MP1527 datasheet(11/14 Pages) MPS

11 / 14 page

MP1527

2A, 1.3MHz

Step-Up Converter

MP1527 Rev 1.8_8/31/05

Monolithic Power Systems, Inc.

Monolithic Power Systems

There is also a right-half-plane zero (fRHPZ) that

exists in all continuous mode (continuous

mode means that the inductor current does not

drop

zero

each

cycle)

step-up

converters.

The frequency of the right half

plane zero is:

fRHPZ = VIN

2*RLOAD / (2π*L*VOUT2)

where L is the value of the inductor.

To stabilize the regulation control loop, the

crossover frequency (The frequency where the

loop gain drop to 0dB or gain of 1, indicated as

fC) should be at least one decade below the

right-half-plane zero and should be at most

75KHz.

fRHPZ is at its lowest frequency at

maximum output load current (RLOAD is at a

minimum)

The crossover frequency is calculated by the

equation:

fC = AVDC*fP1*fP2 / fZ1

fC = GCS*GEA*VIN*VFB*R3 / (2π*C2*VOUT

The known values are:

GCS = 4.3S

GEA = 400µS

VFB = 1.22V

Putting in the known constants:

fC = 3.3x10

-4 *VIN *R3/ (C2*VOUT2)

If the frequency of the right-half-pane zero

fRHPZ is less than 750KHz, then the crossover

frequency should be 1/10 of fRHPZ, and

determine the compensation resistor (R3) with

equation (1). If fRHPZ is greater than or equal to

750KHz, set the crossover frequency to 75KHz

with equation (2).

For fC = fRHPZ / 10, then

R3 = VIN*RLOAD-MIN*C2 / (10GCS*GEA*VFB*L)

The minimum load resistance (RLOAD-MIN) is

equal to the regulated output voltage (VOUT)

divided by the maximum load current ILOAD-MAX.

Substituting that into the above equation:

R3 = VIN*VOUT*C2 /(10GCS*GEA *VFB*L*ILOAD-MAX)

Putting in the known constant values:

(1) R3 ≈ 48*VIN*VOUT*C2 / (L*ILOAD-MAX)

For fC = 75KHz,

fC = (GCS*GEA*VIN*VFB*R3) / (2π*C2*VOUT

Solving for R3,

R3 = (2π*fC*C2*VOUT

2 / (G

CS*GEA*VIN*VFB)

Using 75KHz for fC and putting in the other

known constants:

(2) R3 ≈ 2.2x108*C2*VOUT

2 / V

The value of the compensation resistor is

limited to 10KΩ to prevent overshoot on the

output at turn-on. So if the value calculated for

R3 from either equation (1) or equation (2) is

greater than 10kΩ, use 10KΩ for R3.

Choose C3 to set the zero frequency fZ1 to

one-fourth of the crossover frequency fC:

fZ1 = fC / 4

1 /(2π*C3*R3) = GCS*GEA*VIN*VFB*R3 / (8π*C2*VOUT

Solving for C3:

C3 = 4*C2*VOUT

2 / (G

CS*GEA*VIN*VFB*R3

Entering the known values gives:

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