r
r
High Efficiency 10 LED Boost Converter
FEATURES
Drives High Voltage LED strings (38V)
Up to 87% Efficiency
Low Quiescent Ground Current 0.6mA
Adjustable Output Current
1MHz Fixed Frequency Low noise Operation
Soft start “in-rush” current limiting
Shutdown current less than 1μA
Open LED Overvoltage Protection
Automatic Shutdown at 1.9V (UVLO)
Thermal overload protection
Thin SOT23 5-Lead (1mm max height)
APPLICATIONS
GPS Navigation Systems
Portable Media Players
Handheld Devices, Digital Cameras
Portable Game Machines
ORDERING INFORMATION
DESCRIPTION
The CAT4238 is a DC/DC step-up converter that delivers
an accurate constant current ideal for driving LEDs.
Operation at a fixed 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 resisto
R1. LED currents up to 40mA can be supported over a
wide range of input supply voltages up to 5.5V, making
the device ideal for battery-powered applications. The
CAT4238 high-voltage output stage is perfect for driving
mid-size and large panel displays containing up to ten
white LEDs in series.
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 powe
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.
CAT4238
Part Number Package
CAT4238TD-GT3
* Lead Finish NiPdAu
TSOT23-5
Green*
Quantity
per Reel
3000 MU
Package
Marking
For Ordering Information details, see page 13.
PIN CONFIGURATION
TSOT23 5-Lead
(1mm max height)
1
SW
GND
FB
2
3
Top View
5
VIN
4
SHDN
TYPICAL APPLICATION CIRCUIT
LDVOUT
VIN
C1
4.7µF
OFF
ON
L: Sumida CDC5D23B-470
D: Central CMDSH05-4
C2: Taiyo Yuden UMK212BJ224 (rated 50V)
VIN
CAT4238
47µH
GND
SW
FBSHDN
C2
0.22µF
(300mV)
20mA
R1
15Ω
© Catalyst Semiconductor, Inc. 1 Doc. No. MD-5019 Rev. D
Characteristics subject to change without notice
CAT4238
ABSOLUTE MAXIMUM RATINGS
Parameters Ratings Units
VIN, FB voltage -0.3 to +7 V
¯¯¯¯¯
SHDN
voltage
SW voltage
(1)
-0.3 to +7 V
up to 60 V
Storage Temperature Range -65 to +160 ºC
Junction Temperature Range -40 to +150 ºC
Lead Temperature 300 ºC
Note:
(1) The SW pin voltage is rated up to 39V for external continuous DC voltage.
RECOMMENDED OPERATING CONDITIONS
Typical application circuit with external components are shown on page 1.
Parameters Range Units
VIN up to 5.5 V
SW pin voltage 0 to 38 V
Ambient Temperature Range -40 to +85 ºC
DC ELECTRICAL CHARACTERISTICS
VIN = 3.6V, ambient temperature of 25ºC (over recommended operating conditions unless specified otherwise).
Symbol Parameter Test Conditions Min
IQ Operating Current V
ISD Shutdown Current
= 0.2V
FB
= 0.4V (not switching)
V
FB
= 0V
V
¯¯¯¯¯
SHDN
VFB FB Pin Voltage 10 LEDs with I
= 20mA 285 300 315 mV
LED
0.6
0.1 1 µA
IFB FB pin input leakage 1 µA
I
Programmed LED Current
LED
¯¯¯¯¯
SHDN
¯¯¯¯¯
SHDN
Logic High
Logic Low
VIH
V
IL
R1 = 10Ω
R1 = 15Ω
R1 = 20Ω
Enable Threshold Level
Shutdown Threshold Level
28.5
19
14.25
0.4
FSW Switching Frequency 0.8 1.0 1.3 MHz
DC Maximum Duty Cycle VIN = 3V 92 %
I
Switch Current Limit 350 450 600 mA
LIM
RSW Switch “On” Resistance ISW = 100mA 1.0 2.0
I
Switch Leakage Current Switch Off, VSW = 5V 1 5 µA
LEAK
Thermal Shutdown 150 ºC
Thermal Hysteresis 20 ºC
V
Undervoltage Lockout (UVLO)
UVLO
1.9 V
Threshold
V
Overvoltage Detection Threshold 40 V
OV-SW
V
Output Voltage Clamp “Open LED” with VIN = 5V 43 45 48 V
OCL
Typ
0.1
30
20
15
0.8
0.7
Max Units
1.5
0.6
31.5
21
15.75
1.5
mA
mA
V
V
Ω
Doc. No. MD-5019 Rev. D 2 © Catalyst Semiconductor, Inc.
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 plane on the PCB.
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.
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 overvoltage detection circuit is connected to the SW pin.
When the voltage reaches 40V, 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:
The lower LED cathode is connected to the FB pin.
CAT4238
ILED = 0.3V/R1
BLOCK DIAGRAM
VIN
C1
4.7µF
VIN
SHDN
Vref
Shutdown
300mV
Enable
Thermal
& UVLO
47µH
SW
C2
0.22µF
1MHz
Oscillator
–
A1
+
R
C
C
C
+
A2
–
+
–
Current
Sense
Over Voltage
Protection
PWM &
Logic
Driver
LED
Current
N
1
R
S
GND
FB
R1
15Ω
© Catalyst Semiconductor, Inc. 3 Doc. No. MD-5019 Rev. D
Characteristics subject to change without notice
CAT4238
DEVICE OPERATION
The CAT4238 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 operating from a Li-Ion battery, the device can
deliver 20mA of load current into a string of up to 10
white LEDs. For higher input voltages, the LED
current can be increased.
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 2mA) and with input
voltage above 5.0V, the CAT4238 driving 10 LEDs,
the driver starts pulse skipping. Although the LED
current remains well regulated, some lower frequency
ripple may appear.
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 40V, 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 6μs and keeping it on
for about 1μs.
Switching Waveform VIN = 5.0V, I
= 1.5mA
LED
Doc. No. MD-5019 Rev. D 4 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
TYPICAL CHARACTERISTICS
.
= 3.6V, CIN = 4.7μF, C
V
IN
= 0.22µF, L = 47µH with 10 LEDs at 20mA, T
OUT
CAT4238
= 25ºC, unless otherwise specified.
AMB
Quiescent Current vs. VIN (Not Switching)
150
VFB = 0.4V
125
100
75
QUIESCENT CURRENT [μA]
50
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
FB pin voltage vs. Temperature
303
302
301
Quiescent Current vs. VIN (Switching)
2.0
1.5
1.0
0.5
QUIESCENT CURRENT [mA]
0.0
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
FB Pin Voltage vs. Output Current
310
305
300
299
298
FB PIN VOLTAGE [mV]
297
-50 0 50 100 150
TEMPERATURE [ºC]
Switching Frequency vs. Supply Voltage
1.2
FB PIN VOLTAGE [mV]
300
295
10 LEDs
290
5 1015202530
OUTPUT CURRENT [mA]
Switching Waveforms
1.1
1.0
0.9
SWITCHING FREQUENCY [MHz]
0.8
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
© Catalyst Semiconductor, Inc. 5 Doc. No. MD-5019 Rev. D
Characteristics subject to change without notice
CAT4238
.
TYPICAL CHARACTERISTICS
= 3.6V, CIN = 4.7μF, C
V
IN
= 0.22µF, L = 47µH with 10 LEDs at 20mA, T
OUT
= 25ºC, unless otherwise specified.
AMB
LED Current vs. Input Voltage
40
35
30
25
20
15
10
LED CURRENT [mA]
5
0
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
R1 = 10Ω
VOUT = 33.8V
R1 = 15Ω
VOUT = 33V
R1 = 20Ω
VOUT = 32.5V
Efficiency vs. Load Current (10 LEDs)
100
90
80
70
EFFICIENCY [%]
VIN = 5V
VIN = 3.6V
LED Current Regulation (10mA)
1.0
0.5
0.0
-0.5
10 LEDs @ 10mA
-1.0
LED CURRENT VARIATION [%]
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
Efficiency vs. Input Voltage (10 LEDs)
100
90
80
EFFICIENCY [%]
70
VOUT = 32.5V
10 LEDs @ 15mA
VOUT = 33V
10 LEDs @ 20mA
60
5 1015202530
LED CURRENT [mA]
Power-up with 10 LEDs at 20mA
Switch ON Resistance vs. Input Voltage
Doc. No. MD-5019 Rev. D 6 © Catalyst Semiconductor, Inc.
60
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
2.0
1.5
1.0
0.5
SWITCH RESISTANCE [Ω]
0.0
3.03.54.04.55.05.5
INPUT VOLTAGE [V]
Characteristics subject to change without notice
TYPICAL CHARACTERISTICS
= 3.6V, CIN = 4.7μF, C
V
IN
= 0.22µF, L = 47µH with 10 LEDs at 20mA, T
OUT
CAT4238
= 25ºC, unless otherwise specified.
AMB
Maximum Output Current vs. Input Voltage
60
50
40
30
20
10
OUTPUT CURRENT [mA]
0
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
VOUT = 35V
Shutdown Voltage vs. Input Voltage
1.0
-25°C
0.8
0.6
0.4
85°C
SHUTDOWN VOLTAGE [V]
0.2
3.0 3.5 4.0 4.5 5.0
-40°C
125°C
INPUT VOLTAGE [V]
© Catalyst Semiconductor, Inc. 7 Doc. No. MD-5019 Rev. D
Characteristics subject to change without notice
CAT4238
APPLICATION INFORMATION
External Component Selection
Capacitors
The CAT4238 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 47µH inductor is recommended for most of the
CAT4238 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 CDC5D23B-470 47µH inductor has a rated
current of 490mA and a series resistance (D.C.R.) of
420mΩ 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
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 = 0.3V/LED current
LED current (mA)
R1 (Ω)
5 60
10 30
15 20
20 15
25 12
30 10
Table 1. Resistor R1 and LED current
Doc. No. MD-5019 Rev. D 8 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
OPEN LED PROTECTION
V
In the event of an “Open LED” fault condition, the
CAT4238 will continue to boost the output voltage with
maximum power until the output voltage reaches
approximately 40V. 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 6mW (about 1.6mA
CAT4238
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
Schottky 100V
(Central CMSH1-100)
SW
V
= 300mV
FB
FBSHDN
R1
15Ω
IN
4.7µF
L
47µH
C1
VIN
CAT4238
OFF ON
GND
Open LED Supply Current vs. VIN
without Zener
5.0
4.0
C2
0.22µF
Open LED Switching Waveforms
without Zener
V
OUT
Open LED Output Voltage vs. VIN
without Zener
55
50
3.0
45
2.0
1.0
SUPPLY CURRENT [mA]
0.0
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
40
OUTPUT VOLTAGE [V]
35
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
© Catalyst Semiconductor, Inc. 9 Doc. No. MD-5019 Rev. D
Characteristics subject to change without notice
CAT4238
t
DIMMING CONTROL
There are several methods available to control the
LED brightness.
PWM signal on the SHDN
LED brightness dimming can be done by applying a
PWM signal to the SHDN
repetitively turned on and off, so that the average
current is proportional to the duty cycle. A 100% duty
cycle, with SHDN
¯¯¯¯¯
LEDs at nominal current. Figure 1 shows a 1kHz
signal with a 50% duty cycle applied to the SHDN
The recommended PWM frequency range is from
100Hz to 2kHz.
¯¯¯¯¯
Pin
¯¯¯¯¯
input. The LED current is
always high, corresponds to the
¯¯¯¯¯
pin.
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 CAT4238 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.
PWM
Signal
2.5V
0V
R
A
0.22µF
C1
VIN
SHDN
GND
R
B
CAT4238
FB
i
SW
VFB= 300mV
1kΩ3.1kΩ3.73kΩ
R
2
LED
Curren
R
1
15Ω
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]
Figure 1. Switching Waveform
with 1kHz PWM on SHDN
¯¯¯¯¯
25
20
15
10
5
LED CURRENT [mA]
0
0 102030405060708090100
PWM DUTY CYCLE [%]
Doc. No. MD-5019 Rev. D 10 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
BOARD LAYOUT
The CAT4238 is a high-frequency switching regulator.
The traces that carry the high-frequency switching
current have to be carefully laid out 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 CAT4238 internal switch
is closed. On Figure 4 is shown the current loop, when
the CAT4238 switch is open. Both loop areas should
be as small as possible.
CAT4238
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 CAT4238 allows for direct connection
of the capacitors to ground. The resistor R1 must be
connected directly to the GND pin of the CAT4238
and not shared with the switching current loops and
any other components.
Figure 3. Closed-switch Current Loop
Figure 4. Open-switch Current Loop
Figure 5. Recommended PCB Layout
© Catalyst Semiconductor, Inc. 11 Doc. No. MD-5019 Rev. D
Characteristics subject to change without notice
CAT4238
p
PACKAGE OUTLINE DRAWING
TSOT-23 5-Lead (TD)
(1)(2)
D
e
TOP VIEW
E1 E
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
D 2.90 BSC
E 2.80 BSC
E1 1.60 BSC
e0.95TYP
L 0.30 0.40 0.50
L1 0.60 REF
L2 0.25 BSC
θ 0º 8º
A2
A
b
SIDE VIEW END VIEW
Notes:
(1) All dimensions are in millimeters, angles in degrees.
(2) Complies with JEDEC standard MO-193.
For current Tape and Reel information, download the PDF file from:
://www.catsemi.com/documents/tapeandreel.pdf.
htt
A1
θ
L
L1
c
L2
Doc. No. MD-5019 Rev. D 12 © Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
CAT4238
EXAMPLE OF ORDERING INFORMATION
Prefix
CAT 4238 TD – G T3
Optional
Company ID
Device # Suffix
Product Number
4238
G: NiPdAu
Package
TD: Thin SOT-23 (Lead-free, Halogen-free)
(1)(2) (3)
Lead Finish
Tape & Reel
T: Tape & Reel
3: 3000/Reel
For Product Top Mark Codes, click here:
http://www.catsemi.com/techsupport/producttopmark.asp
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 CAT4238–GT3 (TSOT-23, NiPdAu, Tape & Reel).
(4) For additional package and temperature options, please contact your nearest Catalyst Semiconductor Sales office.
© Catalyst Semiconductor, Inc. 13 Doc. No. MD-5019 Rev. D
Characteristics subject to change without notice
REVISION HISTORY
Date Revision Description
14-Jun-06 A Initial Issue
Update Absolute Maximum Ratings
16-Mar-07 B
Update Figure 2.
Update SOT-23 5 Lead package outline
17-Oct-07 C
Update Absolute Maximum Ratings
Update Package Outline Drawing
03-Jul-08 D Update Output Voltage Clamp
Copyrights, Trademarks and Patents
© 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 product s.
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.
Catalyst Semiconductor, Inc.
Corporate Headquarters
2975 Stender Way
Santa Clara, CA 95054
Phone: 408.542.1000 Document No: MD-5019
Fax: 408.542.1200 Revision: D
www.catsemi.com
Issue date: 07/03/08
Loading...