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Buck regulator stability operating in DCM.

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cmosekv

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I have a simple synchronus voltage mode PWM Buck regualtor 5V to 2.5V design with load at 2.5A, it operates in CCM and it's stable. However, when the load is reduced to 250mA, the regulator is operating in DCM and it's unstable.

Anyone know why and how to solve this issue.
 

If it has a synchronous rectifier, I don't see how the buck would operate discontinuous. The current flow will reverse in the inductor if it approaches zero but that is not discontinuous. Maybe it is due to your load becoming smaller which is changing the transfer function. You may need to stabilize your regulator to operate at lighter loads.
 

snafflekid,

You are correct. There's an error in my simulation making the low side switch reverse conducting thus actually making inductor current goes negative making it DCM. However, when load is increased to 2.5A there's no negative inductor current.
 

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