MP1484 Просмотр технического описания (PDF) - Monolithic Power Systems

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MP1484

3A, 18V, 340KHz Synchronous Rectified Step-Down Converter

Monolithic Power Systems 

MP1484 – 3A, 18V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER

Since the input capacitor (C1) absorbs the input

switching current, it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

When using tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output ripple can be approximated to:

IC1 = ILOAD ×

VOUT

VIN

×⎜⎜⎛1−

⎝

VOUT

VIN

⎟⎞

⎟

⎠

The worst-case condition occurs at VIN = 2VOUT,

where IC1 = ILOAD/2. For simplification, use an

input capacitor with a RMS current rating

greater than half of the maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, a small, high quality ceramic

capacitor, i.e. 0.1μF, should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to

prevent excessive voltage ripple at input. The

input voltage ripple for low ESR capacitors can

be estimated by:

ΔVIN

=

ILOAD

C1× fS

×

VOUT

VIN

× ⎜⎜⎝⎛1 −

VOUT

VIN

⎟⎟⎠⎞

Where C1 is the input capacitance value.

Output Capacitor

The output capacitor (C2) is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended. Under typical application

conditions ， a minimum ceramic capacitor

value of 20 μF is recommended on the output.

Low ESR capacitors are preferred to keep the

output voltage ripple low. The output voltage

ripple can be estimated by:

ΔVOUT

=

VOUT

fS × L

×

⎜⎜⎝⎛1

−

VOUT

VIN

⎟⎟⎠⎞ × ⎜⎜⎝⎛RESR

+

8

×

1

fS ×

C2

⎟⎟⎠⎞

Where C2 is the output capacitance value and

RESR is the equivalent series resistance (ESR)

value of the output capacitor.

When using ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance which is the main cause for the

output voltage ripple. For simplification, the

output voltage ripple can be estimated by:

ΔVOUT

=

VOUT

fS × L

×

⎜⎜⎝⎛1 −

VOUT

VIN

⎟⎟⎠⎞ × RESR

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP1484 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP1484 employs current mode control for easy

compensation and fast transient response. The

system stability and transient response are

controlled through the COMP pin. COMP is the

output of the internal transconductance error

amplifier. A series capacitor-resistor

combination sets a pole-zero combination to

govern the characteristics of the control system.

The DC gain of the voltage feedback loop is

given by:

A VDC

= RLOAD × GCS

× A EA

×

VFB

VOUT

Where VFB is the feedback voltage (0.925V),

AVEA is the error amplifier voltage gain, GCS is

the current sense transconductance and RLOAD

is the load resistor value.

The system has two poles of importance. One

is due to the compensation capacitor (C3) and

the output resistor of the error amplifier, and the

other is due to the output capacitor and the load

resistor. These poles are located at:

fP1

=

GEA

2π × C3 × A VEA

fP2

=

1

2π × C2 × RLOAD

Where GEA is the error amplifier transconductance.

ΔVOUT

=

8

×

VOUT

fS2 × L

×

C2

×

⎜⎜⎝⎛1

−

VOUT

VIN

⎟⎟⎠⎞

MP1484 Rev. 0.9

www.MonolithicPower.com

7

10/23/2008

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© 2008 MPS. All Rights Reserved.

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