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CAT4238AEVB
CAT4238 EVALUATION
I LED (mA) +
V FB (mV) V FB
R1 ) R2
RSET( W )
The CAT4238AGEVB gives the user a way to evaluate
the CAT4238 in a typical application of driving multiple
LEDs.
The following steps are an example of how the user can
evaluate the CAT4238 white LED driver:
Driving 10 LEDs in series and Shutdown Mode
1. Connect the 10-LED string to the CAT4238 output
through J4 header pin connector
2. Set the R2 potentiometer to the middle position
3. Apply the external voltage supply, Vext
(3.0 V < V IN < 5 V) between V IN (T1) and GND (T2)
4. The CAT4238 is in the shutdown mode (SHDN pin is
connected to GND) if the J1 connector (Pin #1 – Pin #2)
is not jumpered
? Connect a current meter between Vext and V IN pad to
measure the shutdown current: I SD << 1 m A
? n this mode of operation the LEDs are disconnected
from the output: LEDs are OFF
5. Connect SHDN pin of CAT4238 to VIN using a jumper
shunt between Pin #1 and Pin #2 of J1 connector
? Observe that LEDs are ON
LED Current Evaluation
1. Programming LED current
LED current is programmed using the external resistors,
RSET = R1 + R2, connected to the FB pin. The voltage
at the FB pin is internally regulated to the value
V FB = 300 mV ? 15 mV.
The current into the LED pin can be set according to the
following equation:
+
? Set the input voltage: ex. V IN = 3.6 V
? Disconnect the jumper from the J4, connector and
insert a current meter between these pins to monitor the
? Monitor the voltage on FB (T5), V OUT (T7), and SW
(T9)
? Verify the internal switch frequency (f SW = 1.0 MHz
typically) using a scope probe connected on SW (T9)
test point (GND = T8)
Figure 3 shows the internal switch output, V SW
(DC coupled, 5 V/div) and the regulated voltage on FB pin,
V FB (DC, 500 mV/div) for V IN = 3.6 V, I LED = 20 mA,
10 LEDs in series connected to the CAT4238 output.
Figure 3. Internal Switch Output and Regulated V FB
Voltage Waveforms (I LED = 20 mA, V IN = 3.6 V)
2. Evaluating LED current regulation versus the input
voltage, VIN
? Set the I LED to a programmed value using the R2
potentiometer (i.e. 10 mA, 20 mA, 30 mA for
V IN = 3.6 V)
? For any I LED value vary the V IN voltage between 3.0 V
and 5.5 V
? Observe the value of I LED measured by the current meter
and V FB using a voltmeter. Figure 4 presents the I LED
current versus V IN . Figure 5 shows the CAT4238 FB
voltage versus input voltage.
?
LED current, I LED
Rotate the potentiometer R2 and observe the I LED value
on the current meter. The current can be adjusted
between 3 mA and 30 mA, approximately. Various
LED current values and the associated RSET values are
listed below:
CAT4238 LED Current versus Input Voltage
(10 LEDs in series)
35
30
25
20
15
I LED (mA)
5
R SET ( W )
59
10
5
0
10
29.8
3.00
3.50
4.00
4.50
5.00
5.50
15
20
25
30
20
15
12
10
V IN (V)
R SET = 29.8 W R SET = 15 W R SET = 10 W
Note: R SET = R1 + R2
Figure 4. LED Current Line Regulation
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