SP7606

12/17/07 rev: A

SP7600

©Copyright 2007 Exar Corporation

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Over current protection

The boost regulator topology inherently does

not have short circuit protection. The SP7606

converter uses a simple comparator circuit to

check for an over current condition on a pulse

by pulse basis. The Vset voltage threshold for

the over-current (OC) pin is set to 0.25V.

Current limit set point is:

Rdson current sense

Typically the converter current limit is set to

about 150% of the normal output current. This

allows the converter to function at maximum

output current without accidentally triggering

the current limit. 150% over current limit takes

into effect the variations in RDSon of the FET,

as well as the inductor inductance values. The

accuracy of the current sensing can be

increased by the use of a sense resistor. The

resistor values tend to be more accurate down

to 1%.

Current sense using resistor

The approximate associated power loss in the

resistor is:

Rsense=Current sense resistor

For continuous conduction mode the ton/T

becomes the duty cycle of the converter. For

DCM mode the value of K can be used.

The other benefit of this combination is that the

OC pin does not see the high voltages and the

converter can be used to generate much

higher voltages then 38V. Please refer to the

high output voltage operation section for a

more detailed explanation.

The over current protection can be disabled by

tying the OC pin to GND.

High Voltage Operation

The converter can be used to boost voltage to

higher than the rated voltage of the DD pin and

the OC pin. To do this two things need to be

done for proper operation.

1) The voltage set resistors need to be

connected directly to the Vfb pin

bypassing the DD internal FET. By

doing this the user can get voltages that

are higher than Vout of 38V. By doing

this the user does not have the output

voltage disconnect feature.

2) The second thing that needs to be done

is the circuit needs to use a current

sense resistor for current limit. This

prevents the OC pin from seeing high