SP7606

12/17/07 rev: A

SP7600

©Copyright 2007 Exar Corporation

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page

CIRCUIT DESCRIPTION THEORY OF OPERATION

The SP7606 converter is a voltage mode boost

controller. The control loop has built in Type 2

compensation coupled with high switching

frequency allows the user to use small

components when designing the output filter.

The

equations

below

show

generic

relationships as applicable to the boost

regulator running in discontinuous conduction

mode (DCM) and continuous conduction mode

(CCM) of operation.

Duty Cycle in continuous conduction mode

(CCM)

 

Where

Vd= Forward voltage drop of D1

Duty Cycle in discontinuous conduction mode

(DCM)

 

    

Where fsw is the switching frequency

L is the inductor

Re is the effective resistance of the small

signal model

Re can be found by using as follows:

Setting the output voltage

The output voltage of the SP7606 can be set

by using an output voltage divider. The internal

reference of this part is set to 0.8V. Due to the

internal compensation, resistor R1 might need

to be chosen according to the desired gain of

the compensation loop. This resistor is

typically between 100K and 1M ohm. Resistor

R2 can be determined by:

Vfb=800mV Feedback Voltage

R1=Top Voltage divider resistor

R2=Bottom Voltage divide resistor

200=the typical impedance of the DD

FET

For typical applications resistor R1 should be

connected between Vout and the DD pin. The

DD pin serves as a disconnect for the output

voltage divider when the SP7606 is disabled.

This feature allows the user to save power

when the converter is not running. The typical

Impedance of the DD FET when enabled is

200 ohms. If the DD pin is not used the resistor

R1 can be connected directly to the Vfb pin to

get the proper output voltage. If this type of

connection is used the Rdson of the DD Fet

can be ignored and the equation becomes:

Vfb=800mV Feedback Voltage

R1=Top Voltage divider resistor

R2=Bottom Voltage divide resistor

If the DD pin is not used the converter can be

used to boost voltages beyond 38V. A more

detailed discussion on this topic can be found

in the section “High Voltages Operation”. A 10

to 22pf decoupling capacitor from the feedback

pin to ground is recommended when the

SP7606 is used with resistor values above 20K

in the voltage divider circuit.