MIC2155 datasheet(21/35 Pages) MICREL

21 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

Each supply contributes approximately half the power:

For VIN1 = 12V and VIN2 = 3.3V

and

Component Selection, Guidelines and Design

Example

The following section outlines a procedure for designing

a two-phase synchronous buck converter using the

MIC2155.

This example will use the following parameters:

VIN = 12V

VOUT = 1.8V

IOUT = 30A

Switching frequency (fS) = 500kHz/channel

(MIC2155)

Output Filter Selection

The output filter is comprised of the output capacitors

and the output inductors. The filter is designed to

attenuate the output voltage ripple to the desired value.

The output filter components also determine how well

the supply responds to output current transients. If

output transients are significant, the output capacitors

should be chosen first to meet the transient specification.

The output inductor is then selected to insure the filter

attenuates the output ripple to meet the specification.

A second, commonly used method of designing the filter

is to select the inductor value to keep the ripple current

between 20% and 30% of the output current for that

channel. Then select the output capacitance to meet the

output voltage ripple specification and output current

transient specification.

Values for inductance, peak and RMS currents are

required to choose the output inductors. The input and

output voltages and the inductance value determine the

peak to peak inductor ripple current. Output capacitor

selection requires calculation of transient current, RMS

capacitor current and output voltage.

There are several tradeoffs to be made when selecting

the output inductor. Generally, higher inductance values

are used with higher input voltages. Larger peak to peak

ripple currents will increase the power dissipation in the

inductor and MOSFETs. Larger output ripple currents

will also require more output capacitance to smooth out

the larger ripple current. Smaller peak to peak ripple

currents require a larger inductance value and therefore

a larger and more expensive inductor.

Higher switching frequencies allow the use of a small

inductance but increase power dissipation in the inductor

core and MOSFET switching loss. The MIC2155

switches at 500kHz/channel and is designed to use a

smaller inductor at the expense of higher switching

losses and slightly lower efficiency. While the 300kHz

MIC2156 was optimized for higher efficiency and higher

output current but its lower switching frequency requires

a larger output inductance to maintain the same peak-to-

peak output ripple current.

The peak output ripple current for a two-phase converter

is shown in Figure 19. The graph shows that peak ripple

current is a function of duty cycle. Since each channel is

180° out of phase with the other, at 50% duty cycle, the

output ripple currents from each channel cancel and

output ripple current is close to zero.

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

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 19. 2 Phase Output Ripple Current vs. Duty Cycle

For this example, with VIN = 12V, VOUT = 1.8V and

efficiency = 88%, the duty cycle is:

OUT

Figure 19 shows the peak-to-peak output ripple current

normalized to:

OUT

The peak-to-peak output ripple current is less than for a

single phase conversion. If VIN varies, the input voltage

that generated the highest ripple current should be used

for the calculation.

MIC2155 Datasheet(PDF) 21 Page - Micrel Semiconductor

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