CAT4237TD

CAT4237 CATALYST | Alldatasheet

Document overview

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• PDF pages: 13

Technical content

Features

• 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

Ordering Information

CAT4237TS TSOT23-5 3000 LS CAT4237TD TSOT23-5 Lead Free1 3000 LT Note 1: NiPdAu Lead Plating Product Description The CAT4237 is a DC/DC step-up converter that delivers an accurate constant current ideal f or 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 c urrents 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 seri es with inherent current matching in LCD backlight applications. 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. In addition to thermal protection and overload current limiting, the device also enters a very low power operating mode during “Open L ED” fault conditions. The device is ho used in a low profile (1mm max height) 5-lead thin SOT23 package for space critical applications. Pin Configuration Top View 5-Lead Thin SOT23 1mm max height Typical Application Circuit L: Sumida CDRH3D16-330 D: Central CMDSH05-4 (rated 40V) C2: Taiyo Yuden UMK212BJ224 (rated 50V)

© 2005 Catalyst Semiconductor, Inc. 2 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice ABSOLUTE MAXIMUM RATINGS Parameter Rating Unit VIN, FB voltage -0.3 to +7 V SHDN* voltage -0.3 to +7 V SW voltage -0.3 to 60 V Storage Temperature Range -65 to +160 °C Junction Temperature Range -40 to +125 °C Lead Temperature 300 °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 Conditions Min Typ Max Unit IQ Operating Current VFB = 0.2V VFB = 0.4V (not switching) 0.6 0.1 1.5 0.6 mA ISD Shutdown Current VSHDN* = 0V 0.1 1 µA VFB FB Pin Voltage 8 LEDs with ILED=20mA 285 300 315 mV IFB FB pin input leakage 1 µA ILED Programmed LED Current R1 = 10Ω R1 = 15Ω R1 = 20Ω 28.5 14.25 31.5 15.75 mA VIH VIL SHDN* Logic High SHDN* Logic Low Enable Threshold Level Shutdown Threshold Level 0.4 0.8 0.7 1.5 V V FSW Switching Frequency 0.8 1.0 1.3 MHz ILIM Switch Current Limit 350 450 600 mA RSW Switch “On” Resistance ISW = 100mA 1.0 2.0 Ω ILEAK Switch Leakage Current Switch Off, VSW = 5V 1 5 µA Thermal Shutdown 150 ºC Thermal Hysteresis 20 ºC VUVLO Undervoltage Lockout (UVLO) Threshold 1.9 V VOV-SW Overvoltage Threshold 35 V

© 2005 Catalyst Semiconductor, Inc. 3 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice PIN DESCRIPTION VIN is the supply input for the internal l ogic 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. SHDN* is the shutdown logic input. When the pin is ti ed to a voltage lower than 0.4V , the device is in shutdown mode , draw ing nearly zero current. When the pin is connected to a voltage higher than 1.5V , the device is enabled. GND is the grou nd reference pin. This pin should be connected directly to the ground place on the PCB. SW pin is connected to the drain of the internal 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. 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 The lower LED cathode is connected to the FB pin. Pin # Name Function 1 SW Switch pin. This is the drain of the internal power switch. 2 GND Ground pin. Connect the pin to the ground plane. 3 FB Feedback pin. Connect to the last LED cathode. 4 SHDN* Shutdown pin (Logic Low). Set high to enable the driver. 5 VIN Power Supply input. BLOCK DIAGRAM

© 2005 Catalyst Semiconductor, Inc. 4 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice 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. 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). During the initial power -up stage, the duty cycle of the internal power switch is l imited to prevent excessive in -rush currents and thereby provide a “soft -start” mode of operation. While in normal operation, t he 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 condi tion, 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 s mall amount of power is transferred to the output. This is achieved by pulsing the switch once every 60 µs and keep it on for about 1 µs only. Thermal overload protection circuitry has been included to prevent the device from operating at unsafe junction temperatures above 150 ºC. In the event of a thermal overload condition the device will automatically shutdown and wait till the junction temperatures cools to 130 ºC before normal operation is resumed. Light load operation Under light load condition (u nder 4mA) and with input voltage above 4.2V, the CAT4237 driving 6 LEDs , the driver start s pulse skipping. Although the LED current remains well regulated, some lower frequency ripple may appear. Switching Waveform VIN = 4.2V, ILED = 4mA

© 2005 Catalyst Semiconductor, Inc. 5 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice 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) 100 120 140 INPUT VOLTAGE [V] INPUT CURRENT [uA] VFB =0.4V (not switching) Quiescent Current vs. VIN (Switching) 0.0 0.5 1.0 1.5 2.0 INPUT VOLTAGE [V] SUPPLY CURRENT [mA] FB Pin Voltage vs. Supply Voltage 285 290 295 300 305 310 315 INPUT VOLTAGE [V] FEEDBACK [mV] 8LEDs at 20mA VOUT= 26V FB Pin Voltage vs. Output Current 285 290 295 300 305 310 315 0 5 10 15 20 25 30 OUTPUT CURRENT [mA] FB PIN VOLTAGE [mV]

8 LEDs

Switching Frequency vs. Supply Voltage 960 980 1000 1020 1040 INPUT VOLTAGE [V] FREQUENCY [kHz] Switching Waveforms

© 2005 Catalyst Semiconductor, Inc. 6 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice 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 vs. Input Voltage (8 LEDS) INPUT VOLTAGE [V] LED CURRENT [mA] RFB = 10Ω RFB = 15Ω RFB = 20Ω LED Current Regulation (20mA) -1.0% -0.5% 0.0% 0.5% 1.0% INPUT VOLTAGE [V] CURRENT VARIATION [%] . 8 LED Efficiency vs. Load Current 5 10 15 20 25 30 LED CURRENT [mA] EFFICIENCY [%] VIN = 3.6V VIN = 4.2V VOUT ~ 27V at 20mA L = 33uH 8 LED Efficiency vs. Input Voltage INPUT VOLTAGE [V] EFFICIENCY [%] 15mA 20mA VOUT ~ 27V at 20mA L = 33uH 7 LED Efficiency vs. Load Current 5 10 15 20 25 30 LED CURRENT [mA] EFFICIENCY [%] VIN = 3.6V VIN = 4.2V

7 LEDs

VOUT ~ 23V at 20mA L = 33uH 6 LED Efficiency vs. Load Current 5 10 15 20 25 30 LED CURRENT [mA] EFFICIENCY [%] VIN = 3.6V VIN = 4.2V

6 LEDs

VOUT ~ 20V at 20mA L = 33uH

© 2005 Catalyst Semiconductor, Inc. 7 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice Typical Characteristics VIN = 3.6V, CIN = 4.7uF, COUT = 0.22µF, L = 33µH, with 8 LEDs at 20mA, TAMB=25ºC, unless otherwise specified. Power-up with 8 LEDs at 20mA Switch ON Resistance vs. Input Voltage 0.0 0.5 1.0 1.5 2.0 INPUT VOLTAGE [V] SWITCH RESISTANCE [ohm] FB pin voltage vs. Temperature 297 298 299 300 301 302 303 -50 0 50 100 150 TEMPERATURE [ºC] FEEDBACK VOLTAGE [mV] VIN=3.6V, 8LEDs ILED=20mA Shutdown Voltage vs. Input Voltage 0.2 0.4 0.6 0.8 1.0 INPUT VOLTAGE [V] SHUTDOWN VOLTAGE [V] 125°C 25°C 85°C -40°C Maximum Output Current vs. Input Voltage 100 120 140 INPUT VOLTAGE [V] MAX OUTPUT CURRENT [mA]. VOUT = 15V VOUT = 20V

© 2005 Catalyst Semiconductor, Inc. 8 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice

APPLICATION INFORMATION

External Component Selection Capacitors The CAT4 237 only requires small ceramic capacitors of 4.7uF on the input and 0.22 µF on the output . Under normal condition, a 4.7µF input capacitor is sufficient. For applications with higher output power, a larger input capacitor of 10 µF may be appropriate. X5R and X7R capacitor types are ideal due to their stability across temperature range. Inductor A 33µH inductor is recommended for most of the CAT4237 applications. In ca ses 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 33 µH inductor has a rated current of 320mA and a series resistance (D.C.R.) of 520mΩ typical. Schottky Diode The current rating of the Schottky diode must exceed the peak current flowing through it. The Schottky diode performance is rated in terms of its forward voltage at a given current. In order to achie ve 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. 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: R1 = 0.3V/LED current LED current (mA) R1 (W) 5 60 10 30 15 20 20 15 25 12 30 10 Table 1. Resistor R1 and LED current

© 2005 Catalyst Semiconductor, Inc. 9 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice OPEN LED PROTECTION In the event of an “Open LED” fault condition, the CAT4 237 will continue to boost the output voltage with maximum power until the output voltage reaches approximate ly 35V. Once the output exceeds this level, the internal circuitry immediately place s 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 clamp s 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 0.0 0.5 1.0 1.5 2.0 INPUT VOLTAGE [V] SUPPLY CURRENT [mA] Open LED Output Voltage vs. VIN without Zener INPUT VOLTAGE [V] OUTPUT VOLTAGE [V]

© 2005 Catalyst Semiconductor, Inc. 12 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice PACKAGE OUTLINES 5-LEAD SOT-23 All dimensions in millimeters

© 2005 Catalyst Semiconductor, Inc. 13 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice

REVISION HISTORY

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Sunnyvale, CA 94089 Phone: 408.542.1000 Fax: 408.542.1200 www.catalyst-semiconductor.com Publication #: 5013 Revision: Rev. A Issue date: 10/14/05