CAT4237
© Catalyst Semiconductor, Inc. 1 Doc. No. MD-5013, Rev. B
Characteristics subject to change without notice
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
RoHS-compliant
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
Part Number Package
Quantity
per Reel
Package
Marking
CAT4237TD-T3 TSOT23-5
(1) 3000 LT
CAT4237TD-GT3 TSOT23-5
(2) 3000 UD
Notes: (1) Matte-Tin Plated Finish (RoHS-compliant).
(2) NiPdAu Plated Finish (RoHS-compliant).
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.
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 (1mm max height) 5-lead thin
SOT23 package for space critical applications.
For Ordering Information details, see page 13.
PIN CONFIGURATION TYPICAL APPLICATION CIRCUIT
5-Lead Thin SOT23
(1mm max height)
Top View
L: Sumida CDRH3D16-330
D: Central CMDSH05-4 (rated 40V)
C2: Taiyo Yuden UMK212BJ224 (rated 50V)
High Voltage CMOS Boost White LED Driver
VIN
FB
2
3
1
SHDN
5
4
SW
GND
VIN
CAT4237
V
IN
LDV
OUT
R1
15Ω
20mA
V
FB
= 300mV
4.7µF
3V to
4.2V C
1
0.22µF
C
2
FBSHDN
SW
GND
ON
33µH
OFF
CAT4237
Doc. No. MD-5013, Rev. B 2 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
ABSOLUTE MAXIMUM RATINGS
Parameters Ratings Units
VIN, FB voltage -0.3 to +7 V
SHDN
¯¯¯¯¯ voltage -0.3 to +7 V
SW voltage -0.3 to 55 V
Storage Temperature Range -65 to +160 ºC
Junction Temperature Range -40 to +150 ºC
Lead Temperature 300 ºC
RECOMMENDED OPERATING CONDITIONS
Typical application circuit with external components are shown on page 1.
Parameters Range Units
VIN 2.8 to 5.5 V
SW pin voltage 0 to 30 V
Ambient Temperature Range -40 to +85 ºC
6, 7 or 8 LEDs 1 to 40 mA
Note:
(1) Thin SOT23-5 package thermal resistance θJA = 135°C/W when mounted on board over a ground plane.
DC ELECTRICAL 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
19
14.25
30
20
15
31.5
21
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
CAT4237
© Catalyst Semiconductor, Inc. 3 Doc. No. MD-5013, Rev. B
Characteristics subject to change without notice
PIN DESCRIPTION
VIN is the supply input for the internal logic 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 (4.7µF) 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
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.
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 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 DESCRIPTIONS
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
V
IN
V
IN
Enable
Current
Sense
300mV
R1
15Ω
LED
Current
4.7µF
C1
RCN1
R
S
C
C
SHDN
Thermal
Shutdown
& UVLO
V
REF
1MHz
Oscillator
Over Voltage
Protection
PWM &
Logic
Driver
0.22µF
C2
GND
SW
FB
33µ
H
+
–
+
–
+
–
A1
A2
CAT4237
Doc. No. MD-5013, Rev. B 4 © Catalyst Semiconductor, Inc.
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 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 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.
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.
Figure 1. Switching Waveform VIN = 4.2V,
ILED = 4mA
CAT4237
© Catalyst Semiconductor, Inc. 5 Doc. No. MD-5013, Rev. B
Characteristics subject to change without notice
TYPICAL CHARACTERISTICS
VIN = 3.6V, CIN = 4.7μF, COUT = 0.22µF, L = 33µH with 8 LEDs at 20mA, TAMB = 25ºC, unless otherwise specified.
Quiescent Current vs. VIN (Not Switching)
0
20
40
60
80
100
120
140
2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8
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
2.53.03.54.04.55.0
INP UT VO L TA GE [ V ]
SUPPLY CURRENT [mA]
FB pin voltage vs. Supply Voltage
285
290
295
300
305
310
315
2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8
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 Switching Waveforms
960
980
1000
1020
1040
2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8
INPUT VOLTAGE [V]
FREQUENCY [kHz]
CAT4237
Doc. No. MD-5013, Rev. B 6 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
TYPICAL CHARACTERISTICS
VIN = 3.6V, CIN = 4.7μF, COUT = 0.22µF, L = 33µH with 8 LEDs at 20mA, TAMB = 25ºC, unless otherwise specified.
LED Current vs. Input Voltage (8 LEDS)
0
5
10
15
20
25
30
35
2.5 3.0 3.5 4.0 4.5 5.0
INPUT VOLTAGE [V]
LED CURRENT [mA]
R
FB
= 10Ω
R
FB
= 15Ω
R
FB
= 20Ω
LED Current Regulation (20mA)
-1.0%
-0.5%
0.0%
0.5%
1.0%
3.0 3.3 3.6 3.9 4.2 4.5 4.8
INPUT VOLTAGE [V]
CURRENT VARIATION [%] .
8 LED Efficiency vs. Load Current
65
70
75
80
85
90
5 1015202530
L ED CU RREN T [mA]
EF FICIEN C Y [ %]
VIN = 3.6V
VIN = 4.2V
8 LEDs
VOUT ~ 27V at 20mA
L = 33uH
8 LED Efficiency vs. Input Voltage
65
70
75
80
85
90
3.0 3.5 4.0 4.5 5.0
INPUT VOLTAGE [V]
EF FICIENCY [ %]
15mA
20mA
8 LEDs
VOUT ~ 27V at 20mA
L = 33uH
7 LED Efficiency vs. Load Current
65
70
75
80
85
90
5 1015202530
LED CURRENT [mA]
EF FI C I EN C Y [% ]
VIN = 3.6V
VIN = 4.2V
7 LEDs
VOUT ~ 23V at 20mA
L = 33uH
6 LED Efficiency vs. Load Current
65
70
75
80
85
90
5 1015202530
L ED CU RREN T [mA]
EF FICIEN CY [% ]
VIN = 3.6V
VIN = 4.2V
6 LEDs
VOUT ~ 20V at 20mA
L = 33uH
CAT4237
© Catalyst Semiconductor, Inc. 7 Doc. No. MD-5013, Rev. B
Characteristics subject to change without notice
TYPICAL CHARACTERISTICS
VIN = 3.6V, CIN = 4.7μF, 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
2.5 3.0 3.5 4.0 4.5
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]
V
IN
=3.6V, 8LEDs
I
LED
=20mA
Shutdown Voltage vs. Input Voltage
0.2
0.4
0.6
0.8
1.0
3.0 3.5 4.0 4.5 5.0
INPUT VOLTAGE [V]
SHUTDOWN VOLTAGE [V]
125°
C
25°C
85°C
-40°C
Maximum Output Current vs. Input Voltage
0
20
40
60
80
100
120
140
2.53.03.54.04.55.0
INPUT VOLTAGE [V]
MAX OUTPUT CURRENT [mA]
VOUT = 15V
VOUT = 20V
CAT4237
Doc. No. MD-5013, Rev. B 8 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
APPLICATION INFORMATION
External Component Selection
Capacitors
The CAT4237 only requires small ceramic capacitors
of 4.7µF 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.
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.
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 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 recom–
mended 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
Table 1. Resistor R1 and LED current
LED current (mA) R1 (Ω)
5 60
10 30
15 20
20 15
25 12
30 10
CAT4237
© Catalyst Semiconductor, Inc. 9 Doc. No. MD-5013, Rev. B
Characteristics subject to change without notice
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
VIN
CAT4237
V
IN
L
Schottky 100V
(Central CMSH1-100)
V
OUT
R1
15Ω
V
FB
= 300mV
4.7µF
C
1
0.22µF
C
2
FBSHDN
SW
GND
OFF ON
33µH
Open LED Switching waveforms
without Zener
Open LED Supply Current vs. VIN
without Zener
0.0
0.5
1.0
1.5
2.0
2.5 3.0 3.5 4.0 4.5 5.0
INPUT VOLTAGE [V]
SU PPLY CURRENT [mA]
Open LED Output Voltage vs. VIN
without Zener
30
35
40
45
50
2.5 3.0 3.5 4.0 4.5 5.0
INP UT V O LTA GE [ V ]
OU T P U T V OLT AG E [V ]
CAT4237
Doc. No. MD-5013, Rev. B 10 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
DIMMING CONTROL
There are several methods available to control the
LED brightness.
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 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 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
0mA to 20mA. The PWM signal frequency can vary
from very low frequency up to 100kHz.
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.
FILTERED PWM DIMMING [0V to 2.5V]
LED CURRENT [mA]
25
20
15
10
5
0
0 102030405060708090100
PWM DUTY CYCLE [%]
V
FB
= 300mV
15Ω
1kΩ3.1kΩ3.73kΩ
0V
2.5V
0.22µF
C1
R
A
R
B
R
2
iR1
VIN
CAT4237
FB
SHDN
SW
GND
VIN
PWN
Signal
LED
Curren
t
CAT4237
© Catalyst Semiconductor, Inc. 11 Doc. No. MD-5013, Rev. B
Characteristics subject to change without notice
BOARD LAYOUT
The CAT4237 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 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.
closed
open
Figure 3. Closed-switch Current Loop Figure 4. Open-switch Current Loop
CAT4237
Doc. No. MD-5013, Rev. B 12 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
PACKAGE OUTLINES DRAWING
TSOT-23 5-Lead (TD) (1)(2)
Notes:
(1) All dimensions are in millimeters. Angles in degrees.
(2) Complies with JEDEC standard MO-229.
For current Tape and Reel information, download the PDF file from:
http://www.catsemi.com/documents/tapeandreel.pdf.
θ
E1 E
A2
A1
e
b
D
c
A
TOP VIEW
SIDE VIEW END VIEW
L1
LL2
SYMBOL MIN NOM MAX
A1.00
A1 0.01 0.05 0.10
A2 0.80 0.87 0.90
b 0.30 0.45
c 0.12 0.15 0.20
D2.90 BSC
E2.80 BSC
E1 1.60 BSC
e0.95TYP
L 0.30 0.40 0.50
L1 0.60 REF
L2 0.25 BSC
θ0º 8º
CAT4237
Doc. No. MD-5013, Rev. B 13 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
EXAMPLE OF ORDERING INFORMATION1
Notes:
(1) All packages are RoHS-compliant (Lead-free, Halogen-free).
(2) The standard lead finish is NiPdAu.
(3) The device used in the above example is a CAT4237TD–GT3 (TSOT-23, NiPdAu Plated Finish, Tape & Reel 3000).
(4) For Matte-Tin package option, please contact your nearest Catalyst Semiconductor Sales office.
Prefix Device # Suffix
CAT 4237 TD – G T3
Product Number
4237
Tape & Reel
T: Tape & Reel
3: 3000/Reel
Lead Finish
G: NiPdAu
Blank: Matte-Tin(4)
Package
TD: TSOT-23
Optional
Company ID
REVISION HISTORY
Date Rev. Reason
12/13/2005 00 Initial Issue
01/31/2007 A
Update Absolute Maximum Ratings
Update Recommended Operating Conditions
Change document number from 25094 to 5013, Rev. A
01/07/2008 B
Add NiPdAu lead finish
Add Extended Temperature range
Update Package Outline Drawing
Add Example of Ordering Information
Add “MD-“ to Document Number
Catalyst Semiconductor, Inc.
Corporate Headquarters
2975 Stender Way
Santa Clara, CA 95054
Phone: 408.542.1000 Document No: MD-5013
Fax: 408.542.1200 Revision: B
2Hwww.catsemi.com Issue date: 01
/
07
/
08
Copyrights, Trademarks and Pat ents
© Catalyst Semiconductor, Inc.
Trademarks and registered trademarks of Catalyst Semiconductor include each of the following:
Adaptive Analog™, Beyond Memory™, DPP™, EZDim™, LDD™, MiniPot™, Quad-Mode™ and Quantum Charge Programmable™
Catalyst Semiconductor has been issued U. S. and foreign patents and has patent applications pending that protect its products.
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.
