MIC23451 datasheet(13/28 Pages) MICROCHIP

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DS20006662A-page 13

MIC23451

5.0

APPLICATIONS INFORMATION

The MIC23451 is a triple high performance DC-to-DC

step down regulator that offers a small solution size.

Supporting three outputs with currents up to 2A inside

a 4 mm × 4 mm FQFN package, the IC requires only

five external components per channel while meeting

today’s miniature portable electronic device needs.

Using the HyperLight Load® switching scheme, the

MIC23451 can maintain high efficiency throughout the

entire load range while providing ultra-fast load

transient response. The following sections provide

additional device application information.

5.1

Input Capacitor

A 2.2 µF or greater ceramic capacitor should be placed

close to the PVIN pin for each channel and its

corresponding PGND pin for bypassing. For example,

the Murata GRM188R61E475KE11D, size 0603,

4.7 µF ceramic capacitor is ideal, based on

performance, size, and cost. An X5R or X7R

temperature rating is recommended for the input

capacitor. Y5V temperature rating capacitors, in

addition to losing most of their capacitance over

temperature, can also become resistive at high

frequencies. This reduces their ability to filter out

high-frequency noise.

5.2

Output Capacitor

The MIC23451 is designed for use with a 2.2 µF or

greater ceramic output capacitor. Increasing the output

capacitance lowers output ripple and improves load

transient response, but could also increase solution

size or cost. A low equivalent series resistance (ESR)

ceramic output capacitor, such as the Murata

GRM188R61E475KE11D, size 0603, 4.7 µF ceramic

capacitor, is recommended based on performance,

size, and cost. Both the X7R or X5R temperature rating

capacitors are recommended. The Y5V and Z5U

temperature rating capacitors are not recommended

due to their wide variation in capacitance over

temperature and increased resistance at high

frequencies.

5.3

Inductor Selection

When selecting an inductor, it is important to consider

the following factors (not necessarily in order of

importance):

• Inductance

• Rated current value

• Size requirements

• DC resistance (DCR)

The MIC23451 is designed for use with a 0.47 µH to

2.2 µH inductor. For faster transient response, a

0.47 µH inductor yields the best result. On the other

hand, a 2.2 µH inductor yields lower output voltage

ripple. For the best compromise of these, a 1 µH is

generally recommended.

Maximum current ratings of the inductor are generally

given in two forms: permissible DC current and

saturation current. Permissible DC current can be rated

either for a 40°C temperature rise or a 10% to 20% loss

in inductance. Make sure the inductor selected can

handle the maximum operating current. When

saturation current is specified, make sure that there is

enough margin, so that the peak current does not

cause the inductor to saturate. Peak current can be

calculated as shown in Equation 5-1:

EQUATION 5-1:

As this equation shows, the peak inductor current is

inversely proportional to the switching frequency and

the inductance; the lower the switching frequency or

the inductance the higher the peak current. As input

voltage increases, the peak current also increases.

The size of the inductor depends on the requirements

of the application. Refer to the Typical Application

Schematic and Bill of Materials sections for details.

DC resistance (DCR) is also important. While DCR is

inversely proportional to size, DCR can represent a

significant efficiency loss. Refer to the Efficiency

Considerations section.

The transition between high loads (CCM) to HyperLight

Load® (HLL) mode is determined by the inductor ripple

current and the load current, as shown in Figure 5-1.

FIGURE 5-1:

Transition between CCM

Mode and HLL Mode.

The diagram shows the signals for high-side switch

drive (HSD) for tON control, the inductor current, and

the low-side switch drive (LSD) for tOFF control.

IPEAK

IOUT VOUT

1 VOUT

–

VIN



2 f



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











LOAD INCREASING

IN HLL MODE

ON FIXED, TOFF VARIABLE)

IN CCM MODE

ON VARIABLE, TOFF FIXED)

OUT

HSD

INDUCTOR

LSD

–50mA

HSD

INDUCTOR

LSD

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