MP1410 datasheet(5/9 Pages) MPS

5 / 9 page

MP1410

2A Step-Down

Switch-Mode Regulator

PRELIMINARY

MP1410 Rev 1.0_ 05/29/02

www.monolithicpower.com

Monolithic Power Systems

Application Information (Continued)

The characteristics of the output capacitor also

effect the stability of the regulation control

system. Ceramic, tantalum, or low-ESR

electrolytic capacitors are recommended.

In the case of ceramic capacitors, the

impedance at the switching frequency is

dominated by the capacitance, and so the

output voltage ripple is mostly independent of

the ESR. The output voltage ripple is

estimated to be:

VRIPPLE ~= 1.4 * VIN * (fLC/fSW)^2

Where VRIPPLE is the output ripple voltage, VIN

is the input voltage, fLC is the resonant

frequency of the LC filter, fSW is the switching

frequency.

In the case of tantalum or low-ESR electrolytic

capacitors, the ESR dominates the impedance

at the switching frequency, and so the output

ripple is calculated as:

VRIPPLE ~= ∆I * RESR

Where VRIPPLE is the output voltage ripple, ∆I is the

inductor ripple current, and RESR is the equivalent

series resistance of the output capacitors.

Output Rectifier Diode

The output rectifier diode supplies the current

to the inductor when the high-side switch is off.

To reduce losses due to the diode forward

voltage and recovery times, use a Schottky

rectifier.

Choose a rectifier who’s maximum reverse

voltage rating is greater than the maximum

input voltage, and who’s current rating is

greater than the maximum load current.

Table 1 provides a list of manufacturer’s and

their websites.

Table 1: Schottky Diode Manufacturers

Manufacturer

Website

Diodes, Inc.

www.diodes.com

Fairchild Semiconductor

www.fairchildsemi.com

General Semiconductor

www.gensemi.com

International Rectifier

www.irf.com

On Semiconductor

www.onsemi.com

Pan Jit International

www.panjit.com.tw

Compensation

The output of the transconductance error

amplifier is used to compensate the regulation

system.

Typically compensation capacitors,

CC sets the dominant pole. The compensation

resistor sets a zero that should have the same

frequency as the pole set by the load

resistance and the output capacitor.

If the

output capacitor is not ceramic type, then there

may need to be another capacitor from COMP

to GND (CCA) to compensate for the zero

produced by the output capacitor and its ESR.

One of the critical parameters is the DC loop

gain. This can be determined by the equation:

AVL = (VFB / VOUT) * AEA * ACS * RL

Where AVL is the loop gain, VFB is the feedback

threshold, 1.22V, VOUT is the regulated output

voltage, AEA is the error amplifier voltage gain,

ACS is the current sense gain, and RL is the

load resistance, or VOUT / ILOAD.

Simplifying the equation:

AVL = AEA * ACS * (VFB / ILOAD(MAX)) ~= 1663 /

ILOAD(MAX)

Another critical parameter is the desired

crossover frequency.

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