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CAT4238 High Efficiency 10 LED Boost Converter
Description
The CAT4238 is a DC/DC step-up converter that delivers an accurate constant current ideal for driving LEDs. Operation at a fixed switching frequency of 1 MHz 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 40 mA can be supported over a wide range of input supply voltages up to 5.5 V, making the device ideal for battery-powered applications. The CAT4238 high-voltage output stage is perfect for driving mid-size and large panel displays containing up to ten white LEDs in series. 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 LED" fault conditions. The device is housed in a low profile (1 mm max height) 5-lead thin SOT23 package for space critical applications.
Features
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5 1 TSOT-23 TD SUFFIX CASE 419AE
PIN CONNECTIONS
1 SW GND FB (Top View) SHDN VIN
MARKING DIAGRAM
* * * * * * * * * * * *
Drives High Voltage LED Strings (38 V) Up to 87% Efficiency Low Quiescent Ground Current 0.6 mA Adjustable Output Current 1 MHz Fixed Frequency Low Noise Operation Soft Start "In-rush" Current Limiting Shutdown Current Less than 1 mA Open LED Overvoltage Protection Automatic Shutdown at 1.9 V (UVLO) Thermal Overload Protection Thin SOT23 5-lead (1 mm Max Height) These Devices are Pb-Free, Halogen Free/BFR Free and are RoHS Compliant GPS Navigation Systems Portable Media Players Handheld Devices, Digital Cameras Portable Game Machines
MUYM
MU = Specific Device Code Y = Production Year (Last Digit) M = Production Month (1-9, A, B, C)
ORDERING INFORMATION
Device CAT4238TD-GT3 Package TSOT-23 (Pb-Free) Green* Shipping 3,000/ Tape & Reel
* Lead Finish NiPdAu
Applications
* * * *
(c) Semiconductor Components Industries, LLC, 2010
February, 2010 - Rev. 4
1
Publication Order Number: CAT4238/D
CAT4238
VIN C1 4.7 mF VIN L 47 mH SW D C2 0.22 mF VOUT
CAT4238 OFF ON SHDN GND FB VFB = 300 mV R1 15 W 20 mA
L: Sumida CDC5D23B-470 D: Central CMDSH05-4 C2: Taiyo Yuden UMK212BJ224 (rated 50 V)
Figure 1. Typical Application Circuit Table 1. ABSOLUTE MAXIMUM RATINGS
Parameters VIN, FB voltage SHDN voltage SW voltage (Note 1) Storage Temperature Range Junction Temperature Range Lead Temperature Ratings -0.3 to +7 -0.3 to +7 up to 60 -65 to +160 -40 to +150 300 Units V V V _C _C _C
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect device reliability. 1. The SW pin voltage is rated up to 39 V for external continuous DC voltage.
Table 2. RECOMMENDED OPERATING CONDITIONS
Parameters VIN SW pin voltage Ambient Temperature Range NOTE: Typical application circuit with external components is shown above. Range up to 5.5 0 to 38 -40 to +85 Units V V _C
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CAT4238
Table 3. DC ELECTRICAL CHARACTERISTICS
(VIN = 3.6 V, ambient temperature of 25C (over recommended operating conditions unless otherwise specified)) Symbol IQ ISD VFB IFB ILED Parameter Operating Current Shutdown Current FB Pin Voltage FB pin input leakage Programmed LED Current R1 = 10 W R1 = 15 W R1 = 20 W Enable Threshold Level Shutdown Threshold Level 28.5 19 14.25 0.4 0.8 VIN = 3 V 350 ISW = 100 mA Switch Off, VSW = 5 V 30 20 15 0.8 0.7 1.0 92 450 1.0 1 150 20 1.9 40 "Open LED" with VIN = 5 V 43 45 48 600 2.0 5 Test Conditions VFB = 0.2 V VFB = 0.4 V (not switching) VSHDN = 0 V 10 LEDs with ILED = 20 mA 285 Min Typ 0.6 0.1 0.1 300 Max 1.5 0.6 1 315 1 31.5 21 15.75 1.5 1.3 Units mA mA mV mA mA
VIH VIL FSW DC ILIM RSW ILEAK
SHDN Logic High SHDN Logic Low Switching Frequency Maximum Duty Cycle Switch Current Limit Switch "On" Resistance Switch Leakage Current Thermal Shutdown Thermal Hysteresis
V MHz % mA W mA C C V V V
VUVLO VOV-SW VOCL
Undervoltage Lockout (UVLO) Threshold Overvoltage Detection Threshold Output Voltage Clamp
Pin Description VIN is the supply input for the internal logic. The device is compatible with supply voltages down to 2.8 V and up to 5.5 V. It is recommended that a small bypass ceramic capacitor (4.7 mF) be placed between the VIN and GND pins near the device. If the supply voltage drops below 1.9 V, the device stops switching. SHDN is the shutdown logic input. When the pin is tied to a voltage lower than 0.4 V, the device is in shutdown mode, drawing nearly zero current. When the pin is connected to a voltage higher than 1.5 V, the device is enabled. GND is the ground 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 40 V, the device enters a low power operating mode preventing the SW voltage from exceeding the maximum rating. FB feedback pin is regulated at 0.3 V. A resistor connected between the FB pin and ground sets the LED current according to the formula:
I LED + 0.3 V R1
The lower LED cathode is connected to the FB pin.
Table 4. PIN DESCRIPTIONS
Pin # 1 2 3 4 5 Name SW GND FB SHDN VIN 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.
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CAT4238
Block Diagram
VIN C1 4.7 mF 1 MHz Oscillator VREF 300 mV - A1 + Enable Thermal Shutdown & UVLO RC CC RS + GND + A2 - PWM & Logic N1 Over Voltage Protection Driver 47 mH SW C2 0.22 mF
VIN
LED Current
SHDN
Current Sense
Figure 2. Block Diagram
Device Operation The CAT4238 is a fixed frequency (1 MHz), 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 0.3 V across the feedback resistor connected to the feedback pin (FB). The value of the resistor sets the LED current accordingly (0.3 V/R1). 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 operating from a Li-Ion battery, the device can deliver 20 mA of load current into a string of up to 10 white LEDs. For higher input voltages, the LED current can be increased. In the event of an "Open LED" fault condition, where the feedback control loop becomes open, the output voltage will continue to increase. Once this voltage exceeds 40 V, 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 every 6 ms and keeping it on for about 1 ms.
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- FB R1 15 W
Thermal overload protection circuitry has been included to prevent the device from operating at unsafe junction temperatures above 150C. In the event of a thermal overload condition the device will automatically shutdown and wait till the junction temperatures cools to 130C before normal operation is resumed.
Light Load Operation
Under light load condition (under 2 mA) and with input voltage above 5.0 V, the CAT4238 driving 10 LEDs, the driver starts pulse skipping. Although the LED current remains well regulated, some lower frequency ripple may appear.
Figure 3. Switching Waveform VIN = 5.0 V, ILED = 1.5 mA
CAT4238
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 47 mH with 10 LEDs at 20 mA, TAMB = 25C, unless otherwise specified.) 150 QUIESCENT CURRENT (mA) QUIESCENT CURRENT (mA) VFB = 0.4 V 125 2.0
1.5
100
1.0
75
0.5
50
3.0
3.5
4.0
4.5
5.0
5.5
0
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 4. Quiescent Current vs. VIN (Not Switching)
303 302 FB PIN VOLTAGE (mV) FB PIN VOLTAGE (mV) 305 301 300 299 298 297 -50 290 310
Figure 5. Quiescent Current vs. VIN (Switching)
300
295 10 LEDs
0
50 TEMPERATURE (C)
100
150
5
10
15
20
25
30
OUTPUT CURRENT (mA)
Figure 6. FB Pin Voltage vs. Temperature
1.2 SWITCHING FREQUENCY (MHz) SW 20V/div
Figure 7. FB Pin Voltage vs. Output Current
1.1
1.0
Inductor Current 100mA/div
0.9
0.8
VOUT AC coupled 500mV/div 3.0 3.5 4.0 4.5 5.0 5.5 500 ns/div
INPUT VOLTAGE (V)
Figure 8. Switching Frequency vs. Supply Voltage
Figure 9. Switching Waveforms
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CAT4238
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 47 mH with 10 LEDs at 20 mA, TAMB = 25C, unless otherwise specified.) 40 LED CURRENT VARIATION (%) 35 LED CURRENT (mA) 30 25 20 15 10 5 0 3.0 3.5 4.0 4.5 5.0 5.5 R1 = 10 W VOUT = 33.8 V R1 = 15 W VOUT = 33 V 1.0
0.5
0
R1 = 20 W VOUT = 32.5 V
-0.5 10 LEDs @ 10 mA -1.0 3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 10. LED Current vs. Input Voltage
100 100
Figure 11. LED Current Regulation (10 mA)
EFFICIENCY (%)
EFFICIENCY (%)
90
VIN = 5 V
90
VOUT = 32.5 V 10 LEDs @ 15 mA VOUT = 33 V 10 LEDs @ 20 mA
80 VIN = 3.6 V 70
80
70
60
5
10
15
20
25
30
60
3.0
3.5
4.0
4.5
5.0
5.5
LED CURRENT (mA)
INPUT VOLTAGE (V)
Figure 12. Efficiency vs. Load Current (10 LEDs)
2.0 SWITCH RESISTANCE (W)
Figure 13. Efficiency vs. Input Voltage (10 LEDs)
1.5
1.0
0.5
0
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
Figure 14. Power-up with 10 LEDs at 20 mA
Figure 15. Switch ON Resistance vs. Input Voltage
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CAT4238
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 47 mH with 10 LEDs at 20 mA, TAMB = 25C, unless otherwise specified.) 60 SHUTDOWN VOLTAGE (V) OUTPUT CURRENT (mA) 50 40 30 20 VOUT = 35 V 10 0 1.0
0.8
-40C -25C
0.6
85C 125C
0.4
3.0
3.5
4.0
4.5
5.0
5.5
0.2
3.0
3.5
4.0 INPUT VOLTAGE (V)
4.5
5.0
INPUT VOLTAGE (V)
Figure 16. Maximum Output Current vs. Input Voltage
Figure 17. Shutdown Voltage vs. Input Voltage
Application Information
External Component Selection Capacitors
The CAT4238 only requires small ceramic capacitors of 4.7 mF on the input and 0.22 mF on the output. Under normal condition, a 4.7 mF input capacitor is sufficient. For applications with higher output power, a larger input capacitor of 10 mF may be appropriate. X5R and X7R capacitor types are ideal due to their stability across temperature range.
Inductor
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 1 MHz. Central Semiconductor Schottky diode CMDSH05-4 (500 mA 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 + 0.3 V current LED
A 47 mH inductor is recommended for most of the CAT4238 applications. In cases where the efficiency is critical, inductances with lower series resistance are preferred. Inductors with current rating of 300 mA or higher are recommended for most applications. Sumida CDC5D23B-470 47 mH inductor has a rated current of 490 mA and a series resistance (D.C.R.) of 420 mW typical.
Schottky Diode
Table 5. RESISTOR R1 AND LED CURRENT
LED Current (mA) 5 10 15 20 25 30 R1 (W) 60 30 20 15 12 10
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
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CAT4238
Open LED Protection In the event of an "Open LED" fault condition, the CAT4238 will continue to boost the output voltage with maximum power until the output voltage reaches approximately 40 V. 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 6 mW (about 1.6 mA input current with a 3.6 V supply). The SW pin clamps at a voltage below its maximum rating of 60 V. There is no need to use an external zener diode between Vout and the FB pin. A 50 V rated C2 capacitor is required to prevent any overvoltage damage in the open LED condition.
L VIN C1 4.7 mF VIN SW CAT4238 OUTPUT VOLTAGE (V) OFF ON SHDN GND FB VFB = 300 mV R1 15 W 50 47 mH Schottky 100 V (Central CMSH1-100) VOUT C2 0.22 mF 5.0 4.0 3.0 2.0 1.0 0
SUPPLY CURRENT (mA)
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
Figure 20. Open LED Supply Current vs. VIN without Zener
55
45
Figure 18. Open LED Protection without Zener
40
35 SW 10 V/div
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
Figure 21. Open LED Output Voltage vs. VIN without Zener
2 ms/div
Figure 19. Open LED Switching Waveforms without Zener
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CAT4238
Dimming Control There are several methods available to control the LED brightness.
PWM Signal on the SHDN Pin
VIN SHDN GND FB PWM Signal 2.5 V 0V 3.73 kW RA C1 3.1 kW RB
SW
CAT4238
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 22 shows a 1 kHz signal with a 50% duty cycle applied to the SHDN pin. The recommended PWM frequency range is from 100 Hz to 2 kHz.
VFB = 300 mV 1 kW i R2
LED Current R1 15 W
0.22 mF
Figure 23. Circuit for Filtered PWM Signal
A PWM signal at 0 V DC, or a 0% duty cycle, results in a max LED current of about 22 mA. A PWM signal with a 93% duty cycle or more, results in an LED current of 0 mA.
25 20 15 10 5
Figure 22. Switching Waveform with 1 kHz PWM on SHDN Filtered PWM Signal
LED CURRENT (mA)
0
0
10
20
30
40
50
60
70
80
90 100
PWM DUTY CYCLE (%)
A filtered PWM signal used as a variable DC voltage can control the LED current. Figure 23 shows the PWM control circuitry connected to the CAT4238 FB pin. The PWM signal has a voltage swing of 0 V to 2.5 V. The LED current can be dimmed within a range from 0 mA to 20 mA. The PWM signal frequency can vary from very low frequency up to 100 kHz.
Figure 24. Filtered PWM Dimming (0 V to 2.5 V)
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CAT4238
Board Layout The CAT4238 is a high-frequency switching regulator. The traces that carry the high-frequency 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 25 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 25 corresponds to the current path when the CAT4238 internal switch is closed. On Figure 26 is shown the current loop, when the CAT4238 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 CAT4238 allows for direct connection of the capacitors to ground. The resistor R1 must be connected directly to the GND pin of the CAT4238 and not shared with the switching current loops and any other components.
Figure 25. Closed-switch Current Loop
Figure 26. Open-switch Current Loop
Figure 27. Recommended PCB Layout
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CAT4238
PACKAGE DIMENSIONS
TSOT-23, 5 LEAD CASE 419AE-01 ISSUE O SYMBOL
A A1 A2 b c D E1 E E E1 e L L1 L2 0.30 0.01 0.80 0.30 0.12 0.15 2.90 BSC 2.80 BSC 1.60 BSC 0.95 TYP 0.40 0.60 REF 0.25 BSC 0.50 0.05 0.87
D e
MIN
NOM
MAX
1.00 0.10 0.90 0.45 0.20
TOP VIEW
0
8
A2 A
q
b
A1 L1
L
c
L2
SIDE VIEW Notes: (1) All dimensions are in millimeters. Angles in degrees. (2) Complies with JEDEC MO-193.
END VIEW
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CAT4238
Example of Ordering Information (Note 4)
Prefix CAT Device # 4238 Suffix TD -G T3
Company ID (Optional)
Product Number 4238
Lead Finish G: NiPdAu
Tape & Reel (Note 6) T: Tape & Reel 3: 3,000 / Reel
Package TD: Thin SOT-23 (Lead-free, Halogen-free)
2. 3. 4. 5. 6.
All packages are RoHS-compliant (Lead-free, Halogen-free). The standard lead finish is NiPdAu. The device used in the above example is a CAT4238TD-GT3 (TSOT-23, NiPdAu, Tape & Reel, 3,000/Reel). For additional package and temperature options, please contact your nearest ON Semiconductor Sales office. For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging Specifications Brochure, BRD8011/D.
ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. "Typical" parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including "Typicals" must be validated for each customer application by customer's technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC 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 SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT: Literature Distribution Center for ON Semiconductor P.O. Box 5163, Denver, Colorado 80217 USA Phone: 303-675-2175 or 800-344-3860 Toll Free USA/Canada Fax: 303-675-2176 or 800-344-3867 Toll Free USA/Canada Email: orderlit@onsemi.com N. American Technical Support: 800-282-9855 Toll Free USA/Canada Europe, Middle East and Africa Technical Support: Phone: 421 33 790 2910 Japan Customer Focus Center Phone: 81-3-5773-3850 ON Semiconductor Website: www.onsemi.com Order Literature: http://www.onsemi.com/orderlit For additional information, please contact your local Sales Representative
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CAT4238/D

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