LM5576: Ripple current

Ryo,

This is not a device limitation, it is related to the current mode topology.

This device is a current mode controller, the control loop works best when when device is continuously conducting.

In buck converters at any point the output current is the 1/2Iripple + Ipk(-) . 

The peak to peak ripple current Iripple is controlled by the VIN and VOUT voltages, switching frequency, and inductor.

The Continuous conduction mode (CCM) boundary is exactly when the load current Io == 0.5 * Iripplepk-pk.

This is also exactly when valley current I_pk- is 0A.

If the output current Io falls below 0.5 * Iripple, then the valley of Iripple(I_pk-) would go negative, below 0A.

However this device has a lowside diode instead of a mosfet, and cannot conduct negative current, so the ripple current is cannot go below zero.

To avoid overcharging the output in light load conditions, instead the SW will just not turn on in the next cycle, and the device will operate in a discontinuous conduction mode (DCM), where the switching frequency is reduced.

The recommended ripple current is 500mA because the designed minimum output current is 250mA, that way the device runs in continuous conduction mode and fixed frequency when the output current is at least 250mA.

Running in DCM is not bad, however the switching frequency will not be as designed, and if the peak current is much larger than the output current you could see larger output voltage ripple.

If you have further questions let me know,

-Orlando

Ryo,

0.5 Ap-p ripple is the requirement for minimum load current = 250mA for the converter to always be in CCM.

The converter is still working if the load current falls below 250mA, just in DCM.

Most applications do not require the converter always in CCM, and sizing for CCM would be impractical for applications with small minimum load currents.

Instead I suggest you size your ripple current to be 40% of the MAX output current.

So if you are using the full 3A output that would be a 1.2A ripple current.

Please see http://www.ti.com/lit/an/slva057/slva057.pdf for how DCM works.

Once the inductor current reaches zero, diode conduction stops, and with 0 inductor current, SW node voltage will decay to VOUT voltage (with LC ring).

-Orlando