CAT4237TS [CATALYST]
High Voltage CMOS Boost White LED Driver; 高电压CMOS升压型白光LED驱动器
| 型号： | CAT4237TS |
| 厂家： | 
CATALYST SEMICONDUCTOR |
| 描述： | High Voltage CMOS Boost White LED Driver |
| 文件： | 总13页 (文件大小：533K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
CAT4237
High Voltage CMOS Boost White LED Driver
Features
Product Description
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Drives 6 to 8 White LEDs in series from 3V
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.
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 1mA
Open LED low power mode
Automatic Shutdown at 1.9V (UVLO)
Thermal shutdown protection
Thin SOT23 5-Lead (1mm max height)
Applications
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Color LCD and keypad Backlighting
Cellular Phones
Handheld Devices
Digital Cameras
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.
PDAs
Portable Game Machine
Ordering Information
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.
Part
Package
Quantity Package
per Reel Marking
Number
CAT4237TS TSOT23-5
CAT4237TD TSOT23-5
Lead Free1
3000
3000
LS
LT
Note 1: NiPdAu Lead Plating
Pin Configuration
Typical Application Circuit
Top View
5-Lead Thin SOT23
1mm max height
L: Sumida CDRH3D16-330
D: Central CMDSH05-4 (rated 40V)
C2: Taiyo Yuden UMK212BJ224 (rated 50V)
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
CAT4237
ABSOLUTE MAXIMUM RATINGS
Parameter
VIN, FB voltage
SHDN* voltage
SW voltage
Storage Temperature Range
Junction Temperature Range
Lead Temperature
Rating
-0.3 to +7
-0.3 to +7
-0.3 to 60
-65 to +160
-40 to +125
300
Unit
V
V
V
°C
°C
°C
RECOMMENDED OPERATING CONDITIONS
Typical application circuit with external components are shown on page 1.
Parameter
VIN
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
Unit
V
V
°C
mA
ELECTRICAL OPERATING CHARACTERISTICS
VIN = 3.6V, ambient temperature of 25ºC (over recommended operating conditions unless specified
otherwise)
Symbol Parameter
Conditions
VFB = 0.2V
VFB = 0.4V (not switching)
VSHDN* = 0V
8 LEDs with ILED=20mA
Min
Typ
0.6
0.1
0.1
300
Max
1.5
0.6
1
315
1
Unit
mA
IQ
Operating Current
ISD
VFB
IFB
Shutdown Current
FB Pin Voltage
FB pin input leakage
µA
mV
µA
285
28.5
19
14.25
30
20
15
31.5
21
15.75
R1 = 10W
R1 = 15W
R1 = 20W
ILED
Programmed LED Current
mA
VIH
VIL
SHDN* Logic High
SHDN* Logic Low
Enable Threshold Level
Shutdown Threshold Level
0.8
0.7
1.5
V
V
0.4
0.8
FSW
ILIM
Switching Frequency
Switch Current Limit
Switch “On” Resistance
Switch Leakage Current
Thermal Shutdown
1.0
450
1.0
1
1.3
600
2.0
5
MHz
mA
350
RSW
ILEAK
ISW = 100mA
W
µA
ºC
ºC
V
Switch Off, VSW = 5V
150
20
Thermal Hysteresis
VUVLO
Undervoltage Lockout
(UVLO) Threshold
1.9
VOV-SW Overvoltage Threshold
35
V
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
CAT4237
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 (4.7uF) be placed between the VIN
and GND pins near the device. If the supply
voltage drops below 1.9V, the device stops
switching.
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.
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.
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
GND is the ground reference pin. This pin
should be connected directly to the ground
place on the PCB.
The lower LED cathode is connected to the FB pin.
Pin #
Name
SW
GND
FB
SHDN*
VIN
Function
1
2
3
4
5
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.
BLOCK DIAGRAM
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
CAT4237
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.
every 60ms and keep it on for about 1ms
only.
Thermal overload protection circuitry has
been included to prevent the device from
operating at unsafe junction temperatures
above 150ºC.
overload condition
In the event of a thermal
the device will
automatically shutdown and wait till the
junction temperatures cools to 130ºC before
normal operation is resumed.
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).
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.
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
Switching Waveform VIN = 4.2V, ILED = 4mA
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
CAT4237
Typical Characteristics
VIN = 3.6V, CIN = 4.7uF, COUT = 0.22µF, L = 33µH with 8 LEDs at 20mA, TAMB=25ºC, unless otherwise specified.
Quiescent Current vs. VIN (Not Switching)
Quiescent Current vs. VIN (Switching)
140
2.0
120
100
80
1.5
1.0
0.5
0.0
60
40
VFB =0.4V
(not switching)
20
0
2.5
3.0
3.5
4.0
4.5
5.0
2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8
INPUT VOLTAGE [V]
INPUT VOLTAGE [V]
FB Pin Voltage vs. Supply Voltage
FB Pin Voltage vs. Output Current
315
315
8LEDs at 20mA
VOUT= 26V
310
310
8 LEDs
305
305
300
295
290
300
295
290
285
285
0
5
10
15
20
25
30
2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8
INPUT VOLTAGE [V]
OUTPUT CURRENT [mA]
Switching Frequency vs. Supply Voltage
Switching Waveforms
1040
1020
1000
980
960
2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8
INPUT VOLTAGE [V]
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
CAT4237
Typical Characteristics
VIN = 3.6V, CIN = 4.7uF, COUT = 0.22µF, L = 33µH, with 8 LEDs at 20mA, TAMB=25ºC, unless otherwise specified.
LED Current vs. Input Voltage (8 LEDS)
LED Current Regulation (20mA)
1.0%
35
30
RFB = 10
W
0.5%
0.0%
25
20
15
10
5
RFB = 15
W
RFB = 20
W
-0.5%
-1.0%
0
3.0 3.3 3.6 3.9 4.2 4.5 4.8
INPUT VOLTAGE [V]
2.5
3.0
3.5
4.0
4.5
5.0
INPUT VOLTAGE [V]
8 LED Efficiency vs. Load Current
8 LED Efficiency vs. Input Voltage
90
90
20mA
VIN = 4.2V
85
85
15mA
80
80
75
70
65
VIN = 3.6V
75
8 LEDs
8 LEDs
70
VOUT ~ 27V at 20mA
VOUT ~ 27V at 20mA
L = 33uH
L = 33uH
65
3.0
3.5
4.0
4.5
5.0
5
10
15
20
25
30
INPUT VOLTAGE [V]
LED CURRENT [mA]
7 LED Efficiency vs. Load Current
6 LED Efficiency vs. Load Current
90
90
VIN = 4.2V
VIN = 4.2V
85
85
VIN = 3.6V
VIN = 3.6V
80
80
75
75
7 LEDs
VOUT ~ 23V at 20mA
6 LEDs
VOUT ~ 20V at 20mA
70
70
L = 33uH
65
L = 33uH
65
5
10
15
20
25
30
5
10
15
20
25
30
LED CURRENT [mA]
LED CURRENT [mA]
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
CAT4237
Typical Characteristics
VIN = 3.6V, CIN = 4.7uF, 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
2.0
1.5
1.0
0.5
0.0
2.5
3.0
3.5
4.0
4.5
INPUT VOLTAGE [V]
FB pin voltage vs. Temperature
303
Shutdown Voltage vs. Input Voltage
1.0
302
301
300
299
298
297
-40°C
25°C
0.8
0.6
0.4
0.2
C
125°
4.5
85°C
VIN=3.6V, 8LEDs
ILED=20mA
3.0
3.5
4.0
5.0
-50
0
50
100
150
INPUT VOLTAGE [V]
TEMPERATURE [ºC]
Maximum Output Current vs. Input Voltage
140
120
VOUT = 15V
100
80
60
40
VOUT = 20V
20
0
2.5
3.0
3.5
4.0
4.5
5.0
INPUT VOLTAGE [V]
© 2005 Catalyst Semiconductor, Inc.
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Characteristics subject to change without notice
CAT4237
APPLICATION INFORMATION
External Component Selection
Capacitors
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
The CAT4237 only requires small ceramic
capacitors of 4.7uF 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.
CMDSH05-4
(500mA
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:
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 520mW typical.
R1 = 0.3V/LED current
LED current (mA)
R1 (W)
60
5
10
15
20
25
30
30
20
15
12
Schottky Diode
10
Table 1. Resistor R1 and LED current
The current rating of the Schottky diode
must exceed the peak current flowing
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
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 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
Open LED Switching waveforms
without Zener
Open LED Supply Current vs. VIN
without Zener
Open LED Output Voltage vs. VIN
without Zener
50
45
40
35
30
2.0
1.5
1.0
0.5
0.0
2.5
3.0
3.5
4.0
4.5
5.0
2.5
3.0
3.5
4.0
4.5
5.0
INPUT VOLTAGE [V]
INPUT VOLTAGE [V]
© 2005 Catalyst Semiconductor, Inc.
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Characteristics subject to change without notice
CAT4237
DIMMING CONTROL
There are several methods available to
control the LED brightness.
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 0 to
20mA. The PWM signal frequency can vary
from very low frequency up to 100kHz.
§
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 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.
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
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
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 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.
Figure 3. Closed-switch current loop
Figure 4. Open-switch current loop
open
closed
© 2005 Catalyst Semiconductor, Inc.
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Characteristics subject to change without notice
CAT4237
PACKAGE OUTLINES
5-LEAD SOT-23
All dimensions in millimeters
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
CAT4237
REVISION HISTORY
Date
Revision
Reason
10/14/2005
A
Initial issue
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Corporate Headquarters
1250 Borregas Avenue
Sunnyvale, CA 94089
Phone: 408.542.1000
Publication #: 5013
Revision:
Rev. A
Fax: 408.542.1200
Issue date: 10/14/05
www.catalyst-semiconductor.com
© 2005 Catalyst Semiconductor, Inc.
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Doc. No. 5013, Rev. A
Characteristics subject to change without notice
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