7

FN9122.2

April 13, 2005

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 ISL6522A 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 ISL6522A 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

maximum voltage rating of the ISL6522A may be exceeded

and the IC will experience a catastrophic failure and the

converter will no longer be operational. Ensuring that there is a

path for the current to follow other than the capacitance on the

rail will prevent these failure modes.

Application Guidelines

Layout Considerations

As in any high frequency switching converter, layout is very

important. Switching current from one power device to

another can generate voltage transients across the

impedances of the interconnecting bond wires and circuit

traces. These interconnecting impedances should be

minimized by using wide, short printed circuit traces. The

critical components should be located as close together as

possible using ground plane construction or single point

grounding.

Figure 5 shows the critical power components of the

converter. To minimize the voltage overshoot the

interconnecting wires indicated by heavy lines should be part

of ground or power plane in a printed circuit board. The

components shown in Figure 6 should be located as close

Locate the ISL6522A within three inches of the MOSFETs,

Q1 and Q2. The circuit traces for the MOSFETs’ gate and

source connections from the ISL6522A must be sized to

handle up to 1A peak current.

Figure 6 shows the circuit traces that require additional

layout consideration. Use single point and ground plane

construction for the circuits shown. Minimize any leakage

close to the SS pin because the internal current source is

only 10

to the BOOT and PHASE pins.

•

()

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

=

()

∆I

() 2

⁄

+

>

PGND

LGATE

UGATE

PHASE

Q1

Q2

D2

FIGURE 5. PRINTED CIRCUIT BOARD POWER AND

GROUND PLANES OR ISLANDS

RETURN

ISL6522A

ISL6522A