7

FN9030.8

March 10, 2006

Overcurrent Protection

The overcurrent function protects the converter from a

shorted output by using the upper MOSFETs on-resistance,

converter’s efficiency and reduces cost by eliminating a

current sensing resistor.

The overcurrent function cycles the soft-start function in a

programs the overcurrent trip level. An internal 200

µA

10

µA current sink and inhibits PWM operation. The soft-start

the error amplifier clamped to the SS voltage. Should an

complete its cycle. Figure 4 shows this operation with an

overload condition. Note that the inductor current increases

overcurrent trip. The converter dissipates very little power

with this method. The measured input power for the

conditions of Figure 4 is 2.5W.

The overcurrent function will trip at a peak inductor current

is typical). The OC trip point varies mainly due to the

from the equation above with:

2. Determine

,

where

∆I is the output inductor ripple current.

For an equation for the ripple current see the section under

component guidelines titled Output Inductor Selection.

A small ceramic capacitor should be placed in parallel with

presence of switching noise on the input voltage.

Current Sinking

The ISL6522 incorporates a MOSFET shoot-through

protection method which allows a converter to sink current

as well as source current. Care should be exercised when

designing a converter with the ISL6522 when it is known that

the converter may sink current.

When the converter is sinking current, it is behaving as a boost

converter that is regulating its input voltage. This means that

that is being down-converted. If there is nowhere for this current

a voltage limiting protection device, or other methods, the

the voltage level of the rail is boosted to a level that exceeds the

maximum voltage rating of the MOSFETs or the input

capacitors, damage may occur to these parts. If the bias

FIGURE 3. SOFT-START INTERVAL

VOLTAGE

TIME

t

1

C

SS

I

SS

----------- V

OSC MIN

()

⋅

=

t

2

t

1

–

C

SS

I

SS

-----------

V

OUT

SteadyState

V

IN

------------------------------------------------

∆V

OSC

⋅⋅

==

Where:

STEADY STATE

0A

0V

TIME (20ms/DIV)

5A

10A

15A

2V

4V

FIGURE 4. OVERCURRENT OPERATION

•

()

---------------------------------------------------

=

()

∆I

() 2

⁄

+

>

ISL6522