Texas Instruments tps61040EVM-002, Руководство пользователя

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Setup
2-4
2.2.3 PWM Dimming Using Enable
The third method for dimming the LEDs is to inject a PWM voltage into the EN
pin of the TPS61040. When the EN pin is high, the supply turns on and the
output current is at the programmed maximum current. When the EN pin is low,
the supply turns off and the output current goes to 0 mA. If the frequency of
the PWM voltage is greater than 100 Hz, the human eye can not detect the on
and off state of the LED current. The human eye averages the on and off state
of the LED and sees a dimmed diode rather than a pulsed brightness. This
dimming method provides a controlled inrush current at turnon, but limits the
maximum PWM frequency to about 200 Hz, due to the fact that the IC enters
soft start during the first 1.5 ms of each pulse. A PWM duty cycle of 0%
produces 0 mA of output current; a duty cycle of 100% produces the maximum
programmed output current. For this dimming method, remove R2 and C2,
short R3, and inject the PWM signal into the center pin of JP1. R4 is calculated
by the following equation where Vref is the reference voltage of the TPS61040
(1.233 V) and Imax is the maximum desired output current. The reference
designators correspond to the EVM schematic shown in Chapter 4.
R4
+
V
REF
Imax
For an output current of 20 mA, R4 = 61.6
Ω
.
2.2.4 PWM Dimming Using Injected Voltage on FB
The fourth method for dimming the LEDs is to inject a PWM control voltage into
the FB pin of the TPS61040. When the control voltage is low, the output current
is at its maximum programmed value. When the control voltage is high, the
output current is 0 mA. As with the PWM dimming using the enable pin, the
PWM frequency should be at least 100 Hz. Because the device does not enter
soft start during each pulse cycle, the maximum PWM frequency can be
increased to over 20 kHz. The equations for determining the resistor values
are the same as for the Analog Dimming With Analog Voltage method. For this
dimming method, remove R1 and C2, and inject the PWM signal into the DIM1
pin on J3.
2.3 Setup
After the EVM has been modified for the appropriate dimming method,
connect an input supply between J1 and J2. Connect the LEDs between the
LED anode and LED cathode connection points. The EVM operates between
1.8 V and 6.0 V. Ensure that the input voltage never exceeds the TPS61040
absolute maximum input voltage rating of 7.0 V. Move the adjust jumper from
the Off position to the On position to enable the supply.