30127104
FIGURE 4. Adjusting the VDHC Pin Voltage
To adjust V
VDHC
on the evaluation board, an additional resis-
tor divider (R
A
and R
B
) can be added across the VDHC test
pad and VCC or AGND terminals on the board. The values of
of R
A
and R
B
should be below 100k
Ω
and 16k
Ω
respectively
to ensure the accuracy of the headroom voltage under steady
state. The V
VDHC
is governed by the following equation:
(6)
where
(7)
Connecting the LED Strings
The LM3464A evaluation board is designed to drive 4 com-
mon anode LEDs strings of 12 serial LEDs per string. The
board includes four turret connectors, LED1, LED2, LED3 and
LED4 for cathode connections of the LED strings. The anode
of the LED strings should connect to the positive power output
terminal of the AC/DC power supply. By default, the output
current for each output channel is set at 350mA. The output
currents of the LM3464A evaluation board can be pro-
grammed individually by changing the value of the resistors
RISNS1, RISNS2, RISNS3 and RISNS4 accordingly. The
LED driving current is governed by the following equation:
(8)
Adjusting Frequency Response of
the LM3464A Circuit
The frequency response of the LM3464A evaluation board
can be adjusted by changing the value of the capacitor,
C
DHC
. Higher capacitance of C
DHC
results in slower frequency
response of the LM3464A driver stage. In order to ensure
stable system operation, it is recommended to set the domi-
nant pole of the LM3464A one decade lower than the domi-
nant pole of the AC/DC converter. The default value of the
C
DHC
on the evaluation board is 0.22uF. For applications with
slow response AC/DC power supply (e.g. converters with ac-
tive PFC), the value of CDHC should be increased to make
the frequency response of the board slower than the response
of the AC/DC power supply. The cut-off frequency of the
LM3464A driver stage is governed by the following equation:
(9)
7
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