MIC23451 datasheet(13/20 Pages) MICREL

13 / 20 page

Micrel, Inc.

MIC23451

November 5, 2013

Revision 1.2

Application Information

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

step down regulator offering a small solution size.

Supporting three outputs with currents up to 2A inside a

4mm × 4mm QFN 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.

Input Capacitor

A 2.2µF 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 GRM188R60J475ME19D, size 0603, 4.7µF

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.

Output Capacitor

The MIC23451 is designed for use with a 2.2µF 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

the

Murata

GRM188R60J475ME84D, size 0603, 4.7µF 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.

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 0.47µH to

2.2µH inductor. For faster transient response, a 0.47µH

inductor yields the best result. On the other hand, a

2.2µH inductor yields lower output voltage ripple. For the

best compromise of these, a 1µH 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 2:

























−

OUT

PEAK

Eq. 2

As Equation 2 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 2.

Figure 2. 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.

In HLL mode, the inductor is charged with a fixed TON

pulse on the high-side switch (HSD). After this, the LSD

is switched on and current falls at a rate of VOUT/L. The

controller remains in HLL mode while the inductor falling

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