MIC2155 datasheet(23/33 Pages) MICREL

23 / 33 page

Micrel, Inc.

MIC2155/2156

May 2009

M9999-052709-A

(408) 944-0800

ΔVOPP is the peak to peak output voltage ripple

IPP is the peak to peak ripple current as see by the

capacitors

fS is the per channel switching frequency

Notice the calculation is performed at 2x the switching

frequency since the capacitors see ripple current from

both phases.

For this example, using ΔVOPP = 10mV, the minimum

COUT is:

kHz

500

COUT

≥

A capacitance value this low is usually not used in high

current converters because of transient output current

requirements.

For this example, 500µF total capacitance is used. It is

split up into (4) 47µF Ceramic capacitors and (2) 150µF

Aluminum Polymer capacitors

The total output ripple is a combination of the ESR and

the output capacitance. The total ripple is calculated

below:

[]2

ESR

OUT

⎥

⎦

⎤

⎢

⎣

⎡

To increase reliability, the recommended voltage rating

of capacitor should be twice the output voltage for a

tantalum and 20% greater for an aluminum electrolytic or

Ceramic.

The output capacitor RMS current is calculated below:

)

RMS

(

COUT

The power dissipated in the output capacitors can be

calculated by the equation below:

()

ESR

)

RMS

(

COUT

)

COUT

(

DISS

Inductor Current Sense Components

The RC circuit values that sense current across the

inductor can be calculated once the inductor is selected.

The circuit is shown in Figure 20.

Output Inductor and

Winding Resistance

Figure 20. Inductor Current Sense

The inductor has the following values:

L = 1.0µH, R L= 1.9mΩ

Proper sensing of the DC voltage across the inductor

requires the RL/L time constant be equal to the R1×C1

time constant.

A good range of values for C1 is 0.1µF to 1µF. For this

example C1 is chosen as 0.22µF. R1 is:

Input Capacitor Selection

In addition to high frequency Ceramic capacitors, a

larger bulk capacitance, either Ceramic or Al. El. should

be used to help attenuate ripple on the input and to

supply current to the input during large output current

transients. The input capacitors must be rated for the

RMS input current of the power supply. RMS input

capacitor current is determined at the maximum output

current. The graph in Figure 21 shows the normalized

RMS input ripple current vs. duty cycle. Data is

normalized to the output current.

For a 2 phase converter operating at 17% duty cycle, the

input RMS current is determined from the graph:

OUT

RMS

CIN

≈

The power dissipated in the input capacitor is:

(

)

ESR

)

RMS

(

CIN

)

CIN

(

DISS

0.1

0.2

0.3

0.4

0.5

0.6

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

DUTY CYCLE

Single Phase

2 Phase

Figure 21. RMS Input Current vs. Duty Cycle

MOSFET Selection

External N-Channel logic level power MOSFETs must be

used for the high and low side switches. The MOSFET

gate to source drive voltage of the MIC2155 is regulated

by an internal 5V VDD regulator. Logic level MOSFETs,

whose operation is specified at VGS = 4.5V must be

used. This resistance is used to calculate the losses

MIC2155 Datasheet(PDF) 23 Page - Micrel Semiconductor

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