2

AT9933DB1

Testing the Demo Board

Normal Operation: Connect the input source and the output

LEDs as shown in the Connection Diagram and enable the

board. The LEDs will glow with a steady intensity. Connect-

ing an Ammeter in series with the LEDs will allow measure-

ment of the LED current. The current will be 350mA +/- 5%.

Open LED test: Connect a voltmeter across the output ter-

minals of the AT9933DB1. Start the demoboard normally

and once the LED current reaches steady state, unplug one

end of the LED string from the demoboard. The output volt-

age will rise to about 33V and stabilize.

Short Circuit Test: When the AT9933DB1 is operating in

steady state, connect a jumper across the terminals of the

LED string. Notice that the switching frequency drops, but

the average output current remains the same.

PWM Dimming: With the input voltage to the board discon-

nected, apply a TTL compatible, push-pull square wave sig-

nal between PWMD and GND terminals of connector J3 as

shown in the Connection Diagram. Turn the input voltage

back on and adjust the duty cycle and / or frequency of the

PWM dimming signal. The output current will track the PWM

dimming signal. Note that although the converter operates

perfectly well at 1kHz PWM dimming frequency, the best

PWM dimming ratios can be obtained at lower frequencies

like 100 or 200Hz

Typical Results

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.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.

Fig.3 shows the output current variation over the input volt-

age range. The LED current has a variation of about 2mA

over the entire voltage range.

The waveforms in Fig.4 show the drain voltage of the FET

(channel 1 (blue); 10V/div) and the LED current (channel 4

(green); 100mA/div) at three different operating conditions

– 9V in, 13.5V in and 16V in.

Fig. 5 shows the operation of the converter during cold crank

conditions as the input voltage decreases from 13.5V to

6V and increases back to 13.5V. In these cases, the input

current reaches the limit set and the output current drops

correspondingly. Thus, the LEDs continue to glow, but with

reduced intensity. Once the voltage ramps back up, the out-

put current goes back to its normal value and the converter

comes out of the input current limit.

Fig.6 shows the LED current during an input step change

from 13.5V to 42V and back to 13.5V (similar to a clamped

load dump). It can be seen that the LED current drops briefly

when the input voltage jumps, but there are no overshoots.

Fig. 7a shows the operation of the converter during an Open

LED condition and Fig. 7b shows the operation during output

short circuit condition. In both cases, it can be seen that the

AT9933DB1 can easily withstand faults and come back into

normal operation almost instantly.

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. 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.