© Semiconductor Components Industries, LLC, 2011
November, 2011 − Rev. 3
1Publication Order Number:
CAT4237/D
CAT4237
High Voltage CMOS Boost
White LED Driver
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 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 from 2.8 V to 5.5 V, 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 (1 mm
max height) 5−lead thin SOT23 package for space critical
applications.
Features
•Drives 6 to 8 White LEDs in Series from 3 V
•Up to 87% Efficiency
•Low Quiescent Ground Current 0.6 mA
•Adjustable Output Current (up to 40 mA)
•High Frequency 1 MHz Operation
•High Voltage Power Switch
•Shutdown Current Less than 1 mA
•Open LED Low Power Mode
•Automatic Shutdown at 1.9 V (UVLO)
•Thermal Shutdown Protection
•Thin SOT23 5−lead (1 mm Max Height)
•These Devices are Pb−Free, Halogen Free/BFR Free and are RoHS
Compliant
Applications
•Color LCD and Keypad Backlighting
•Cellular Phones
•Handheld Devices
•Digital Cameras
•PDAs
•Portable Game Machine
http://onsemi.com
TSOT−23
TD SUFFIX
CASE 419AE
PIN CONNECTIONS
UDYM
MARKING DIAGRAMS
Device Package Shipping (Note 4)
ORDERING INFORMATION (Note 3)
CAT4237TD−T3
(Note 1)
TSOT−23
(Pb−Free)
3,000/
Tape & Reel
LT = CAT4237TD−T3
UD = CAT4237TD−GT3
Y = Production Year (Last Digit)
M = Production Month (1−9, A, B, C)
1
5
(Top View)
VIN
SHDN
SW
GND
FB
1
LTYM
CAT4237TD−GT3
(Note 2)
TSOT−23
(Pb−Free)
3,000/
Tape & Reel
1. Matte−Tin Plated Finish (RoHS−compliant).
2. NiPdAu Plated Finish (RoHS−compliant)
3. For detailed information and a breakdown of
device nomenclature and numbering systems,
please see the ON Semiconductor Device No-
menclature document, TND310/D, available at
www.onsemi.com
4. For information on tape and reel specifications, in-
cluding part orientation and tape sizes, please
refer to our Tape and Reel Packaging Specifica-
tions Brochure, BRD8011/D.
CAT4237
http://onsemi.com
2
Figure 1. Typical Application Circuit
L: Sumida CDRH3D16−330
D: Central CMDSH05−4 (rated 40 V)
C2: Taiyo Yuden UMK212BJ224 (rated 50 V)
VIN
CAT4237
LD
15 W
20 mA
4.7 mF
3 V to
4.2 V
0.22 mF
FB
SW
GND
ON
33 mH
OFF VFB = 300 mV
SHDN
VOUT
C1
VIN
C2
R1
Table 1. 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 _C
Junction Temperature Range −40 to +150 _C
Lead Temperature 300 _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.
Table 2. RECOMMENDED OPERATING CONDITIONS
Parameters Range Units
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
NOTE: Typical application circuit with external components is shown above.
5. Thin SOT23−5 package thermal resistance qJA = 135°C/W when mounted on board over a ground plane.
CAT4237
http://onsemi.com
3
Table 3. DC ELECTRICAL CHARACTERISTICS
(VIN = 3.6 V, ambient temperature of 25°C (over recommended operating conditions unless otherwise specified))
Symbol Parameter Conditions Min Typ Max Unit
IQOperating Current VFB = 0.2 V
VFB = 0.4 V (not switching)
0.6
0.1
1.5
0.6
mA
ISD Shutdown Current VSHDN = 0 V 0.1 1 mA
VFB FB Pin Voltage 8 LEDs with ILED = 20 mA 285 300 315 mV
IFB FB pin input leakage 1mA
ILED Programmed LED Current R1 = 10 W
R1 = 15 W
R1 = 20 W
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
FSW Switching Frequency 0.8 1.0 1.3 MHz
ILIM Switch Current Limit 350 450 600 mA
RSW Switch “On” Resistance ISW = 100 mA 1.0 2.0 W
ILEAK Switch Leakage Current Switch Off, VSW = 5 V 1 5 mA
Thermal Shutdown 150 °C
Thermal Hysteresis 20 °C
VUVLO Undervoltage Lockout (UVLO) Threshold 1.9 V
VOV-SW Overvoltage Threshold 35 V
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 35 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:
ILED +
0.3 V
R1
The lower LED cathode is connected to the FB pin.
Table 4. 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.
CAT4237
http://onsemi.com
4
Block Diagram
Enable
Current
Sense
300 mV
R1
15 W
LED
Current
4.7 mF
C1
Thermal
Shutdown
& UVLO
1 MHz
Oscillator
Over Voltage
Protection
PWM &
Logic
Driver
0.22 mF
C2
GND
SW
FB
33 mH
+
–
+
–
+
–
A1
A2
VIN
VIN
VREF
SHDN
RC
CC
RS
N1
Figure 2. Block Diagram
Device Operation
The CAT4237 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 in normal operation, the device can deliver up to
40 mA of load current into a string of up to 8 white LEDs.
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 35 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 60 ms and
keep it on for about 1 ms only.
Thermal overload protection circuitry has been included
to prevent the device from operating at unsafe junction
temperatures above 150°C. In the event of a thermal
overload condition the device will automatically shutdown
and wait till the junction temperatures cools to 130°C before
normal operation is resumed.
Light Load Operation
Under light load condition (under 4 mA) and with input
voltage above 4.2 V, the CAT4237 driving 6 LEDs, the
driver starts pulse skipping. Although the LED current
remains well regulated, some lower frequency ripple may
appear.
Figure 3. Switching Waveform VIN = 4.2 V,
ILED = 4 mA
CAT4237
http://onsemi.com
5
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.)
Figure 4. Quiescent Current vs. VIN
(Not Switching)
Figure 5. Quiescent Current vs. VIN
(Switching)
INPUT VOLTAGE (V) INPUT VOLTAGE (V)
4.84.2 4.53.93.63.33.02.7
0
20
40
60
80
100
120
140
5.04.54.03.53.02.5
0
0.5
1.0
1.5
2.0
Figure 6. FB Pin Voltage vs. Supply Voltage Figure 7. FB Pin Voltage vs. Output Current
INPUT VOLTAGE (V) OUTPUT CURRENT (mA)
4.5 4.84.23.93.63.33.02.7
285
290
295
300
305
310
315
302520151050
285
290
295
300
305
310
315
Figure 8. Switching Frequency vs. Supply
Voltage
Figure 9. Switching Waveforms
INPUT VOLTAGE (V)
0.5 msec/div
4.54.23.9 4.83.63.33.02.7
960
980
1000
1020
1040
INPUT CURRENT (mA)
SUPPLY CURRENT (mA)
FEEDBACK (mV)
FB PIN VOLTAGE (mV)
FREQUENCY (kHz)
VFB = 0.4 V
(not switching)
8 LEDs at 20 mA
VOUT = 26 V
SW pin
20V/div
Inductor
Current
100mA/div
VOUT
AC coupled
200mV/div
8 LEDs
CAT4237
http://onsemi.com
6
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.)
Figure 10. LED Current vs. Input Voltage
(8 LEDs)
Figure 11. LED Current Regulation (20 mA)
INPUT VOLTAGE (V) INPUT VOLTAGE (V)
5.04.54.03.53.02.5
0
5
10
15
20
25
30
35
4.84.54.23.93.63.33.0
−1.0
−0.5
0
0.5
1.0
Figure 12. 8 LED Efficiency vs. Load Current Figure 13. 8 LED Efficiency vs. Input Voltage
LED CURRENT (mA) INPUT VOLTAGE (V)
30252015105
65
70
75
80
85
90
5.04.54.03.53.0
65
70
75
80
85
90
Figure 14. 7 LED Efficiency vs. Load Current Figure 15. 6 LED Efficiency vs. Load Current
LED CURRENT (mA) LED CURRENT (mA)
30252015105
65
70
75
80
85
90
30252015105
65
70
75
80
85
90
LED CURRENT (mA)
CURRENT VARIATION (%)
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
RFB = 10 W
RFB = 15 W
RFB = 20 W
VIN = 4.2 V
VIN = 3.6 V
8 LEDs
VOUT ~ 27 V at 20 mA
L = 33 mH
15 mA
20 mA
8 LEDs
VOUT ~ 27 V at 20 mA
L = 33 mH
7 LEDs
VOUT ~ 23 V at 20 mA
L = 33 mH
VIN = 4.2 V
VIN = 3.6 V
6 LEDs
VOUT ~ 20 V at 20 mA
L = 33 mH
VIN = 4.2 V
VIN = 3.6 V
CAT4237
http://onsemi.com
7
TYPICAL CHARACTERISTICS
(VIN = 3.6 V, CIN = 4.7 mF, COUT = 0.22 mF, L = 33 mH with 8 LEDs at 20 mA, TAMB = 25°C, unless otherwise specified.)
Figure 16. Power−up with 8 LEDs at 20 mA Figure 17. Switch ON Resistance vs. Input
Voltage
50 msec/div INPUT VOLTAGE (V)
4.54.03.53.02.5
0
0.5
1.0
1.5
2.0
Figure 18. FB Pin Voltage vs. Temperature Figure 19. Shutdown Voltage vs. Input Voltage
TEMPERATURE (°C) INPUT VOLTAGE (V)
150100500−50
297
298
299
300
301
302
303
5.04.54.03.53.0
0.2
0.4
0.6
0.8
1.0
Figure 20. Maximum Output Current vs. Input
Voltage
INPUT VOLTAGE (V)
5.04.54.03.53.02.5
0
20
40
60
80
100
120
140
SWITCH RESISTANCE (W)
FEEDBACK VOLTAGE (mV)
SHUTDOWN VOLTAGE (V)
MAX OUTPUT CURRENT (mA)
EN
5V/div
VOUT
10V/div
Input
Current
100mA/
div
VIN = 3.6 V, 8 LEDs
ILED = 20 mA
25°C
−40°C
85°C
125°C
VOUT = 15 V
VOUT = 20 V
CAT4237
http://onsemi.com
8
Application Information
External Component Selection
Capacitors
The CAT4237 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
A 33 mH 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 300 mA or higher
are recommended for most applications. Sumida
CDRH3D16−330 33 mH inductor has a rated current of
320 mA and a series resistance (D.C.R.) of 520 mW 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 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
LED current
Table 5. RESISTOR R1 AND LED CURRENT
LED Current (mA) R1 (W)
5 60
10 30
15 20
20 15
25 12
30 10
CAT4237
http://onsemi.com
9
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 35 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 4 mW (about 1 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.
Figure 21. Open LED Protection without Zener
VIN
CAT4237
L
Schottky 100 V
(Central CMSH1−100)
R1
15 W
4.7 mF0.22 mF
FB
SW
GND
OFF ON
33 mH
VIN
C1
VOUT
SHDN VFB = 300 mV
C2
Figure 22. Open LED Switching Waveforms without
Zener
10 msec/div
SW PIN
10 V/div
Figure 23. Open LED Supply Current vs. VIN without
Zener
INPUT VOLTAGE (V)
5.04.54.03.53.02.5
0
0.5
1.0
1.5
2.0
SUPPLY CURRENT (mA)
Figure 24. Open LED Output Voltage vs. VIN without
Zener
INPUT VOLTAGE (V)
5.04.54.03.53.02.5
30
35
40
45
50
OUTPUT VOLTAGE (V)
CAT4237
http://onsemi.com
10
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 25
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.
Figure 25. Switching Waveform with 1 kHz PWM on
SHDN
Filtered PWM Signal
A filtered PWM signal used as a variable DC voltage can
control the LED current. Figure 26 shows the PWM control
circuitry connected to the CAT4237 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 26. Circuit for Filtered PWM Signal
1 kW
3.1 kW
0 V
2.5 V
0.22 mF
C1 i
VIN
CAT4237
FB
SW
GND
PWN
Signal LED
Current
RB
3.73 kW
SHDN
R1
15 W
R2
RA
VFB = 300 mV
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.
Figure 27. Filtered PWM Dimming (0 V to 2.5 V)
LED CURRENT (mA)
25
20
15
10
5
0
0 10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE (%)
CAT4237
http://onsemi.com
11
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 28 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 28
corresponds to the current path when the CAT4237 internal
switch is closed. On Figure 29 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.
Figure 28. Closed−switch Current Loop Figure 29. Open−switch Current Loop
Closed Open
CAT4237
http://onsemi.com
12
PACKAGE DIMENSIONS
TSOT−23, 5 LEAD
CASE 419AE−01
ISSUE O
E1 E
A2
A1
e
b
D
c
A
TOP VIEW
SIDE VIEW END VIEW
L1
LL2
Notes:
(1) All dimensions are in millimeters. Angles in degrees.
(2) Complies with JEDEC MO-193.
SYMBOL
θ
MIN NOM MAX
q
A
A1
A2
b
c
D
E
E1
e
L
0º 8º
L1
L2
0.01
0.80
0.30
0.12
0.30
0.05
0.87
0.15
2.90 BSC
2.80 BSC
1.60 BSC
0.95 TYP
0.40
0.60 REF
0.25 BSC
1.00
0.10
0.90
0.45
0.20
0.50
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.
CAT4237/D
PUBLICATION ORDERING INFORMATION
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: 81−3−5817−1050
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
ON Semiconductor Website: www.onsemi.com
Order Literature: http://www.onsemi.com/orderlit
For additional information, please contact your local
Sales Representative
