MP1527DM datasheet(12/14 Pages) MPS

12 / 14 page

MP1527

2A, 1.3MHz

Step-Up Converter

MP1527 Rev 1.8_8/31/05

Monolithic Power Systems, Inc.

Monolithic Power Systems

C3 ≈ 1.9x103 C2 VOUT

2 / (VIN R32)

In some cases, if an output capacitor with high

capacitance

and

high

equivalent

series

resistance (ESR) is used, then a second

compensation capacitor (from COMP to

SGND) is required to compensate for the zero

introduced by the output capacitor ESR. The

extra capacitor is required if the ESR zero is

less than 4x the crossover frequency.

The

ESR zero frequency is:

fZESR = 1 / (2π*C2*RESR)

The

second

compensation

capacitor

required if:

4*fC ≥ fZESR

4*GCS*GEA*VIN*VFB*R3

(2π*C2*VOUT

≥

(2π*C2*RESR)

Simplifying:

(8.4x10-3*VIN*R3*RESR )/ VOUT

2 ≥ 1

If this is the case, calculate the second

compensation capacitor by the equation:

R3*C4 = C2*RESR

C4 = (C2*RESR) / R3

Example

Given:

Input Voltage (VIN): 5V

Output Voltage (VOUT): 12V

Maximum Load Current (ILOAD-MAX): 500mA

Output Capacitor (C2): 10µF (ESR=10mΩ

Maximum)

Inductor Value (L): 4.7µH

Find the frequency of the right-half-plane zero:

fRHPZ = VIN

2 / (2π*L*VOUT*ILOAD-MAX)

fRHPZ = (5V)

2 /

(2π*4.7µH*12V*500mA)=141KHz

The frequency of the right-half-plane zero is

less than 750khz, so use equation (1) to

determine the compensation resistor R3:

R3 ≈ 48*VIN*VOUT*C2 / (L*ILOAD-MAX)

R3 ≈ 48*5*12*10µF/(4.7µH*500mA) =12.3KΩ

(use 10KΩ)

Find the compensation capacitor C3:

C3 ≈ 1.9x10

3*C2*V

OUT

2 / (V

IN*R3

C3 ≈ 1.9x10

3*10µF (12V)2 / (5 * 10KΩ2) = 5.4nF

(use the nearest standard value, 5.6nF)

Determine

the

second

compensation

capacitor is required:

8.4x10

-3 * 5V * 5.6KΩ * 10mΩ / 12V2 = 0.016 ≤ 1

Therefore no second compensation capacitor

is required.

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