CAT4238
© 2006 Catalyst Semiconductor, Inc. 1 Doc. No. 5019 Rev. A
Characteristics subject to change without notice
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
Part Number Package Quantity
per Reel
Package
Marking
CAT4238TD-GT3 TSOT23-5
Green* 3000 MU
* NiPdAu Plated Finish
For Ordering Information details, see page 13.
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
r
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
r
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.
PIN CONFIGURATION TYPICAL APPLICATION CIRCUIT
5-Lead Thin SOT23
(1mm max height)
High Efficiency 10 LED Boost Converter
VIN
FB
2
3
1
SHDN
5
4
SW
GND
VIN
CAT4238
VIN
LDVOUT
R1
15Ω
20mA
(300mV)
4.7µF
C1
0.22µF
C2
FBSHDN
SW
GND
ON
47µH
OFF
L: Sumida CDC5D23B-470
D: Central CMDSH05-4
C2: Taiyo Yuden UMK212BJ224 (rated 50V)
CAT4238
Doc. No. 5019 Rev. A 2 © 2006 Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
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 45 V
Storage Temperature Range -65 to +160 ºC
Junction Temperature Range -40 to +125 ºC
Lead Temperature 300 ºC
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 Typ Max Units
IQ Operating Current VFB = 0.2V
VFB = 0.4V (not switching)
0.6
0.1
1.5
0.6
mA
ISD Shutdown Current VSHDN
¯¯¯¯¯ = 0V 0.1 1 µA
VFB FB Pin Voltage 10 LEDs with ILED = 20mA 285 300 315 mV
IFB FB pin input leakage 1 µA
ILED Programmed LED Current R1 = 10Ω
R1 = 15Ω
R1 = 20Ω
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
V
FSW Switching Frequency 0.8 1.0 1.3 MHz
DC Maximum Duty Cycle VIN = 3V 92 %
ILIM Switch Current Limit 350 450 600 mA
RSW Switch “On” Resistance ISW = 100mA 1.0 2.0 Ω
ILEAK Switch Leakage Current Switch Off, VSW = 5V 1 5 µA
Thermal Shutdown 150 ºC
Thermal Hysteresis 20 ºC
VUVLO Undervoltage Lockout (UVLO)
Threshold
1.9 V
VOV-SW Overvoltage Detection Threshold 40 V
VOCL Output Voltage Clamp “Open LED” 42 V
CAT4238
© 2006 Catalyst Semiconductor, Inc. 3 Doc. No. 5019 Rev. A
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.
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 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:
ILED = 0.3V/R1
The lower LED cathode is connected to the FB pin.
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.
BLOCK DIAGRAM
VIN
VIN
Enable
Current
Sense
300mV
R1
15Ω
LED
Current
4.7µF
C1
RCN1
R
S
C
C
SHDN
Thermal
Shutdown
& UVLO
Vref
1MHz
Oscillator
Over Voltage
Protection
PWM &
Logic
Driver
0.22µF
C2
GND
SW
FB
47µH
+
–
+
–
+
–
A1
A2
CAT4238
Doc. No. 5019 Rev. A 4 © 2006 Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
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.
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.
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.
Switching Waveform VIN = 5.0V, ILED = 1.5mA
CAT4238
© 2006 Catalyst Semiconductor, Inc. 5 Doc. No. 5019 Rev. A
Characteristics subject to change without notice
TYPICAL CHARACTERISTICS
VIN = 3.6V, CIN = 4.7μF, COUT = 0.22µF, L = 47µH with 10 LEDs at 20mA, TAMB = 25ºC, unless otherwise specified.
Quiescent Current vs. VIN (Not Switching)
50
75
100
125
150
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
QUIESCENT CURRENT [μA]
.
VFB = 0.4V
Quiescent Current vs. VIN (Switching)
0.0
0.5
1.0
1.5
2.0
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
QUIESCENT CURRENT [mA]
FB pin voltage vs. Temperature
297
298
299
300
301
302
303
-50 0 50 100 150
TEMPERATURE [ºC]
FB PIN VOLTAGE [mV]
FB Pin Voltage vs. Output Current
290
295
300
305
310
5 1015202530
OUTPUT CURRENT [mA]
FB PIN VOLTAGE [mV]
10 LEDs
Switching Frequency vs. Supply Voltage
Switching Waveforms
0.8
0.9
1.0
1.1
1.2
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
SWITCHING FREQUENCY [MHz]
CAT4238
Doc. No. 5019 Rev. A 6 © 2006 Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
60
70
80
90
100
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
EFFICIENCY [%]
VOUT = 33V
10 LEDs @ 20mA
VOUT = 32.5V
10 LEDs @ 15mA
TYPICAL CHARACTERISTICS
VIN = 3.6V, CIN = 4.7μF, COUT = 0.22µF, L = 47µH with 10 LEDs at 20mA, TAMB = 25ºC, unless otherwise specified.
LED Current vs. Input Voltage
0
5
10
15
20
25
30
35
40
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
LED CURRENT [mA]
R1 = 10Ω
VOUT = 33.8V
R1 = 15Ω
VOUT = 33V
R1 = 20Ω
VOUT = 32.5V
LED Current Regulation (10mA)
Efficiency vs. Load Current (10 LEDs)
Efficiency vs. Input Voltage (10 LEDs)
Power-up with 10 LEDs at 20mA
Switch ON Resistance vs. Input Voltage
60
70
80
90
100
5 1015202530
LED CURRENT [mA]
EFFICIENCY [%]
VIN = 3.6V
VIN = 5V
-1.0
-0.5
0.0
0.5
1.0
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
LED CURRENT VARIATION [%]
.
10 LEDs @ 10mA
0.0
0.5
1.0
1.5
2.0
3.03.54.04.55.05.5
INPUT VOLTAGE [V]
SWITCH RESISTANCE [Ω]
CAT4238
© 2006 Catalyst Semiconductor, Inc. 7 Doc. No. 5019 Rev. A
Characteristics subject to change without notice
TYPICAL CHARACTERISTICS
VIN = 3.6V, CIN = 4.7μF, COUT = 0.22µF, L = 47µH with 10 LEDs at 20mA, TAMB = 25ºC, unless otherwise specified.
Maximum Output Current vs. Input Voltage
0
10
20
30
40
50
60
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
OUTPUT CURRENT [mA]
VOUT = 35V
Shutdown Voltage vs. Input Voltage
0.2
0.4
0.6
0.8
1.0
3.0 3.5 4.0 4.5 5.0
INPUT VOLTAGE [V]
SHUTDOWN VOLTAGE [V]
-40°C
-25°C
85°C
125°C
CAT4238
Doc. No. 5019 Rev. A 8 © 2006 Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
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
CAT4238
© 2006 Catalyst Semiconductor, Inc. 9 Doc. No. 5019 Rev. A
Characteristics subject to change without notice
OPEN LED PROTECTION
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
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
0.0
1.0
2.0
3.0
4.0
5.0
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
SUPPLY CURRENT [mA]
Open LED Output Voltage vs. VIN
without Zener
35
40
45
50
55
3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE [V]
OUTPUT VOLTAGE [V]
VIN
CAT4238
V
IN
L
Schottky 100V
(Central CMSH1-100)
V
OUT
R1
15Ω
V
FB
= 300mV
4.7µF
C1
0.22µF
C2
FBSHDN
SW
GND
OFF ON
47µH
CAT4238
Doc. No. 5019 Rev. A 10 © 2006 Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
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 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 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 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.
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.
V
FB
= 300mV
15Ω
1kΩ3.1kΩ3.73kΩ
0V
25V
0.22µF
C1
R
A
R
B
R
2
iR1
VIN
CAT4238
FB
SHDN
SW
GND
VIN
PWN
Signal
LED
Curren
t
FILTERED PWM DIMMING [0V to 2.5V]
LED CURRENT [mA]
25
20
15
10
5
0
0 102030405060708090100
PWM DUTY CYCLE [%]
CAT4238
© 2006 Catalyst Semiconductor, Inc. 11 Doc. No. 5019 Rev. A
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.
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
CAT4238
Doc. No. 5019 Rev. A 12 © 2006 Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
PACKAGE OUTLINES
5-LEAD SOT-23
SYMBOL MIN NOM MAX
A — — 1.0
A1 0.01 0.05 0.1
A2 0.80 0.87 0.9
b 0.30 — 0.45
c 0.12 0.15 0.20
D 2.90 BSC
E 2.80 BSC
E1 1.60 BSC
e 1.95 BSC
e1 1.90 BSC
L 0.30 0.40 0.50
L1 0.60 REF
L2 0.25 BSC
q 0º 8º
Notes:
(1) All dimensions are in millimeters, angles in degrees.
(2) Refer JEDEC MO-193.
A2 A
A1
D
e
b
E
E1
e1
e
L2
GAUGE
PLANE
L
L1
c
θ
CAT4238
© 2006 Catalyst Semiconductor, Inc. 13 Doc. No. 5019 Rev. A
Characteristics subject to change without notice
EXAMPLE OF ORDERING INFORMATION
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.
Prefix Device # Suffix
CAT 4238 TD – G T3
Optional
Company ID
Product Number
4238
Tape & Reel
T: Tape & Reel
3: 3000/Reel
Package
TD: Thin SOT-23 (Lead-free, Halogen-free)
Lead Finish
G: NiPdAu
REVISION HISTORY
Date Rev. Reason
06/14/2006 A Initial Issue
Copyrights, Trademarks and Pat ents
Trademarks and registered trademarks of Catalyst Semiconduct or include each of the following:
DPP ™ AE2 ™
Catalyst Semiconductor has been issued U. S. and foreign patents and has patent applications pending that protect its products.
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
1250 Borregas Avenue
Sunnyvale, CA 94089
Phone: 408.542.1000 Document No: 5019
Fax: 408.542.1200 Revision: A
www.catsemi.com Issue date: 06
/
14
/
06
