Quiet Modulation

The figure below shows lamp modulation noise on the X input of an ISL6 (A3030C) for 20% brightness. This noise occurs even with no lamp attached and no X input leads.


Figure: Lamp Modulation Noise on X for A3030C with 20% Brightness, 50-ms Pulses at 10 Hz.
When we modulate the lamp, we apply a 1-MHz clock signal to the gate of Q1 so as to turn on and off the lamp current. During lamp modulation, the A3030C battery current increases by around 5 mA. We suspect that this 5 mA causes the 3V0 output of our TPS70930 to drop by 500 μV, which in turn causes X to step up by 2 mV.

In the A3030C, Q1 is the NDS355AN, with 200 pF gate capacitance. We load our A3030D with NTR4003N for Q1. The figure below shows A3030D modulation noise for random 10-ms pulses.


Figure: Lamp Modulation Noise on X for A3030D with 80% Brightness, 10-ms Random Pulses at 10 Hz. Vertical range is 140 μV.
The new transistor reduces modulation noise to 50 μVpp. The NTR4003N has resistance 1.5 Ω, which is much larger than the 0.2Ω of the NDS355AN, but still insignificant compared to our 60-Ω lamp lead resistance.