CAT4237TD-GT3

CAT4237
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10
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 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.
Figure 25. Switching Waveform with 1 kHz PWM on
SHDN
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 26. Circuit for Filtered PWM Signal
1 kW
3.1 kW
0 V
2.5 V
0.22 mF
C1
i
VIN
CAT4237
FB
SW
GND
PWN
Signal
LED
Current
R
B
3.73 kW
SHDN
R
1
15 W
R
2
R
A
V
FB
= 300 mV
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.
Figure 27. Filtered PWM Dimming (0 V to 2.5 V)
LED CURRENT (mA)
25
20
15
10
5
0
0 10 20 30 40 50 60 70 80 90 100
PWM DUTY CYCLE (%)
CAT4237
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11
Board Layout
The CAT4237 is a highfrequency switching regulator.
The traces that carry the highfrequency 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 C
1
has to be placed as close as possible to the
V
IN
pin and GND. The capacitor C
2
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 R
1
must be connected directly to the
GND pin of the CAT4237 and not shared with the switching
current loops and any other components.
Figure 28. Closedswitch Current Loop Figure 29. Openswitch Current Loop
Closed
Open
CAT4237
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12
PACKAGE DIMENSIONS
TSOT23, 5 LEAD
CASE 419AE01
ISSUE O
E1 E
A2
A1
e
b
D
c
A
TOP VIEW
SIDE VIEW END VIEW
L1
L
L2
Notes:
(1) All dimensions are in millimeters. Angles in degrees.
(2) Complies with JEDEC MO-193.
SYMBOL
θ
MIN NOM MAX
q
A
A1
A2
b
c
D
E
E1
e
L
L1
L2
0.01
0.80
0.30
0.12
0.30
0.05
0.87
0.15
2.90 BSC
2.80 BSC
1.60 BSC
0.95 TYP
0.40
0.60 REF
0.25 BSC
1.00
0.10
0.90
0.45
0.20
0.50
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“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
PUBLICATION ORDERING INFORMATION
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CAT4237TD-GT3

Mfr. #:
Manufacturer:
ON Semiconductor
Description:
LED Lighting Drivers LED Driver,Boost 8 LED,series
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
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