Catalyst CAT37 Cmos white led driver boost converter Datasheet

CAT37
CMOS White LED Driver Boost Converter
FEATURES
■ Low quiescent ground current (0.5mA typical)
■ Drives up to 4 White LEDs in series
■ Power efficiency over 80%
■ Shutdown current less than 1µA
■ Compatible pinout with LT1937
■ Load fault protection against open-circuits
■ Adjustable output current (up to 40mA)
■ Low value external components
■ High frequency 1.2MHz operation
■ Low profile 5-lead SOT23 (1mm max) package
■ Input voltage operation down to 2.5V
Ω) power switch
■ Low resistance (0.5Ω
APPLICATIONS
■ Color LCD and keypad backlighting
■ Digital cameras
■ Cellular phones
■ PDAs/Games
■ Handheld terminals
■ Portable MP3 players
DESCRIPTION
A high voltage output stage allows up to 4 White LEDs
to be driven in series. Series drive provides inherent
current matching.
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.
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.
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.
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
D1
VIN
3 V to 5 V
5
VIN
1
SW
CAT37
OFF ON
4
15mA
FB
SHDN
3
GND
2
SOT23 Pin Numbers
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
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
1
Doc. No. 5005, Rev. S
CAT37
ORDERING INFORMATION
Part Number
Package
Tube
Quantity
Reel
Quantity
Package
Marking Code
CAT37EKK-TE7
5-lead, thin SOT23
—
3000
BF
CAT37TDI-TE7
5-lead, thin SOT23
Lead Free
—
3000
LM
PIN CONFIGURATION
SW
1
GND
2
FB
3
5
VIN
4
SHDN
CAT37
5-Lead SOT-23
1mm maximum height
θ
[Free Air]
PIN DESCRIPTIONS
Part Number
Name
1
SW
Switch pin. This is the drain of the internal power switch. For
minimum EMI, minimize the trace area connected to this pin.
2
GND
Ground pin. Connect pin 2 to ground.
3
FB
4
SHDN
5
VIN
Doc. No. 5005, Rev. S
Function
LED (cathode) connection pin.
Shutdown pin.
Input supply pin. This pin should be bypassed with a
capacitor to ground. A 1µF capacitor mounted close to the
pin is recommended.
2
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
CAT37
ABSOLUTE MAXIMUM RATINGS
VIN , FB, SHDN Voltage ......................................... 8 V
Storage Temperature .......................... -65°C to 160°C
SW Voltage .......................................................... 20 V
ESD Rating - Human Body Model .................... 2000V
Junction Temperature ....................................... 125°C
Lead Soldering Temperature ............................ 300°C
RECOMMENDED OPERATING CONDITIONS
Parameter
Range
Unit
VIN
2.5 to 7
V
Ambient Temperature Range
-40 to +85
° C
Inductor L1
6.8 ±20% typical
µH
Input Capacitor C1
1.0 ±20% typical
µF
Output Capacitor C2
1.0 ±20% typical
µF
ILED with 1 to 4 LEDs in series
0 to 20
mA
ELECTRICAL SPECIFICATIONS
Over recommended operating conditions unless otherwise specified. TA = 25˚C, VIN = 3V and VSHDN = 3V.
Symbol
Parameter
Conditions
Input Voltage Range
Min
Typ
2.5
Max
Units
7
V
IQ
Quiescent Current
VFB = 0.2 V
0.5
0.7
mA
ISD
Shutdown Current
VSHDN = 0 V
0.05
1
µA
VFB
FB Pin Voltage
95
105
mV
IFB
FB Pin Leakage Current
1
2
µA
4 LEDs at 15 mA
Shutdown High Threshold
85
0.85
V
Shutdown Low Threshold
ISHDN
Shutdown Pin Current
0.25
V
1
nA
1.6
MHz
fSW
Boost Converter Frequency
0.8
1.2
DC
Maximum Switch
90
95
400
550
780
mA
ISW = 300 mA
150
200
mV
Switch Off, VSW = 5 V
0.01
5
µA
Figure 1 with
83
%
Duty Cycle
ISWL
Switch Current Limit
Switch Saturation Voltage
Switch Leakage Current
Efficiency
%
specified components
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
3
Doc No. 5005, Rev. S
CAT37
TYPICAL CHARACTERISTICS
VIN = 3.6 V,TAMB = 25˚C, CIN = 4.7 µF, COUT = 1µF, L = 6.8 µH, unless otherwise specified.
LED Current vs. Temperature
LED Current vs. Input Voltage
16.0
30
LED CURRENT [mA]
LED CURRENT [mA]
LEDs
33LEDs
20mA
20mA
20
10mA
10mA
10
15.5
15.0
4 4LEDs
LEDs
14.5
14.0
0
2
3
4
5
-50
6
-25
0
25
50
75
100
INPUT VOLTAGE [V]
FB Pin Voltage vs. LED Current
FB Pin Voltage vs. Temperature
100
105
FB PIN VOLTAGE [mV]
FB PIN VOLTAGE [mV]
110
LEDs
33 LEDs
100
95
90
85
5
10
15
LED CURRENT [mA]
90
85
80
-50
80
0
95
20
Switching Frequency vs. Input Voltage
SWITCHING FREQUENCY [MHz]
SWITCH FREQUENCY [MHz]
1.2
1.1
1.0
0.9
4 LEDs
4 LEDs
1
2
3
4
5
-25
0
25
50
75
100
Switching Frequency vs. Temperature
1.3
0.8
4 LEDs at
4LEDs
at15mA
15mA
VIN = 3.6V
VIN = 3.6V
6
1.3
1.2
1.1
LOAD
= mA
20mA
LOAD
=20
1.0
LOAD
VIN
LOAD= =3 LEDs
3 LEDs
=VIN
3.6V= 3.6V
0.9
0.8
-50
-25
0
25
50
75
100
INPUT VOLTAGE [V]
Doc. No. 5005, Rev. S
4
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
CAT37
TYPICAL CHARACTERISTICS
VIN = 3.6 V,TAMB = 25˚C, CIN = 4.7 µF, COUT = 1µF, L = 6.8 µH, unless otherwise specified.
Efficiency vs. Input Voltage
Efficiency vs. Temperature
86
90
84
EFFICIENCY [%]
85
EFFICIENCY [%]
LOAD = 20 mA
LOAD =20 mA
80
33
LEDS
at at
15mA
LEDs
15mA
VOUT = 10V
75
VOUT = 10V
70
65
82
LOAD
15 mA
LOAD
=15= mA
80
LOAD
= mA
10 mA
LOAD
=10
78
LOAD
=3=LEDs
VIN
LOAD
3 LEDs
= 3.6V
76
60
VIN = 3.6V
74
2
3
4
5
INPUT VOLTAGE [V]
6
-50
-25
0
25
50
TEMPERATURE [ºC]
75
100
PWM on SHDN pin at 1kHz
VSW, IL, & VOUT Waveforms
Enable Power-Up Waveforms
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
5
Doc No. 5005, Rev. S
CAT37
OPERATION
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.
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.
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.
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:
PWM dimming operation can be achieved by switching
the SHDN pin or by pulling the FB pin higher than 95mV.
ILED = 0.095/R1
BLOCK DIAGRAM
L1
VIN
C1
4
D1
5
1
VIN
SW
C2
1.2 MHz
Clock
SHDN
Over Voltage
Protection
(22V)
CAT37
FB
3
+
-
PWM
& Logic
Error
Amp
0.095V
REF
+
ILED
R1
Current
Sense
Amp
GND
2
Note: SOT23 Pin Numbers are shown
Figure 2. CAT37 Block Diagram
Doc. No. 5005, Rev. S
6
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
CAT37
APPLICATION INFORMATION
Table 2. Schottky Diode Suppliers
CAPACITOR SELECTION
Part
Supplier
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 1.0µF output capacitor is recommended
for most applications.
MBR0520
ON Semiconductor
MBR0530
www.onsemi.com
MBR0540
800.282.9855
ZHCS400
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.
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 µF or 4.7 µF
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 3. R1 Resistor Values
Table 1. Ceramic Capacitor Manufacturers
Supplier
Web
Taiyo Yuden
www.t-yuden.com
Murata
www.murata.com
Kemet
www.kemet.com
AVX
www.avxcorp.com
R1 (Ω)
40
30
20
15
12
10
5
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.
DIODE SELECTION
R1= 0.095
ILED
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.
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
ILED (mA)
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.
7
Doc No. 5005, Rev. S
CAT37
LED DIMMING WITH DC SIGNAL
LED DIMMING WITH A LOGIC 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 µA) 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.
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.
LED DIMMING WITH PWM SIGNAL
RINCR=
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.
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.
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.
VDC
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
CAT37
FB
FB
R3
18k
R2
1k
10k
PWM
0.095V
ILED(INCREASE)
CAT37
FB
R3
18k
R2
1k
RINCR
LOGIC
SIGNAL
Figure 3. Dimming with a
DC Voltage
Doc. No. 5005, Rev. S
Figure 4. Dimming with a Filtered
PWM Signal
8
2N7002
R1
Figure 5. Dimming with a
Logic Signal
© 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
Efficiency - Two LEDs
D1
90
VIN
3 V to 4.2 V
VIN = 3.6V
VIN
3.6V
85
1
SW
18k
CAT37
4
SHDN
VDC
Dimming
FB
1k
3
EFFICIENCY [%]
5
VIN
15mA
GND
2
VIN =
VIN
= 3.0V
3.0V
80
75
70
65
60
0
5
10
15
20
25
LED CURRENT [mA]
30
Efficiency - Three LEDs
Three LEDs with DC Level Dimming Control
D1
VIN
3 V to 4.2 V
90
1
SW
18k
CAT37
4
SHDN
FB
1k
3
VIN
= 3.6V
VIN=3.6V
85
VDC
Dimming
15mA
GND
2
EFFICIENCY [%]
5
VIN
80
VIN =VIN=3.0V
3.0V
75
70
65
60
0
Four LEDs with PWM Dimming Control
5
10
15
20
25
LED CURRENT [mA]
30
Efficiency - Four LEDs
D1
VIN
3 V to 4.2 V
90
PWM
SW
18k
CAT37
SHDN
FB
3
1k
15mA
EFFICIENCY [%]
1 10k
5
VIN
4
85
VIN
= 3.6V
VIN=3.6V
80
VIN=3.0V
VIN
= 3.0V
75
70
65
GND
2
60
0
5
10
15
20
25
30
LED CURRENT [mA]
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
9
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
1.75 MAX
1.50 MIN
0.20 MAX
0.09 MIN
0.90 MAX
0.80 MIN
1.00 MAX
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
10
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
CAT37
REVISION HISTORY
Date
Rev.
Reason
9/25/2003
H
Updated Ordering Information - part numers
Changed L1 for each figure from 22 µH to 6.8 µH
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 µF to 1 µF
Changed C2 for each typical application circuit from 2.2 µF to 1 µF
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
I
Updated Ordering Information - part numbers
11/6/2003
J
Updated Features
Changed R1 for Typical Application figure and Typical Application
Circuit figures from 6.34Ω to 6.8Ω
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
L
Updated FB Pin Leakage Current in Electrical Specifications
12/18/2003
M
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
O
Updated Electrical Specifications (changed VRSET to VFB in IQ
6/15/2004
P
Reoved Preliminary data sheet designation
9/22/2004
Q
Added Green Packages in all areas
11/1/2004
R
Eliminated TDFN (3x4.9mm) package
07/07/2005
S
Update Features
Updated Ordering Information
Update Description
Update Ordering Information
Update Pin Configuration
Update Pin Description
Update Block Diagram
Update Package Outlines
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
11
Doc No. 5005, Rev. S
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Publication #:
Revison:
Issue date:
5005
S
07/07/05