MIC2155 datasheet(24/35 Pages) MICREL

24 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

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 two-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

during the MOSFET’s conduction time. If operating at

4.5VIN, without connecting VDD to VIN, the gate drive

voltage to the high-side FET could be as low as 3.2V.

MOSFETs with low VGS enhanced gates should be used

in this situation.

It is important to note the on-resistance of a MOSFET

increases at high junction temperature. A 75°C rise in

junction temperature will increase the channel resistance

of the MOSFET by 40% to 75% of the resistance

specified at 25°C. This change in resistance must be

accounted for when calculating MOSFET power

dissipation.

Total gate charge is the charge required to turn the

MOSFET on and off under specified operating conditions

(VDS and VGS). The gate charge is supplied by the

MIC2155 gate drive circuit. Gate charge can be a

significant source of power dissipation in the controller

due to the high switching frequencies and generally

large MOSFETs that are driven. At low output load this

power dissipation is noticeable as a reduction in

efficiency.

MIC2155 Datasheet(PDF) 24 Page - Micrel Semiconductor

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