SP7606

12/17/07 rev: A

SP7600

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page

The MOSFET needs to be chosen based on

three main criteria. The drain to source voltage

needs to be higher than the output voltage of

the converter. The MOSFET needs to be able

to conduct the peak current that is calculated

in the inductor selection section. The Rdson of

the MOSFET needs to satisfy current limit

criteria. Picking a MOSFET with the lowest Qg

and Crss that meets the above requirement is

crucial to good efficiency. At 1.2MHz the

switching losses become a significant power

loss in the system even compared to the

Rdson of the MOSFET. For continuous

conduction mode the power loss in the

MOSFET is:

Where In max is the maximum input current

D is the duty cycle

Vo is the output voltage

Crss reverse transfer capacitance of MOSFET

fsw is the switching frequency

kt is the temperature dependency of RDSon

kg is the constant inversely proportional to gate

drive current a value of 1.5 should be used.

For discontinuous mode of operation the

power loss in the FET will is similar.

Crss reverse transfer capacitance of MOSFET

Fsw is the switching frequency

Ρ is the temperature dependency of RDSon

Z is the constant inversely proportional to gate drive

current

Where In max is the maximum input current

K is the duty cycle

Vo is the output voltage

Where Iinrms

The gate drive loss is associated with the FET

but it actually is lost in the driver IC. Below is

the formula for the gate charge loss Qg.

5V is the gate drive voltage

Qg is the total gate charge

Fsw is the switching frequency

Input capacitor selection

For both continuous and discontinuous mode

of operation the input capacitor needs to be

chosen based on maximum input voltage

rating and the RMS ripple current and

minimum input capacitance. For DCM mode

the RMS current is given by:

Where K is the conduction time constant

Ip is the peak inductor current

The minimum input capacitance that is

required is:

Ton is the calculated on time

Vin is the minimum input voltage

For CCM of operation the input capacitor ripple

current is:

Fsw is the switching frequency

Vo is the output voltage

Vinmin is the minimum input voltage

L inductor inductance