SP7606

12/17/07 rev: A

SP7600

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page

switch-node

voltages.

The

current

sense resistor schematic setup is shown

in Figure 2.

Schematic for High Voltage Operation

Other Topologies

The SP7606 is not only capable of driving

boost regulator circuits, but can also be used

in flyback and SEPIC topologies. Look for an

application note in the future on EXAR’s web

site

Inductor selection

Typically the inductor needs to be chosen for

its current capability and size.

For most

applications using the SP7606 the inductor

should be chosen so that at light loads the

inductor runs in discontinuous mode and then

enters continuous mode of operation at high

loads. This allows the inductor to be

reasonably sized and helps with compensation

of the overall circuit as well. The procedure for

selecting the inductor current for discontinuous

mode of operation is as follows.

1 The first thing that needs to be determined

is the constant K. K represents a ratio of

MOSFET conduction to diode conduction.

Typically a value of 0.8 can be used, this will

assure that there is about 20% dead time

present to have DCM mode of operation.

2 The on time is calculated is:

Vo is the output voltage

Vinmin is the minimum input voltage

3 The inductor inductance is:

Where

K is .8 ratio of MOSFET and diode conduction

time to T (T=1/fsw)

= output impedance at full load.

Vo is the output voltage

Vinmin is the minimum input voltage

4 The inductor peak current Ip is:

Although

the

SP7606

typical

mode

of

operation is in DCM mode, due to easier

compensation below are the formulas for when

the SP7606 runs in CCM.

For continuous conduction mode the inductor

current is:

fsw is the switching frequency

Vd is forward diode drop of D1

Vo is the output voltage

Vinmin is the minimum input voltage

The approximate peak inductor current is:

MOSFET selection