100 3.0 150 TEMPERATURE [ºC] 3.5 4.0 4.5 INPUT VOLTAGE [V] 5.0 Maximum Output Current vs. Input Voltage MAX OUTPUT CURRENT [mA]. 140 120 VOUT = 15V 100 80 60 40 VOUT = 20V 20 0 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE [V] © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 5.0 7 of 13 Doc. No. 5013, Rev. A CAT4237 APPLICATION INFORMATION External Component Selection through it. The Schottky diode performance is rated in terms of its forward voltage at a given current. In order to achieve the best efficiency, this forward voltage should be as low as possible. The response time is also critical since the driver is operating at 1MHz. Central Semiconductor Schottky diode CMDSH05-4 (500mA rated) is recommended for most applications. Capacitors The CAT4237 only requires small ceramic capacitors of 4.7uF on the input and 0.22µF on the output. Under normal condition, a 4.7µF input capacitor is sufficient. For applications with higher output power, a larger input capacitor of 10µF may be appropriate. X5R and X7R capacitor types are ideal due to their stability across temperature range. LED Current Setting The LED current is set by the external resistor R1 connected between the feedback pin (FB) and ground. The formula below gives the relationship between the resistor and the current: Inductor A 33µH inductor is recommended for most of the CAT4237 applications. In cases where the efficiency is critical, inductances with lower series resistance are preferred. Inductors with current rating of 300mA or higher are recommended for most applications. Sumida CDRH3D16-330 33µH inductor has a rated current of 320mA and a series resistance (D.C.R.) of 520mΩ typical. R1 = 0.3V/LED current LED current (mA) R1 (Ω) 5 60 10 30 15 20 20 15 25 12 30 10 Table 1. Resistor R1 and LED current Schottky Diode The current rating of the Schottky diode must exceed the peak current flowing © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 8 of 13 Doc. No. 5013, Rev. A CAT4237 OPEN LED PROTECTION In the event of an “Open LED” fault condition, the CAT4237 will continue to boost the output voltage with maximum power until the output voltage reaches approximately 35V. Once the output exceeds this level, the internal circuitry immediately places the device into a very low power mode where the total input power is limited to about 4mW (about 1mA input current with a 3.6V supply). The SW pin clamps at a voltage below its maximum rating of 60V. There is no need to use an external zener diode between Vout and the FB pin. A 50V rated C2 capacitor is required to prevent any overvoltage damage in the open LED condition. Open LED Protection without Zener Open LED Switching waveforms without Zener Open LED Supply Current vs. VIN without Zener Open LED Output Voltage vs. VIN without Zener 50 OUTPUT VOLTAGE [V] SUPPLY CURRENT [mA] 2.0 1.5 1.0 0.5 45 40 35 30 0.0 2.5 3.0 3.5 4.0 4.5 2.5 5.0 © 2005 Catalyst Semiconductor, Inc. 3.5 4.0 4.5 5.0 INPUT VOLTAGE [V] INPUT VOLTAGE [V] Characteristics subject to change without notice 3.0 9 of 13 Doc. No. 5013, Rev. A CAT4237 DIMMING CONTROL There are several methods available to control the LED brightness. § the CAT4237 FB pin. The PWM signal has a voltage swing of 0V to 2.5V. The LED current can be dimmed within a range from 0 to 20mA. The PWM signal frequency can vary from very low frequency up to 100kHz. PWM signal on the SHDN* pin LED brightness dimming can be done by applying a PWM signal to the SHDN* input. The LED current is repetitively turned on and off, so that the average current is proportional to the duty cycle. A 100% duty cycle, with SHDN* always high, corresponds to the LEDs at nominal current. Figure 1 shows a 1kHz signal with a 50% duty cycle applied to the SHDN* pin. The recommended PWM frequency range is from 100Hz to 2kHz. Figure 2. Circuit for filtered PWM signal A PWM signal at 0V DC, or a 0% duty cycle, results in a max LED current of about 22mA. A PWM signal with a 93% duty cycle or more, results in an LED current of 0mA. Figure 1. Switching waveform with 1kHz PWM on SHDN* § Filtered PWM signal A filtered PWM signal used as a variable DC voltage can control the LED current. Figure 2 shows the PWM control circuitry connected to © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 10 of 13 Doc. No. 5013, Rev. A CAT4237 BOARD LAYOUT The CAT4237 is a high-frequency switching regulator. The traces that carry the highfrequency switching current have to be carefully layout on the board in order to minimize EMI, ripple and noise in general. The thicker lines on Figure 3 show the switching current path. All these traces have to be short and wide enough to minimize the parasitic inductance and resistance. The loop shown on Figure 3 corresponds to the current path when the CAT4237 internal switch is closed. On Figure 4 is shown the current loop, when the CAT4237 switch is open. Both loop areas should be as small as possible. Capacitor C1 has to be placed as close as possible to the VIN pin and GND. The capacitor C2 has to be connected separately to the top LED anode. A ground plane under the CAT4237 allows for direct connection of the capacitors to ground. The resistor R1 must be connected directly to the GND pin of the CAT4237 and not shared with the switching current loops and any other components. Figure 4. Open-switch current loop Figure 3. Closed-switch current loop open closed © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 11 of 13 Doc. No. 5013, Rev. A CAT4237 PACKAGE OUTLINES 5-LEAD SOT-23 All dimensions in millimeters © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice 12 of 13 Doc. No. 5013, Rev. A CAT4237 REVISION HISTORY Date 10/14/2005 Revision A Reason Initial issue Copyrights, Trademarks and Patents Trademarks and registered trademarks of Catalyst Semiconductor include each of the following: DPP ™ AE2 ™ Catalyst Semiconductor has been issued U.S. and foreign patents and has patent applications pending that protect its products. For a complete list of patents issued to Catalyst Semiconductor contact the Company’s corporate office at 408.542.1000. CATALYST SEMICONDUCTOR MAKES NO WARRANTY, REPRESENTATION OR GUARANTEE, EXPRESS OR IMPLIED, REGARDING THE SUITABILITY OF ITS PRODUCTS FOR ANY PARTICULAR PURPOSE, NOR THAT THE USE OF ITS PRODUCTS WILL NOT INFRINGE ITS INTELLECTUAL PROPERTY RIGHTS OR THE RIGHTS OF THIRD PARTIES WITH RESPECT TO ANY PARTICULAR USE OR APPLICATION AND SPECIFICALLY DISCLAIMS ANY AND ALL LIABILITY ARISING OUT OF ANY SUCH USE OR APPLICATION, INCLUDING BUT NOT LIMITED TO, CONSEQUENTIAL OR INCIDENTAL DAMAGES. Catalyst Semiconductor products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the Catalyst Semiconductor product could create a situation where personal injury or death may occur. Catalyst Semiconductor reserves the right to make changes to or discontinue any product or service described herein without notice. Products with data sheets labeled "Advance Information" or "Preliminary" and other products described herein may not be in production or offered for sale. Catalyst Semiconductor advises customers to obtain the current version of the relevant product information before placing orders. Circuit diagrams illustrate typical semiconductor applications and may not be complete. Catalyst Semiconductor, Inc. Corporate Headquarters 1250 Borregas Avenue Sunnyvale, CA 94089 Phone: 408.542.1000 Fax: 408.542.1200 www.catalyst-semiconductor.com © 2005 Catalyst Semiconductor, Inc. Characteristics subject to change without notice Publication #: 5013 Revision: Rev. A Issue date: 10/14/05 13 of 13 Doc. No. 5013, Rev. A