MPM3695-100 external ramp

In the “Selecting the Output Capacitor” section of the datasheet it says that when using ceramic output capacitors

The ESR contributes minimally to the output voltage ripple, so an external ramp must be implemented to stabilize the system. Design the external ramp using R4, C4, and the equations above.

However, there’s no mention of R4 or C4 anywhere else in the datasheet, and the equations mentioned don’t really shed any light on the ramp design. Also, in the “Operation” section on page 18 it says

Internal ramp compensation guarantees stable operation for applications using zero-ESR ceramic output capacitors.

and later on the same page

The MPM3695-100 operates with zero-ESR ceramic output capacitors by using internal ramp compensation. A triangular RAMP signal is generated internally, then superimposed onto the FB signal.

which sounds like no external ramp compensation is needed.

So is an external ramp needed or not? If so, what’s the design process? Any help would be appreciated.

Hello stephen.harvey,

Typically, with ceramic caps that have too low ESR you will need to use ramp compensation. With the MPM3695-100, there is internal ramp compensation. However, if that is not enough, then you will need to use external ramp compensation methods that is outlined on page 45. You may be able to get away without needing external ramp compensation if you use a combination of POSCAPs and ceramics caps.

You can read more about why ramp compensation is needed for COT converters here: Advantages of Constant-On-Time Control in DC/DC Converters | Article | MPS.

Vinh Tran

Thanks for the response; that link was helpful in clarifying the purpose of the external ramp. However, the design procedure for this external ramp is still not clear to me. Page 45 of the datasheet is not helpful as all it says is “Design the external ramp using R4, C4, and the equations above”, but, as I stated in my original post, there’s no mention of R4 or C4 anywhere else in the datasheet and the “equations above” are for estimating ripple voltage, not designing the ramp.

The article you linked to shows a ramp generator circuit that can be made by placing an RC network between the switched node, the output, and the feedback pin, but I don’t see how this helps as the switched nodes are not exposed on the MPM3695-100. Even if the switched nodes were exposed, the article only gives the topology and provides no info on computing the necessary values.

If you can provide further info about the specifics of designing this ramp generator, that would be appreciated. Again, the information in the datasheet is not sufficient for me to design this.

Hello stephen.harvey,

The Vout node is used to connect the ramp compensation circuit. It is connected to the output of the integrated inductor. This can be seen on the block diagram.

Please read this for how to design the ramp compensation circuit: Designing a Stable COT Converter for a Desired Load and Line Regulation.

Vinh Tran

Could you elaborate on this? The document you linked only shows examples of the compensation circuit being connected between the switched node, the Vout node and the FB node - I don’t see how I can connect a compensation circuit without access to the switched node.

To elaborate, take a look at Figure 10 in the document you reference. There is no SW pin in the MPM3695-100, so where am I meant to connect R4? I could just omit it and only connect C4 and R9, but that doesn’t seem like it would generate much of a ramp.