Texas Instruments TPS61042EVM-226, User Manual, Page: 13 / 22

EVM Operation
2-3 Setup
-
I
o_min
is the minimum output current
-
I
o_max
is the maximum output current
-
Reference designators correspond to the EVM schematic shown in
Chapter 4
R
2
+
V
ref
ǒ
Io_max R
3
*
I
o_min
R
3
)
V
adj_min
*
V
adj_max
Ǔ
I
o_min
ǒ
V
ref
*
V
adj_min
Ǔ
)
Io_max
ǒ
V
adj_max
*
V
ref
Ǔ
R
1
+
V
ref
R
3
)
V
ref
R
2
*
V
adj_max
R
2
I
o_min
R
3
*
V
ref
)
V
adj_max
If V
ref
= 0.25 V, V
adj_min
= 0 V, V
adj_max
= 3.3 V, I
o_min
= 0 A, I
o_max
= 20 mA,
and R3 = 121 k
Ω
, R2 is calculated to be 9.9 k
Ω
, and R1 is calculated to be
13.5
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 upon 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 TPS61042 as in the
Analog Dimming with Analog Voltage method described above. The output
current decreases as the duty cycle increases. 0% duty cycle delivers
maximum LED current and 100% duty cycle delivers minimum LED current.
For this dimming method the PWM signal must be filtered and converted to its
analog equivalent before it is applied to the EVM. The filtered signal is then
applied to 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 following equation converts
the PWM control voltage into its equivalent analog control voltage where:
-
V
pwm_pk
is the peak to peak voltage of the injected PWM signal
-
D is the duty cycle of the injected PWM signal
-
V
min
is the minimum voltage of the injected PWM signal
V
analog
= V
pwm_pk
×
D + V
min