Texas Instruments tps61040EVM-002, Руководство пользователя

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EVM Operation
2-3 Setup and Test Results
seen at the FB pin, which lowers the LED current. If the resistor values are
chosen correctly, the analog control voltage varies the output current between
0 mA and maximum programmed current. This dimming method is the default
configuration of the EVM, and is accomplished by injecting an analog voltage
into the DIM1 pin on J3. When using this method, R1 and C2 do not affect the
operation of the EVM. The EVM is designed so that an analog voltage of 0 V
to 3.3 V varies the LED current from 20 mA to 0 mA. Use the following
equations to calculate the required resistor values.
Where:
-
V
REF
is the TPS61040 reference voltage = 1.233 V.
-
V
adj_min
is the minimum adjust voltage.
-
V
adj_max
is the maximum adjust voltage.
-
I
out_min
is the minimum output current.
-
I
out_max
is the maximum output current.
-
R2 is a dimming resistor.
-
Ref Des corresponds to the EVM schematic shown in Chapter 4.
R3
+
V
REF
ǒ
I
o_min
R2
)
V
adj_max
*
I
o_max
R2
*
V
adj_min
Ǔ
ǒ
V
FB
I
o_max
*
V
adj_max
I
o_max
*
V
FB
I
o_min
)
V
adj_min
I
o_min
Ǔ
R4
+
V
FB
R2
)
V
FB
R3–V
adj_max
R3
I
o_min
R2–V
FB
)
V
adj_max
For the EVM, V
REF
= 1.233 V, V
adj_min
= 0 V, V
adj_max
= 3.3 V, I
out_min
= 0 A,
I
out_max
= 20 mA, and R2 = 249 k
Ω
. Using these values, R3 is calculated to be
148.3 k
Ω
, and R4 is calculated to be 98.4
Ω
.
2.2.2 Analog Dimming With PWM Voltage
The second method for dimming the LEDs is to inject a pulse width modulated
(PWM) voltage for analog dimming. With this method, an RC filter is used to
convert the PWM control voltage into an analog voltage. The component
values of the RC filter depend on the frequency of the PWM voltage and the
amount of allowable ripple on the converted analog signal. The converted
analog voltage is then injected into the FB pin of the TPS61040 as in the
Analog Dimming With Analog Voltage method. The output current decreases
as the duty cycle increases. Inject the PWM control voltage into the DIM2 pin
on J3. Assuming that the PWM control voltage amplitude varies between 0 V
and 3.3 V, the resistor values calculated in the Analog Dimming with Analog
Voltage method may still be used. The PWM control voltage is converted to
its equivalent analog control voltage using the following equation.
Vanalog = Vpwm_pk
×
D + Vmin
Where:
-
Vpwm_pk is the peak-to-peak voltage of the injected PWM signal.
-
D is the duty cycle of the injected PWM signal.
-
Vmin is the minimum voltage of the injected PWM signal.