AT9933DB1
Testing the Demo Board
Normal Operation: Connect the input source and the output Fig.3 shows the output current variation over the input volt-
LEDs as shown in the Connection Diagram and enable the age range. The LED current has a variation of about 2mA
board. The LEDs will glow with a steady intensity. Connect- over the entire voltage range.
ing an Ammeter in series with the LEDs will allow measure-
ment of the LED current. The current will be 350mA +/- 5%. The waveforms in Fig.4 show the drain voltage of the FET
(channel 1 (blue); 10V/div) and the LED current (channel 4
Open LED test: Connect a voltmeter across the output ter- (green); 100mA/div) at three different operating conditions
minals of the AT9933DB1. Start the demoboard normally – 9V in, 13.5V in and 16V in.
and once the LED current reaches steady state, unplug one
end of the LED string from the demoboard. The output volt- Fig. 5 shows the operation of the converter during cold crank
age will rise to about 33V and stabilize.
conditions as the input voltage decreases from 13.5V to
6V and increases back to 13.5V. In these cases, the input
Short Circuit Test: When the AT9933DB1 is operating in current reaches the limit set and the output current drops
steady state, connect a jumper across the terminals of the correspondingly. Thus, the LEDs continue to glow, but with
LED string. Notice that the switching frequency drops, but reduced intensity. Once the voltage ramps back up, the out-
the average output current remains the same.
put current goes back to its normal value and the converter
comes out of the input current limit.
PWM Dimming: With the input voltage to the board discon-
nected, apply a TTL compatible, push-pull square wave sig- Fig.6 shows the LED current during an input step change
nal between PWMD and GND terminals of connector J3 as from 13.5V to 42V and back to 13.5V (similar to a clamped
shown in the Connection Diagram. Turn the input voltage load dump). It can be seen that the LED current drops briefly
back on and adjust the duty cycle and / or frequency of the when the input voltage jumps, but there are no overshoots.
PWM dimming signal. The output current will track the PWM
dimming signal. Note that although the converter operates Fig. 7a shows the operation of the converter during an Open
perfectly well at 1kHz PWM dimming frequency, the best LED condition and Fig. 7b shows the operation during output
PWM dimming ratios can be obtained at lower frequencies short circuit condition. In both cases, it can be seen that the
like 100 or 200Hz
AT9933DB1 can easily withstand faults and come back into
normal operation almost instantly.
Typical Results
Fig. 8 shows the PWM dimming performance of the
AT9933DB1 with a 100Hz, 3.3V square wave signal. The
converter can easily operate at PWM dimming duty cycles
from 1% - 99%.
Fig.1 shows the efficiency plot for the AT9933DB1 over the
input voltage range. The converter has efficiencies greater
than 80% over 13V input. Note that these measurements
so not include the 0.3W - 0.5W loss in the reverse blocking
diode.
Fig. 9 shows the rise and fall times of the output current dur-
ing PWM dimming. The converter has nearly symmetric rise
and fall times of about 25µs. These rise and fall times can
be reduced (if desired) by reducing the output capacitance
C10. However, this will lead to increased ripple in the output
current.
Fig.2 shows the variation of the switching frequency over
the input votage range. The frequency varies from 300kHz
to 500kHz over the entire input voltage range and avoids the
restricted frequency band of 150kHz to 300kHz and the AM
band greater than 530kHz. This makes it easier to meet the
conducted and radiated EMI specifications for the automo-
tive industry.
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SUPERTEX [ Supertex, Inc ]
