APW7074 datasheet(13/21 Pages) ANPEC

13 / 21 page

Rev. A.4 - Aug., 2008

APW7074

www.anpec.com.tw

Function Description (Cont.)

Over-Current Protection (Cont.)

- The minimum I

OCSET (170µA) and minimum ROCSET

should be used in the above equation.

- Note that the I

LIMIT is the current flow through the

upper MOSFET; I

LIMIT must be greater than maximum out-

put current add the half of inductor ripple current.

An over current condition will shut down the device and

discharge the C

SS with a 10µA sink current and then ini-

tiate the soft-start sequence. If the over current condition

is not removed during the soft-start interval, the device

will be shut down while the over current is detected and

the SS still rises to 4V to complete its cycle. The soft-start

function will be cycled until the over-current condition is

removed. Both over-current protections have the same

behavior while an over current condition is detected.

DS(ON)

LIMIT

0.29V

For the over-current is never occurred in the normal oper-

ating load range; the parameters R

DS(ON) and ILIMIT in the

above equation also have the same notices as the previ-

ous section.

Under Voltage Protection

The FB pin is monitored during converter operation by

their own Under Voltage (UV) comparator. If the FB volt-

age drops below 50% of the reference voltage (50% of 0.

8V = 0.4V), a fault signal is internally generated, and the

device turns off both high-side and low-side MOSFET

and the converter’s output is latched to float.

Over-Current Protection (monitor lower MOSFET)

The other over-current protection monitors the output cur-

rent by using the voltage drop across the lower MOSFET’s

DS(ON) and this voltage drop will be compared with the

internal 0.29V reference voltage. If the voltage drop across

the lower MOSFET’s R

DS(ON) is larger than 0.29V, an over-

current condition is detected. The threshold of the over

current limit is given by:

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