Post Go back to editing

Hi,

The Datasheet says this device has the automatic transition between PFM and PWM.
But I want to konw how much output curennt trigers PFM mode in general.
Because this device is usd for the analog circuit directly in my system.

I know the transition current depends on Vin, Vout and inductor.
But it would be great if there is a way to predict it.

Best regards,
Tarzan

• Hi Tarzan,

If you are worried about PFM, then you might consider switching to an ADP2370.  This part has a mode pin that allow it operate in force PWM mode so there is no transition to worry about.  This part will also probably give you better efficiency since it is synchronous and will be a smaller solution.

In terms of when the ADP2300 will go into PFM.  If you use the ADISimpower tool and look at the efficiency curve for your design, you will often see an abrupt change in slope.  This is when the part goes from CCM to DCM.  PFM will only occur once the pulse width goes below the minimum ontime

If you use the equations below you can calculate t1 (on time).  If you set t1 = minimum on time from the datasheet you can solve for the approximate Iout where PFM will occur.

V1 = Vin - Vout - Iout * (FET1_RDSon + dcr)

V2 = -Vout - Vf - Iout * dcr

t1 = Sqr(2 * Iout * L * Ts / (V1 - V1 ^ 2 / V2))

t2 = (-V1 / V2) * t1

t3 = Ts - t1 - t2

V1= ontime voltage across inductor

V2 = off time voltage across inductor

Iout = output current

Vf = forward voltage of diode

t1 = ontime on switch

t2 = on time of diode

SQR = square root function

Ts = period of clock (1/Fsw)

Fsw = switching frequency

DCR = DCR of inductor

FET1_RDSon = rdson on the high side FET

L = inductance

Regards,

Kevin

• Hi kevin,

Thank you for the detailed explanation.

I see your points, so I will check how it actuallly works on the eval-board.

Best regards,

Tarzan

• This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

Thank you,