I am using the TPS61500 eval board to drive 7 LEDs in series. I have replaced various components on the eval board as shown in the schematic below:
I'm not using the eval board LEDs and am connecting my own 7 LED string via the OUT and FB headers. I have the current limit programed by the 0.68Ohm FB resistor to be 294mA.
A normal startup waveform looks like this:
The yellow trace is the FB voltage. I apply a 10kHz, 3.0V square wave to the EN pin with 85% duty. The FB voltage rises to 0.85*200 = 170mV.
The green trace is the VOUT net, rising to roughly 17V
The blue trace is the input current.
This startup looks good and there's nothing to worry about. The device is using the analog dimming method well. I am using a power supply set to 4V and 3.0A current limit.
However, sometimes the device goes into this strange mode when starting up:
The 4.0V input voltage is always on and I am just toggling a 85% duty square wave from a function generator to the EN pin.
I don't think I am hitting any of the device protection modes, such as the output overvoltage protection. I am using IHLP3232CZER100M11 for the inductor. I tried changing the PWM frequency and it didn't notice a change. It enters this regulation mode maybe 4/5 times.
If I decrease the duty to 10%, this happens less frequently (maybe 1/15 times). It can always hit 85%+ duty when I start from a lower duty and ramp up.
I calculated the max output current according to Section 8.2.1.2.4 "Computing the Maximum Output Current" of the datasheet, I am below the theoretical maximum.
Is there some startup requirement that I need to meet?
Martin
