Can someone give me more detailed description of LTM4644 overcurrent protection function?
How does it work? (working theory)
Response time (turn-off time)
Recovery time (turn-on time)
Each phase of the LTM4644 uses a constant on-time (& 'constant' frequency) valley current mode controller, where the inductor's valley current is sensed each cycle [ note : the inductor ripple current can be estimated by iLpp = (Vin - Vout) / (fsw x L) x (Vout/Vin), the inductor valley current is the load current minus half the ripple. Fsw = 1MHz, L = 1uH ].
In the datasheet electrical table the typical 'Ioutpk' (DC Output Current Limit) spec is 7A (for 12Vin to 1.5Vout operating condition) which corresponds to a typical valley current limit of 6A. If loaded beyond the valley current limit, since the inductor current is limited, the output voltage will start to fall accordingly. If the feedback pin voltage falls to below ~0.35V the part then also enables it's foldback current limiting feature which linearly reduces the valley current limit further (to about 40% of the original valley current limit @ 0V on FB pin), further reducing the output and input power during the short circuit condition.
This overcurrent protection is fast, current is sensed every switching cycle. The rate at which the output falls during a short circuit is dependent on the short impedance and output capacitance, recovery from the short circuit condition also depends on the control loop bandwidth.
Example captures of short circuit and recovery behavior are also available on page 5 of the datasheet. For more detail you can also simulate a short circuit event for the LTM4644 using LTSpice.
I simulated a short circuit event on Vout1 with the demo circuit http://www.analog.com/media/en/simulation-models/LTspice-demo-circuits/LTM4644_DC1900A.asc . I still see large current (10A) flowing out of Vout1 after Vout1 shorts to GND with 0.01 Ohms.
Thanks for the message. The spice model for the LTM4644 spice model regarding short circuit current limit is now under review. In the meantime the short circuit behavior is better represented by simulating with an example LTC3605A spice model. Thanks.
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