10

SC220

dition. When the current flowing through the high-side

PMOS switch exceeds the current limit, the PMOS switch

is turned off. The high-side switch is then held off for a

period sufficient to allow inductor current to decay. When

the output voltage is close to the nominal regulation

point, this will look similar to constant current limiting. As

the output voltage falls, the imposed off-time is such that

the frequency will drop, thus the inductor will have appro-

priate time to reset. This may cause the average current to

reduce, giving a mild ‘fold-back’ characteristic to the

current limit, where the average load current drops as the

output voltage collapses (although the peak current

maintains its nominal value). In a short-circuit condition,

the system runs continuously at this reduced frequency.

Load Transient Response

The SC220 features excellent regulation during line and

load transients. This is due to the nature of the proprietary

control method and the high di/dt allowed by the use of a

small inductor. This allows for best performance while

providing the benefit of using a small low cost output

OUT

shows only tens of millivolts of ripple

1mA to 650mA within hundreds of nanoseconds, using

typical a 1μF output capacitor. (See the typical perfor-

mance curves on page 7).

X-EMI™ Inductor Technology

The SC220 is the industry’s first buck regulator that enables

designers to draw their own inductors directly on the PC

board. This patented technology is called X-EMI™ inductor

technology and is different from conventional PCB trace

inductors. Conventional PCB trace inductors can be used

with high-frequency switchers, but they exhibit significant

EMI issues. X-EMI™ inductor technology solves these EMI

problems and can meet or exceed the EMI performance of

conventional chip inductors.

inductors adjacent to each other in anti-phase position,

where the magnetic fields of each air-core inductor par-

tially cancel one another to reduce EMI. The net flux from

the two inductors, partially cancels the wide leakage paths

caused by the wide geometry, but is still able to store the

energy of the inductors in series. This approach uses mul-

tiple air core inductors implemented using printed circuit

traces. Because these traces may be located on internal

layers of the PCB, inductors can be designed with almost

no impact on the available PCB area for components on

the surface of the board. (See the efficiency curves using

X-EMI™ inductor technology on page 6).

External Components

The input and output capacitors used for the operation of

the SC220 should be multilayer ceramic capacitors, prefer-

ably with X7R or X5R dielectric. The SC220 is optimized to

operate with typical 1μF input and output capacitance. In

practice, the self-resonant frequency of surface mount

capacitors of this type is well below the 20MHz ripple fre-

quency, so efforts to minimize output ripple may best be

focused on minimizing the inductance of the output

capacitor and the length of the circuit path to ground

rather than on increasing the capacitance value of the

output capacitor.

The inductor should be a high current inductor rated for

currents at least up to 1A. Typical power inductors may be

too lossy at 20MHz unless specifically designed for high

frequency operation. For inductor value of 220nH, it may

be preferred to use wire-wound RF chokes, many of which

are rated for appropriate currents.

Some Recommended External Components

Value

Manufacturer

Part Number

Package

L

0.22μH

Coilcraft

XFL2005-221ME

2x2mm

0.24μH

Coilcraft

0603LS-241XJL

0603

C

IN

1μF

Murata

GRM155R61C105KA12

0402

C

OUT

1μF

Murata

GRM155R61C105KA12

0402

Applications Information (Cont.)

Figure 1. Top view of an evaluation board with X-EMI inductor