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CAT4238AEVB
CAT4238 FB Voltage versus Input Voltage
Dimming Control
The LED brightness control can be accomplished by using
I LED = 10 mA
I LED = 20 mA
I LED = 30 mA
a PWM signal applied to the SHDN pin or to the FB pin. The
310
300
290
280
270
260
250
240
230
220
210
200
3.00
3.50
4.00
4.50
5.00
5.50
other method is to use a variable DC voltage applied through
a resistor to FB pin.
1. Dimming using a PWM signal on the SHDN pin
The LEDs are turned off and on at the PWM frequency.
The average current changes with the duty cycle.
Increasing the duty cycle will increase the LED
brightness. The peak current value sets the light spectrum.
I LED S VFi
V IN
I IN
V IN (V)
Figure 5. V FB Voltage Line Regulation
Efficiency Evaluation
The efficiency is evaluated according to the following
equation:
Efficiency (%) + 100
where S VFi = V OUT – V LED , V LED is the voltage measured on
LED (T10) test point.
1. Insert a current meter, CM1, between input supply
voltage, Vext, and V IN pad to monitor the input current, I IN
(or use an embedded power supply current meter)
2. Set the input voltage for V IN = 3.6 V
3. Adjust the R2 potentiometer for the I LED = 5 mA. Observe
the I LED current on the meter, CM2, inserted between Pin
#1 and Pin #2 of J4 connector
4. Measure the I IN current on CM1
5. Monitor the output voltage on V OUT (T7) and V LED voltage
on LED (T10) test points
6. Repeat steps 3 to 5 for I LED = 10 mA, 15 mA, 20 mA,
25 mA and 30 mA
Figure 6 shows the efficiency measured for 2 values of
input voltage, V IN = 3.6 V and V IN = 4.2 V, with 10 LEDs in
series connected to the CAT4238 output.
CAT4238 Efficiency (10 LEDs in series)
? Connect the jumper shunt between Pin #2 and Pin #3 of
the J1 connector
? Apply a pulse signal generator to the GEN/DC (T3)
pad; Frequency = 200 Hz to 2 kHz; Amplitude 0 V to
3 V for V IN = 3.6 V
? Modify the duty cycle between 0% and 100%
? Observe the average current through LEDs measured
by a current meter inserted at J4 connector. For 0% duty
cycle, the I LED will be off (I LED = 0 mA); At the
maximum duty cycle, the LED will be driven at the
maximum current set by the R2 potentiometer
? Monitor the FB voltage compared with the PWM signal
applied on SHDN input
Figure 7 shows the voltage monitored on the FB pin, V FB
(CH2) using a PWM signal applied to the SHDN pin (CH1).
The frequency of the PWM signal is 2 kHz approximately.
90%
85%
80%
75%
70%
V IN = 3.6 V
V IN = 4.2 V
Figure 7. FB Voltage Waveform with PWM
2. Dimming using a DC voltage applied to the FB pin
TA variable external DC voltage is applied on FB pin to
65%
5
10
15
I LED (mA)
20
25
30
adjust the LED current. As the DC voltage is increased,
the voltage drop on resistor R3 is increased and the
voltage drop on RSET = R1+R2 is decreased, thus the
Figure 6. CAT4238 Efficiency Driving
10 LEDs in Series
LED current decreases. The external DC voltage is
applied to FB pin through a series resistor, R5, which sets
the maximum DC voltage.
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