Hello support team,
I'm using an LTM8052 (to be sure, without "A") module in a prototype power supply. The current limits are set to maximum (both CTL_I and CTL_T tied to VREF).
The design includes 200 uF input capacitance and 500 uF output capacitance. No resistive load was connected to the output.
The input voltage (28VDC) is supplied to the module via a power diode to protect the input power supply against occasional voltage rise.
The output voltage set point can be switched between 10V and 16V using a digitally controlled feedback resistor network.
The experiments are as follows:
1. The output voltage set point is switched from 10V to 16V. The output voltage rises in about 600 us, implying a 5A output charging current. This is exactly as expected.
2. The output voltage set point is switched back from 16V to 10V. In this case, the actual output voltage first drops very slowly for about 75 ms, then drops much faster (in about 10 ms) to its new value of 10V. Calculations show that the initial output discharge current (i.e., the current sinking into the power module) is about 10 mA that lasts about 75 ms. Then the current increases to about 250 mA and lasts about 10 ms. To further investigate the circuit, I looked at the ADJ pin, and found out that the voltage jumps from 1.2V to about 1.7V when the set point is switched from 16V to 10V, and remains above 1.2V for 75 + 10 = 85 ms. This behavior indicates that when the output is loaded with some capacitance, and the output voltage set point is caused to decrease, the output voltage remains unregulated for a while.
3. When some resistive load is added to the output, the initial slow discharge stage is omitted, and the 16V to 10V transition takes about 10 ms. But - once again! - the output sinking current is about 250 mA, much lower than the maximum of 7A.
This contradicts to the datasheet in two respects:
a). The output sinking current is much smaller than the specified 7A. I expected that the output capacitor will be discharged with the maximum available sinking current when the set point is decreased, just like the same capacitor is charged with the maximum available sourcing current when the set point is increased.
b). The output voltage can stay unregulated for a while, despite the current limit is not exceeded, and moreover is not even approached.
Please explain this situation.
Thanks in advance and best regards,
Michael Levin
