MIC2133 datasheet(35/50 Pages) MICROCHIP

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DS20006653B-page 35

MIC2133

Assuming the peak-to-peak inductor current ripple is

low, the RMS current rating of the input capacitor can

be estimated from the equation below.

EQUATION 5-19:

The graph in the figure below shows the normalized

RMS input capacitor current vs. duty cycle for both

single-phase and two-phase buck converter operation.

Data are normalized to the output current.

FIGURE 5-2:

Normalized RMS Input

Capacitor Current vs. Duty Cycle.

For a two-phase buck converter operating at duty cycle D,

the input capacitor RMS current can also be

determined from the graph in the figure above, together

with the equation below.

EQUATION 5-20:

The power dissipated in the input capacitor can then be

computed from the equation below.

EQUATION 5-21:

The voltage rating of the input capacitor must be high

enough to withstand the high input voltage. The

recommended voltage rating is at least 1.25 times the

maximum input voltage.

5.4

Switch Power MOSFET Selection

The following parameters are important for MOSFET

selection:

• Voltage rating

• Current rating

• On-resistance

• Total gate charge

The voltage rating for both the high-side and low-side

MOSFETs in the buck converter is essentially equal to

the power stage Input Voltage, VIN. It is recommended

that a safety factor of 30% be added to the VIN(MAX),

while selecting the voltage rating of the MOSFETs to

account for voltage spikes due to circuit parasitic ele-

ments, as shown in the equation below.

EQUATION 5-22:

The peak switch current for both the high-side and

low-side MOSFET in the buck converter is the same and

is equal to the peak inductor current in each phase, as

shown in the equation below.

EQUATION 5-23:

The RMS current rating of the high-side power

MOSFET in each phase channel is approximated in the

equation below.

EQUATION 5-24:

ICIN RMS



IOUT MAX



---

–





---

–





–





Where:

IOUT(MAX) = Maximum Output Current

k = 0,1 for D > k/n and k < n, Maximum

Integer Less than n×D

n = Total Number of Phases

ICIN(RMS) = ICINRMS(NORM)  IOUT(MAX)

Where:

ICINRMS(NORM) = Normalized RMS Input Capacitor

Current at Given Duty Cycle for

Two-Phase Buck Converter from

Figure 5-2

PDISS CIN



ICIN RMS



2 ESR

CIN



VDS RATING



VIN MAX



1.3





ISWHS PK



ISWLS PK



IOUT MAX



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

I

LPP



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

Where:

IOUT(MAX) = Maximum Output Current

n = Total Number of Phases

ΔIL(PP) = Peak-to-Peak Inductor Current per

Phase

ISWHS RMS



IOUT MAX



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

DMAX



Where:

DMAX = Maximum Duty Cycle

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