During 100% load step test in the EVKIT i.e. 3.2A‐>0A load transition, output voltage does not recover to nominal steady state voltage after overshoot and stays at an elevated level, until the next load cycle begins.
The observed MAX17504 load transient response is the expected operation for a large load step case (3.2A‐>0A ) and can be explained as follows:
During a large load dump, output voltage overshoot is high. In order to bring the output voltage back to a nominal level, the low side MOSFET turns on for a longer period of time (LX stays LOW) and the inductor current becomes negative to discharge the output. However, once the inductor current hits the negative current limit, both high side and low side MOSFETs turn off (LX stays at high impedance), and the regulator waits for the output voltage to decay close to nominal voltage so that it can start switching again.
During this time, the output voltage discharge only depends on load current.
For absolute no‐load condition, output voltage stays at higher level until next load step cycle begins. Refer to waveform below for part operation analysis.
If we test 3.2A‐>50mA load transient i.e. increase the minimum load current from 0 to 50mA then we will see that the output voltage can be discharged back to nominal regulation level within 200us during 3.2A‐>50mA
load transient, before next load cycle begins. The results are shown as below:
The standard EVKIT MAX17504EVKITA is designed to support 50% load transient and maintain output voltage deviation to around 3%.
To make the EVKIT perform well for a 100% load step the following changes in the EVKIT are recommended:
1. Output capacitor = 3 * 47uF/1210 (GRM32ER71A476KE)
2. Feedback resistor divider – Rtop = 75kohm, Rbottom = 28kohm
The proposed design has been tested by MAXIM and test result is shown as below: