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CAT37
CMOS White LED Driver Boost Converter FEATURES
I Low quiescent ground current (0.5mA typical) I Power efficiency over 80% I Compatible pinout with LT1937 I Adjustable output current (up to 40mA) I High frequency 1.2MHz operation I Input voltage operation down to 2.5V I Low resistance (0.5) power switch I Drives up to 4 White LEDs in series I Shutdown current less than 1A I Load fault protection against open-circuits I Low value external components I Low profile 5-lead SOT23 (1mm max) package
APPLICATIONS
I Color LCD and keypad backlighting I Cellular phones I Handheld terminals I Digital cameras I PDAs/Games I Portable MP3 players
DESCRIPTION
The CAT37 is a DC/DC step up converter that delivers a regulated output current. Operation at a constant switching frequency of 1.2MHz allows the device to be used with very small value external inductor and ceramic capacitors. The CAT37 is targeted to drive multiple white lightemitting diodes (LEDs) connected in series and provides the necessary regulated current to control the brightness and the color purity. An external resistor R1 controls the output current level. LED currents of up to 40mA can be supported over a wide range of input supply voltages from 2.5V to 7V, making the device ideal for batterypowered applications. A high voltage output stage allows up to 4 White LEDs to be driven in series. Series drive provides inherent current matching. 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 "near zero" quiescent current. In addition to overcurrent limiting protection, the device also includes detection circuitry to ensure protection against open-circuit load fault conditions. The device is available in a low profile (1mm max height) 5-lead thin SOT23 package.
TYPICAL APPLICATION
VIN 3 V to 5 V D1
5 VIN
1 SW
CAT37
OFF ON 4 SHDN FB GND 2
SOT23 Pin Numbers
15mA 3
C1: Taiyo Yuden JMK212BJ475 C2: Taiyo Yuden EMK212BJ105 D1: Zetez ZHCS400 L1: Panasonic ELJEA6R8 or equivalent
Figure 1. Driver for Four High-Brightness White LEDs
(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
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Doc. No. 5005, Rev. S
CAT37
ORDERING INFORMATION
Part Number CAT37EKK-TE7 CAT37TDI-TE7 Package 5-lead, thin SOT23 5-lead, thin SOT23 Lead Free Tube Quantity -- -- Reel Quantity 3000 3000 Package Marking Code BF LM
PIN CONFIGURATION
SW GND FB 1 2 3 CAT37 4 SHDN 5 VIN
5-Lead SOT-23 1mm maximum height
[Free Air]
PIN DESCRIPTIONS
Part Number 1 2 3 4 5 Name SW GND FB SHDN VIN Function Switch pin. This is the drain of the internal power switch. For minimum EMI, minimize the trace area connected to this pin. Ground pin. Connect pin 2 to ground. LED (cathode) connection pin. Shutdown pin. Input supply pin. This pin should be bypassed with a capacitor to ground. A 1F capacitor mounted close to the pin is recommended.
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(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
CAT37
ABSOLUTE MAXIMUM RATINGS
VIN , FB, SHDN Voltage ......................................... 8 V SW Voltage .......................................................... 20 V Junction Temperature ....................................... 125C Storage Temperature .......................... -65C to 160C ESD Rating - Human Body Model .................... 2000V Lead Soldering Temperature ............................ 300C
RECOMMENDED OPERATING CONDITIONS
Parameter VIN Ambient Temperature Range Inductor L1 Input Capacitor C1 Output Capacitor C2 ILED with 1 to 4 LEDs in series Range 2.5 to 7 -40 to +85 6.8 20% typical 1.0 20% typical 1.0 20% typical 0 to 20 Unit V C H F F mA
ELECTRICAL SPECIFICATIONS
Over recommended operating conditions unless otherwise specified. TA = 25C, VIN = 3V and VSHDN = 3V.
Symbol Parameter Input Voltage Range IQ ISD VFB IFB Quiescent Current Shutdown Current FB Pin Voltage FB Pin Leakage Current Shutdown High Threshold Shutdown Low Threshold ISHDN fSW DC Shutdown Pin Current Boost Converter Frequency Maximum Switch Duty Cycle ISWL Switch Current Limit Switch Saturation Voltage Switch Leakage Current Efficiency ISW = 300 mA Switch Off, VSW = 5 V Figure 1 with specified components 400 550 150 0.01 83 780 200 5 mA mV A % 0.8 90 1.2 95 0.85 0.25 1 1.6 VFB = 0.2 V VSHDN = 0 V 4 LEDs at 15 mA 85 Conditions Min 2.5 0.5 0.05 95 1 Typ Max 7 0.7 1 105 2 Units V mA A mV A V V nA MHz %
(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
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Doc No. 5005, Rev. S
CAT37
TYPICAL CHARACTERISTICS
VIN = 3.6 V,TAMB = 25C, CIN = 4.7 F, COUT = 1F, L = 6.8 H, unless otherwise specified.
LED Current vs. Input Voltage 30 33 LEDs LEDs
20mA 20mA
LED Current vs. Temperature 16.0
LED CURRENT [mA]
LED CURRENT [mA]
15.5
20
15.0
4 4 LEDs LEDs
10mA 10mA
10
14.5
0 2 3 4 5 6
INPUT VOLTAGE [V]
14.0 -50 -25 0 25 50 75 100
FB Pin Voltage vs. LED Current
110 100
FB Pin Voltage vs. Temperature
FB PIN VOLTAGE [mV]
FB PIN VOLTAGE [mV]
105 100 95 90 85 80 0 5 10 15 LED CURRENT [mA] 20 3 LEDs 3 LEDs
95
90
85
4 LEDs at 15mA 4LEDs at 15mA VIN = 3.6V VIN = 3.6V -25 0 25 50 75 100
80 -50
Switching Frequency vs. Input Voltage
Switching Frequency vs. Temperature
SWITCHING FREQUENCY [MHz]
1.3
1.3 1.2 1.1 LOAD = 20mA LOAD =20 mA 1.0 0.9 0.8 -50 -25 0 25 50 75 100
LOAD = = LEDs VIN LOAD 3 3 LEDs =VIN = 3.6V 3.6V
SWITCH FREQUENCY [MHz]
1.2 1.1 1.0 0.9 4 LEDs 4 LEDs 0.8 1 2 3 4 5 6
INPUT VOLTAGE [V]
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(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
CAT37
TYPICAL CHARACTERISTICS
VIN = 3.6 V,TAMB = 25C, CIN = 4.7 F, COUT = 1F, L = 6.8 H, unless otherwise specified.
Efficiency vs. Input Voltage 90 85 86 84 Efficiency vs. Temperature
LOAD =20 mA
LOAD = 20 mA
EFFICIENCY [%]
EFFICIENCY [%]
80 75 70 65 60 2 3 4 5 INPUT VOLTAGE [V] 6
3 3 LEDs at 15mA LEDS at 15mA VOUT = 10V
82 80 78 76 74 -50 -25
LOAD =3= 3 LEDs LOAD LEDs VIN = 3.6V
LOAD = 15 LOAD =15 mA mA LOAD = 10 LOAD =10 mA mA
VOUT = 10V
VIN = 3.6V
0 25 50 TEMPERATURE [C]
75
100
VSW, IL, & VOUT Waveforms
PWM on SHDN pin at 1kHz
Enable Power-Up Waveforms
(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
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Doc No. 5005, Rev. S
CAT37
OPERATION
The CAT37 device is a high efficiency, constant frequency, current regulating boost driver for white LEDs. The device includes a switch and an internally compensated loop for the regulation of the current in the LEDs. Operation can be best understood by examining the block diagram. The FB pin is regulated at 95 mV and the current through the external resistor will set the regulated current in the LEDs at: ILED = 0.095/R1 While maintaining LED current regulation, the CAT37 boost converter automatically adjusts the FB pin voltage to be as low as possible. A low FB pin voltage ensures high efficiency. Current through the internal power switch is continuously monitored cycle-by-cycle. If the current limit is exceeded, the switch is immediately turned off, protecting the device, for the remainder of the cycle. PWM dimming operation can be achieved by switching the SHDN pin or by pulling the FB pin higher than 95mV.
BLOCK DIAGRAM
L1 VIN C1 5 VIN 4 SHDN
Over Voltage Protection (22V)
D1 1 SW 1.2 MHz Clock FB 3
0.095V REF
C2
PWM & Logic
ILED
Error Amp
CAT37
+ Current Sense Amp
GND 2
Note: SOT23 Pin Numbers are shown
Figure 2. CAT37 Block Diagram
Doc. No. 5005, Rev. S
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+ -
R1
(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
CAT37
APPLICATION INFORMATION
CAPACITOR SELECTION Low ESR (equivalent series resistance) capacitors should be used at the output to minimize the output ripple voltage. The low ESR and small package options available with multilayer ceramic capacitors make them excellent choices. The X5R and X7R capacitor types are preferred because they retain their capacitance over wider voltage and temperature ranges than the Y5V or Z5U types. A 1.0F output capacitor is recommended for most applications. The voltage rating of the output capacitor C2 depends on the number of LEDs driven in the series. A 16 V ceramic capacitor is recommended when driving 3 or 4 LEDs. Low profile ceramic capacitors with a 1mm maximum height/thickness are available for designs height requirements. Ceramic capacitors also make a good choice for the input capacitor, which should be mounted as close as possible to the CAT37. A 1 F or 4.7 F input capacitor is recommended. Table 1 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers for detailed information as new products and package options are introduced regularly. Table 1. Ceramic Capacitor Manufacturers Supplier Taiyo Yuden Murata Kemet AVX DIODE SELECTION Schottky diodes, with their low forward voltage drop and fast switching speed, are the ideal choice for high efficiency applications. Table 2 shows several different Schottky diodes that work well with the CAT37. Make sure that the diode has a voltage rating greater than the output voltage. The diode conducts current only when the power switch is turned off (typically less than onethird the time), so a 0.4A or 0.5A diode will be sufficient for most designs. Web www.t-yuden.com www.murata.com www.kemet.com www.avxcorp.com
Table 2. Schottky Diode Suppliers Part MBR0520 MBR0530 MBR0540 ZHCS400 Supplier ON Semiconductor www.onsemi.com 800.282.9855 Zetex
LED CURRENT PROGRAMMING The LED current is programmed with a single resistor connected to the FB pin. The FB pin is internally regulated to 95 mV, which sets the current flowing through R1 and the LEDs as equal to 0.095/R1. For the best accuracy, a 1% or better resistor is recommended. Table 3 shows several typical 1% R1 values. Table 3. R1 Resistor Values ILED (mA) 40 30 20 15 12 10 5 R1 () 2.37 3.16 4.75 6.34 7.87 9.53 19.1
For other LED current values, use the following equation to choose R1.
R1= 0.095 ILED
Most white LEDs are driven at maximum currents of 15mA to 20mA. Some higher power designs will use two parallel strings of LEDs for greater light output, resulting in 30mA to 40mA (two strings of 15mA to 20mA) flowing through the R1 resistor.
(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
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Doc No. 5005, Rev. S
CAT37
LED DIMMING WITH DC SIGNAL Dimming the LEDs can be done by applying a variable DC voltage as shown on Figure 3. As the VDC increase the voltage across R1 decreases and therefore lower the LED current. The resistors R2 and R3 must be large enough so that their current (tens of A) is much smaller than the LED current but much larger than the FB leakage current (IFB). When adjusting VDC between 0V and 2V, the resistors shown on Figure 3 will set the LED current between 0mA and 15mA. LED DIMMING WITH PWM SIGNAL PWM brightness control provides the widest dimming range (greater than 20:1). By turning the LEDs ON and OFF using the control signal, the LEDs operate at either zero or full current, but their average current changes with the PWM signal duty cycle. Typically, a 5kHz to 40kHz PWM signal is used. A filtered PWM signal is used to dim the LEDs, as shown in Figure 4. In addition to providing the widest dimming range, PWM brightness control also ensures the "purest" white LED color over the entire dimming range. The true color of a white LED changes with operating current, and is the "purest" white at a specific forward current, usually 15mA or 20mA. If the LED current is less than or more than this value, the emitted light becomes more blue. Applications involving color LCDs can find the blue tint objectionable.
LED DIMMING WITH A LOGIC SIGNAL For applications that need to adjust the LED brightness in discrete steps, a logic signal can be used as shown in Figure 5. R1 sets the minimum LED current value (when the NMOS switch is OFF):
R1= 0.095V ILED(MIN)
RINCR determines how much LED current increases when the external NMOS switch is turned ON.
0.095V ILED(INCREASE)
RINCR=
PCB LAYOUT GUIDELINES The CAT37 is a high-frequency switching regulator where proper PCB board layout and component placement can minimize noise and radiation and increase efficiency. To maximize efficiency, the CAT37 design has fast switch rise and fall times. To prevent radiation and high frequency resonance problems minimize the length and area of all traces connected to the SW pin and use a ground plane under the switching regulator. The SW pin, schottky diode and capacitor C2 signal path should be kept as short as possible. The ground connection for the R1 resistor should be tied directly to the GND pin and not be shared with other components.
CAT37 FB VDC R3 18k R2 1k 10k PWM R3 18k
CAT37 FB R2 1k LOGIC SIGNAL
CAT37 FB RINCR 2N7002 R1
Figure 3. Dimming with a DC Voltage
Figure 4. Dimming with a Filtered PWM Signal
Figure 5. Dimming with a Logic Signal
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(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
CAT37
TYPICAL APPLICATION CIRCUITS
The application diagrams are shown for the SOT23 pin numbers. Two LEDs with DC Level Dimming Control
D1 VIN 3 V to 4.2 V
Efficiency - Two LEDs
90 85
EFFICIENCY [%]
VIN 3.6V VIN = 3.6V
5 VIN
1 SW
VDC Dimming 18k
80 75 70 65 60 0 5
VIN = 3.0V VIN = 3.0V
CAT37
4 SHDN FB GND 2
3
1k
15mA
10 15 20 25 LED CURRENT [mA]
30
Three LEDs with DC Level Dimming Control
D1 VIN 3 V to 4.2 V
Efficiency - Three LEDs
90
EFFICIENCY [%]
5 VIN 1 SW VDC Dimming 18k FB GND 2 3 1k
85 80 75 70 65 60 0 5
VIN = 3.6V VIN=3.6V
VIN=3.0V VIN = 3.0V
CAT37
4 SHDN
15mA
10 15 20 25 LED CURRENT [mA]
30
Four LEDs with PWM Dimming Control
D1 VIN 3 V to 4.2 V PWM 5 VIN 1 10k SW
Efficiency - Four LEDs
90 85 VIN = 3.6V VIN=3.6V
EFFICIENCY [%]
80 75 70 65 60 0 5 10 15 20 25 30 LED CURRENT [mA] VIN=3.0V VIN = 3.0V
CAT37
4 SHDN FB GND 2
18k 3 1k
15mA
(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
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Doc No. 5005, Rev. S
CAT37
PACKAGE OUTLINES 5-LEAD THIN SOT-23 (TS, TD, GTD)
0.95 REF
0.50 MAX 0.35 MIN
PIN 1
3.00 MAX 2.60 MIN
1.90 REF
3.00 MAX 2.80 MIN 0.20 MAX 0.09 MIN 1.00 MAX 0.90 MAX 0.80 MIN
1.75 MAX 1.50 MIN
0.20 GAUGE PLANE 0.15 MAX 0.00 MIN 0.70 MAX 0.50 MIN 0.60 MAX 0.10 MIN
Note: 1. All dimensions are in millimeters.
Doc. No. 5005, Rev. S
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(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
CAT37
REVISION HISTORY
Date 9/25/2003 Rev. H Reason Updated Ordering Information - part numers Changed L1 for each figure from 22 H to 6.8 H Changed VIN for Typical Application from 2.7V to 4.2 Vto 3V to 5V Changed C1 for each typical application circuit from 4.7 F to 1 F Changed C2 for each typical application circuit from 2.2 F to 1 F Changed VIN for each for each typical application circuit from 1.8 V to 3 V to 3 V to 4.2 V 9/25/2003 11/6/2003 I J Updated Ordering Information - part numbers Updated Features Changed R1 for Typical Application figure and Typical Application Circuit figures from 6.34 to 6.8 Updated Description (2V to 2.5V input supply voltage) Updated Ordering Information (added package marking code) Updated Electrical Specifications Updated Table 4 (R1 Resistor Value) with new values Updated LED dimming with logic signal (0.095V to 0.105V) 12/8/2003 K Updated Typical Application figures Updated Electrical Specifications Updated Operation text Updated LED current programming text Updated Table 4 (R1 Resistor Value) with new values Updated LED dimming equations Updated Typical Application Circuit figures 12/10/2003 12/18/2003 L M Updated FB Pin Leakage Current in Electrical Specifications Added Typical Characteristics graphs (pages 4-6) Added Efficiency graphs to Typical Application Circuits Changed package drawing for 5-lead SOT-23 12/22/2003 N Updated Absolute Maximum Ratings Deleted temp range from Electrical Specifications Added Recommended Operating Conditions Updated Block Diagram 6/9/2004 6/15/2004 9/22/2004 11/1/2004 07/07/2005 O P Q R S Updated Electrical Specifications (changed VRSET to VFB in IQ Reoved Preliminary data sheet designation Added Green Packages in all areas Eliminated TDFN (3x4.9mm) package Updated Ordering Information Update Features Update Description Update Ordering Information Update Pin Configuration Update Pin Description Update Block Diagram Update Package Outlines
(c) 2005 by Catalyst Semiconductor, Inc. Characteristics subject to change without notice
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Doc No. 5005, Rev. S
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Publication #: Revison: Issue date:
5005 S 07/07/05

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