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.
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5
1
TSOT−23
TD SUFFIX
CASE 419AE
PIN CONNECTIONS
1
VIN
SW
GND
SHDN
FB
(Top View)
MARKING DIAGRAMS
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
•
•
•
•
•
•
November, 2011 − Rev. 3
UDYM
LT = CAT4237TD−T3
UD = CAT4237TD−GT3
Y = Production Year (Last Digit)
M = Production Month (1−9, A, B, C)
ORDERING INFORMATION (Note 3)
Package
Shipping (Note 4)
CAT4237TD−T3
(Note 1)
TSOT−23
(Pb−Free)
3,000/
Tape & Reel
CAT4237TD−GT3
(Note 2)
TSOT−23
(Pb−Free)
3,000/
Tape & Reel
Device
1. Matte−Tin Plated Finish (RoHS−compliant).
2. NiPdAu Plated Finish (RoHS−compliant)
Color LCD and Keypad Backlighting
Cellular Phones
Handheld Devices
Digital Cameras
PDAs
Portable Game Machine
© Semiconductor Components Industries, LLC, 2011
LTYM
3. For detailed information and a breakdown of
device nomenclature and numbering systems,
please see the ON Semiconductor Device Nomenclature document, TND310/D, available at
www.onsemi.com
4. 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.
1
Publication Order Number:
CAT4237/D
CAT4237
L
VIN
3 V to
4.2 V
D
33 mH
C1
4.7 mF
C2
SW
VIN
VOUT
0.22 mF
CAT4237
OFF ON
SHDN
GND
FB
VFB = 300 mV
20 mA
R1
15 W
L: Sumida CDRH3D16−330
D: Central CMDSH05−4 (rated 40 V)
C2: Taiyo Yuden UMK212BJ224 (rated 50 V)
Figure 1. Typical Application Circuit
Table 1. ABSOLUTE MAXIMUM RATINGS
Ratings
Units
VIN, FB voltage
Parameters
−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
300
_C
Lead Temperature
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
VIN
SW pin voltage
Ambient Temperature Range
6, 7 or 8 LEDs
Range
Units
2.8 to 5.5
V
0 to 30
V
−40 to +85
_C
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.
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CAT4237
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
IQ
Operating 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
300
315
mV
IFB
FB pin input leakage
1
mA
ILED
Programmed LED Current
VIH
VIL
R1 = 10 W
R1 = 15 W
R1 = 20 W
SHDN Logic High
SHDN Logic Low
Enable Threshold Level
Shutdown Threshold Level
285
28.5
19
14.25
30
20
15
31.5
21
15.75
mA
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:
I LED +
0.3 V
R1
The lower LED cathode is connected to the FB pin.
Table 4. PIN DESCRIPTIONS
Pin #
Name
Function
1
SW
2
GND
Ground pin. Connect the pin to the ground plane.
3
FB
Feedback pin. Connect to the last LED cathode.
4
SHDN
5
VIN
Switch pin. This is the drain of the internal power switch.
Shutdown pin (Logic Low). Set high to enable the driver.
Power Supply input.
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CAT4237
Block Diagram
33 mH
VIN
SW
C2
C1
4.7 mF
0.22 mF
1 MHz
Oscillator
300 mV
–
A1
+
Enable
SHDN
Thermal
Shutdown
& UVLO
Driver
+
RC
LED
Current
PWM &
Logic
A2
–
N1
CC
RS
Current
Sense
GND
–
VREF
+
VIN
Over Voltage
Protection
FB
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.
R1
15 W
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
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CAT4237
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.)
140
2.0
SUPPLY CURRENT (mA)
INPUT CURRENT (mA)
120
100
80
60
VFB = 0.4 V
(not switching)
40
1.5
1.0
0.5
20
0
2.7
3.0
3.3
3.6
3.9
4.2
4.5
0
4.8
3.5
4.0
4.5
INPUT VOLTAGE (V)
Figure 4. Quiescent Current vs. VIN
(Not Switching)
Figure 5. Quiescent Current vs. VIN
(Switching)
8 LEDs at 20 mA
VOUT = 26 V
FB PIN VOLTAGE (mV)
310
305
300
295
8 LEDs
305
300
295
290
290
2.7
3.0
3.3
3.6
3.9
4.2
4.5
285
4.8
0
5
10
15
20
25
INPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
Figure 6. FB Pin Voltage vs. Supply Voltage
Figure 7. FB Pin Voltage vs. Output Current
FREQUENCY (kHz)
1040
SW pin
20V/div
1020
Inductor
Current
100mA/div
1000
980
960
5.0
315
310
FEEDBACK (mV)
3.0
INPUT VOLTAGE (V)
315
285
2.5
VOUT
AC coupled
200mV/div
2.7
3.0
3.3
3.6
3.9
4.2
4.5
4.8
0.5 msec/div
INPUT VOLTAGE (V)
Figure 8. Switching Frequency vs. Supply
Voltage
Figure 9. Switching Waveforms
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30
CAT4237
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.)
35
1.0
RFB = 10 W
CURRENT VARIATION (%)
LED CURRENT (mA)
30
25
RFB = 15 W
20
RFB = 20 W
15
10
5
0
2.5
3.0
3.5
4.0
4.5
−0.5
3.0
3.3
3.6
3.9
Figure 10. LED Current vs. Input Voltage
(8 LEDs)
Figure 11. LED Current Regulation (20 mA)
4.8
90
VIN = 4.2 V
EFFICIENCY (%)
75
8 LEDs
VOUT ~ 27 V at 20 mA
L = 33 mH
5
10
15
20 mA
85
VIN = 3.6 V
70
15 mA
80
75
8 LEDs
VOUT ~ 27 V at 20 mA
L = 33 mH
70
20
25
65
30
3.0
3.5
4.0
4.5
LED CURRENT (mA)
INPUT VOLTAGE (V)
Figure 12. 8 LED Efficiency vs. Load Current
Figure 13. 8 LED Efficiency vs. Input Voltage
5.0
90
90
VIN = 4.2 V
VIN = 4.2 V
85
85
VIN = 3.6 V
EFFICIENCY (%)
EFFICIENCY (%)
4.5
INPUT VOLTAGE (V)
80
80
75
7 LEDs
VOUT ~ 23 V at 20 mA
L = 33 mH
70
65
4.2
INPUT VOLTAGE (V)
85
EFFICIENCY (%)
0
−1.0
5.0
90
65
0.5
5
10
15
VIN = 3.6 V
80
75
6 LEDs
VOUT ~ 20 V at 20 mA
L = 33 mH
70
20
25
65
30
5
10
15
20
25
LED CURRENT (mA)
LED CURRENT (mA)
Figure 14. 7 LED Efficiency vs. Load Current
Figure 15. 6 LED Efficiency vs. Load Current
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30
CAT4237
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.)
2.0
SWITCH RESISTANCE (W)
EN
5V/div
VOUT
10V/div
Input
Current
100mA/
div
1.5
1.0
0.5
0
3.5
4.0
INPUT VOLTAGE (V)
Figure 16. Power−up with 8 LEDs at 20 mA
Figure 17. Switch ON Resistance vs. Input
Voltage
4.5
1.0
SHUTDOWN VOLTAGE (V)
302
301
300
299
VIN = 3.6 V, 8 LEDs
ILED = 20 mA
298
0
50
100
25°C
85°C
0.6
125°C
0.4
0.2
150
−40°C
0.8
3.0
3.5
4.0
4.5
5.0
TEMPERATURE (°C)
INPUT VOLTAGE (V)
Figure 18. FB Pin Voltage vs. Temperature
Figure 19. Shutdown Voltage vs. Input Voltage
140
MAX OUTPUT CURRENT (mA)
FEEDBACK VOLTAGE (mV)
3.0
50 msec/div
303
297
−50
2.5
120
100
VOUT = 15 V
80
60
VOUT = 20 V
40
20
0
2.5
3.0
3.5
4.0
4.5
INPUT VOLTAGE (V)
Figure 20. Maximum Output Current vs. Input
Voltage
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5.0
CAT4237
Application Information
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.
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.
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 +
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.
0.3 V
current
LED
Table 5. RESISTOR R1 AND LED CURRENT
LED Current (mA)
R1 (W)
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
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8
5
60
10
30
15
20
20
15
25
12
30
10
CAT4237
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.
C1
SW
VIN
SUPPLY CURRENT (mA)
0.5
2.5
3.0
3.5
4.0
4.5
5.0
Figure 23. Open LED Supply Current vs. VIN without
Zener
C2
4.7 mF
1.0
INPUT VOLTAGE (V)
VOUT
33 mH
1.5
0
Schottky 100 V
(Central CMSH1−100)
L
VIN
2.0
0.22 mF
50
OFF ON
SHDN
GND
FB
OUTPUT VOLTAGE (V)
CAT4237
VFB = 300 mV
R1
15 W
Figure 21. Open LED Protection without Zener
45
40
35
30
2.5
3.0
3.5
4.0
4.5
5.0
INPUT VOLTAGE (V)
Figure 24. Open LED Output Voltage vs. VIN without
Zener
SW PIN
10 V/div
10 msec/div
Figure 22. Open LED Switching Waveforms without
Zener
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CAT4237
Dimming Control
There are several methods available to control the LED
brightness.
VIN
SW
CAT4237
PWM Signal on the SHDN Pin
SHDN
GND FB
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.
PWN
Signal
2.5 V
3.73 kW
3.1 kW
RA
RB
0V
C1
VFB = 300 mV
1 kW
R2
i
LED
Current
R1
15 W
0.22 mF
Figure 26. 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.
LED CURRENT (mA)
25
20
15
10
5
Figure 25. Switching Waveform with 1 kHz PWM on
SHDN
0
0
10
20
30
40
50
60
70
80
90 100
PWM DUTY CYCLE (%)
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 27. Filtered PWM Dimming (0 V to 2.5 V)
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CAT4237
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.
Open
Closed
Figure 28. Closed−switch Current Loop
Figure 29. Open−switch Current Loop
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CAT4237
PACKAGE DIMENSIONS
TSOT−23, 5 LEAD
CASE 419AE−01
ISSUE O
SYMBOL
D
MIN
NOM
A1
0.01
0.05
0.10
A2
0.80
0.87
0.90
b
0.30
c
0.12
A
e
E1
1.00
0.45
0.15
D
2.90 BSC
E
2.80 BSC
E1
1.60 BSC
E
MAX
e
0.20
0.95 TYP
L
0.30
0.40
L1
0.60 REF
L2
0.25 BSC
0º
θ
0.50
8º
TOP VIEW
A2 A
b
q
L
A1
c
L2
L1
SIDE VIEW
END VIEW
Notes:
(1) All dimensions are in millimeters. Angles in degrees.
(2) Complies with JEDEC MO-193.
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
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CAT4237/D
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