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 R
1
. 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
http://onsemi.com
5
1
TSOT−23
TD SUFFIX
CASE 419AE
PIN CONNECTIONS
1
SW
GND
FB
(Top View)
SHDN
VIN
MARKING DIAGRAMS
•
•
•
•
•
•
•
•
•
•
•
•
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
Color LCD and Keypad Backlighting
Cellular Phones
Handheld Devices
Digital Cameras
PDAs
Portable Game Machine
LTYM
UDYM
LT = CAT4237TD−T3
UD = CAT4237TD−GT3
Y = Production Year (Last Digit)
M = Production Month (1−9, A, B, C)
ORDERING INFORMATION
(Note 3)
Device
CAT4237TD−T3
(Note 1)
CAT4237TD−GT3
(Note 2)
Package
TSOT−23
(Pb−Free)
TSOT−23
(Pb−Free)
Shipping
(Note 4)
3,000/
Tape & Reel
3,000/
Tape & Reel
Applications
•
•
•
•
•
•
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.
©
Semiconductor Components Industries, LLC, 2011
November, 2011
−
Rev. 3
1
Publication Order Number:
CAT4237/D
CAT4237
V
IN
3 V to
4.2 V
C
1
VIN
L
33
mH
SW
D
C
2
0.22
mF
V
OUT
4.7
mF
CAT4237
OFF ON
SHDN
GND
FB
V
FB
= 300 mV
R
1
15
W
20 mA
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
Parameters
V
IN
, FB voltage
SHDN voltage
SW voltage
Storage Temperature Range
Junction Temperature Range
Lead Temperature
Ratings
−0.3
to +7
−0.3
to +7
−0.3
to +55
−65
to +160
−40
to +150
300
Units
V
V
V
_C
_C
_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
V
IN
SW pin voltage
Ambient Temperature Range
6, 7 or 8 LEDs
Range
2.8 to 5.5
0 to 30
−40
to +85
1 to 40
Units
V
V
_C
mA
NOTE: Typical application circuit with external components is shown above.
5. Thin SOT23−5 package thermal resistance
q
JA
= 135°C/W when mounted on board over a ground plane.
http://onsemi.com
2
CAT4237
Table 3. DC ELECTRICAL CHARACTERISTICS
(V
IN
= 3.6 V, ambient temperature of 25°C (over recommended operating conditions unless otherwise specified))
Symbol
I
Q
I
SD
V
FB
I
FB
I
LED
Parameter
Operating Current
Shutdown Current
FB Pin Voltage
FB pin input leakage
Programmed LED Current
R1 = 10
W
R1 = 15
W
R1 = 20
W
Enable Threshold Level
Shutdown Threshold Level
28.5
19
14.25
0.4
0.8
350
I
SW
= 100 mA
Switch Off, V
SW
= 5 V
30
20
15
0.8
0.7
1.0
450
1.0
1
150
20
1.9
35
Conditions
V
FB
= 0.2 V
V
FB
= 0.4 V (not switching)
V
SHDN
= 0 V
8 LEDs with I
LED
= 20 mA
285
Min
Typ
0.6
0.1
0.1
300
Max
1.5
0.6
1
315
1
31.5
21
15.75
1.5
1.3
600
2.0
5
Unit
mA
mA
mV
mA
mA
V
IH
V
IL
F
SW
I
LIM
R
SW
I
LEAK
SHDN Logic High
SHDN Logic Low
Switching Frequency
Switch Current Limit
Switch “On” Resistance
Switch Leakage Current
Thermal Shutdown
Thermal Hysteresis
V
MHz
mA
W
mA
°C
°C
V
V
V
UVLO
V
OV-SW
Undervoltage Lockout (UVLO) Threshold
Overvoltage Threshold
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
R
1
The lower LED cathode is connected to the FB pin.
Table 4. PIN DESCRIPTIONS
Pin #
1
2
3
4
5
Name
SW
GND
FB
SHDN
VIN
Function
Switch pin. This is the drain of the internal power switch.
Ground pin. Connect the pin to the ground plane.
Feedback pin. Connect to the last LED cathode.
Shutdown pin (Logic Low). Set high to enable the driver.
Power Supply input.
http://onsemi.com
3
CAT4237
Block Diagram
V
IN
C1
4.7
mF
1 MHz
Oscillator
V
REF
300 mV
–
A1
+
Enable
Thermal
Shutdown
& UVLO
R
C
C
C
R
S
+
GND
+
A2
–
PWM &
Logic
N
1
Over Voltage
Protection
Driver
33
mH
SW
C2
0.22
mF
V
IN
LED
Current
SHDN
Current
Sense
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/R
1
).
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.
http://onsemi.com
4
–
FB
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 V
IN
= 4.2 V,
I
LED
= 4 mA
CAT4237
TYPICAL CHARACTERISTICS
(V
IN
= 3.6 V, C
IN
= 4.7
mF,
C
OUT
= 0.22
mF,
L = 33
mH
with 8 LEDs at 20 mA, T
AMB
= 25°C, unless otherwise specified.)
140
120
SUPPLY CURRENT (mA)
INPUT CURRENT (mA)
100
80
60
40
20
0
2.7
3.0
3.3
3.6
3.9
4.2
4.5
4.8
0
2.5
3.0
3.5
4.0
4.5
5.0
VFB = 0.4 V
(not switching)
1.5
2.0
1.0
0.5
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 4. Quiescent Current vs. V
IN
(Not Switching)
315
310
FEEDBACK (mV)
305
300
295
290
285
8 LEDs at 20 mA
VOUT = 26 V
315
310
FB PIN VOLTAGE (mV)
Figure 5. Quiescent Current vs. V
IN
(Switching)
8 LEDs
305
300
295
290
285
2.7
3.0
3.3
3.6
3.9
4.2
4.5
4.8
0
5
10
15
20
25
30
INPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
Figure 6. FB Pin Voltage vs. Supply Voltage
1040
SW pin
20V/div
Figure 7. FB Pin Voltage vs. Output Current
FREQUENCY (kHz)
1020
1000
Inductor
Current
100mA/div
980
960
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
http://onsemi.com
5
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
