MIC2132 datasheet(33/48 Pages) MICROCHIP

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DS20006654B-page 33

MIC2132

The ESR of the total input capacitance can be

determined in Equation 5-18:

EQUATION 5-18:

The input capacitor must be rated for the input current

ripple. The rated RMS value of the input capacitor cur-

rent is determined at the maximum output current.

Assuming the peak-to-peak inductor current ripple is

low, the RMS current rating of the input capacitor can

be estimated from Equation 5-19:

EQUATION 5-19:

The graph in Figure 5-2 shows the normalized RMS

input capacitor current vs. duty cycle for single-phase,

two-phase, four-phase, six-phase and eight-phase

buck converter operation. Data are normalized to the

output current.

FIGURE 5-2:

Normalized RMS Input

Capacitor Current vs. Duty Cycle.

For a given multiphase buck converter operating at

duty cycle D, the input capacitor RMS current can also

be determined from the graph in Figure 5-2, together

with Equation 5-20.

EQUATION 5-20:

The power dissipated in the input capacitor can then be

computed from Equation 5-21:

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. A safety factor of

30% should be added to the VIN(MAX), while selecting

the voltage rating of the MOSFETs to account for

voltage spikes due to circuit parasitic elements, as

shown in Equation 5-22.

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 Equation 5-23.

EQUATION 5-23:

ESRCIN ≤ VIN ×

IOUT

Where:

IOUT(MAX) = Maximum Output Current

k = 0,1,2,3,.... for D > k/n and k < n, Maximum

Integer Less than nxD

n = Total Number of Phases (n = 2,4,6,8)

ICIN(RMS) ≈ IOUT(MAX) ×









D –×

[]









– D –

√

ICIN(RMS) = ICINRMS(NORM) × IOUT(MAX)

Where:

ICINRMS(NORM) = Normalized RMS Input Capacitor

Current at given Duty Cycle for

Given Multiphase Buck Converter

from Figure 5-2

PDISS(CIN) = ICIN(RMS)2 × ESRCIN

VDS(RATING) ≥ VIN(MAX) × 1.3

Where:

IOUT(MAX) = Maximum Output Current

n = Total Number of Phases

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

ISWHS(PK) = ISWLS(PK) =

IOUT(MAX)

IL(PP)

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