Other Parts Discussed in Thread: LM2596,
Hi TI
I am having trouble with TPS2511 current limiting power switch, used with LM2596-adj for compensating for voltage drop by controlling the LM2596 FB pin from nCS pin via a resistor. This should give me 5.0-5.3 range on LM2596 output. I have followed ref designs for automotive USB charger which have been helpful but nowhere do I see discussion of the problems I am experiencing. I have a board layout according to guidelines in the TPS2511 datasheet, however I am experiencing the following issues:
1. I have Rilim set for 2.1A (24kohm). TPS2511 output switches off instantaneously when I connect a load of approx 500mA or more. I have to disconnect the load for the output to switch back on. Also TPS2511 does not power on with this load connected. I can see the output trying to switch on every 12s (hiccup mode) but it only switches on when I remove the load. Seems like deglitching is required on ILIM pin but up to 1uF cap does not help. Nothing is connected to DP or DM and they are not connected together. I am using a variable electronic load with a MOSFET acting as voltage controlled resistive load element. How can I tell what is causing shutdown? It could be instant overload detect, yes? Or what other mechanism? Anything to do with DP and/or DM?
2. The load compensation mechanism has unexpected behaviour different to data sheet. nCS does not switch down at approx half Ilim, instead nCS output drops slowly as load increases, as though the internal FET is turning on in its linear region. This is not correct surely? What can be causing that? Only very close to current limit does nCS output drop to within spec of the datasheet (max 140mV).
3. The power switch in TPS2511 is not turning on fully, I measure up to 700mV across the device IN to OUT at approx 900mA. This causes thermal shutdown. What can prevent the switch turning on fully? From the datasheet block diag there are many inputs to the driver, any of which could limit the driver from turning the power switch on. I am surely meeting all the power conditions in the design, how do I debug this further?
Your help/insight would be greatly appreciated.
