MP2309 Просмотр технического описания (PDF) - Monolithic Power Systems

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MP2309

1A, 23V, 340KHz Synchronous Rectified Step-Down Converter

Monolithic Power Systems 

TM

MP2309 – 1A, 23V, 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:

IC1 = ILOAD ×

VOUT

VIN

×⎜⎜⎛1−

⎝

VOUT

VIN

⎟⎞

⎟

⎠

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

where IC1 = ILOAD/2. For simplification, choose

the input capacitor whose 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 caused by capacitance can

be estimated by:

∆VIN

=

ILOAD

C1

×

VOUT

VIN

× ⎜⎜⎝⎛1 −

VOUT

VIN

⎟⎟⎠⎞

Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Ceramic, tantalum, or low

ESR electrolytic capacitors are recommended.

Low ESR capacitors are preferred to keep the

output voltage ripple low. The output voltage

ripple can be estimated by:

∆VOUT

=

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.

In the case of ceramic capacitors, the

impedance at the switching frequency is

dominated by the capacitance. The output

voltage ripple is mainly caused by the

capacitance. For simplification, the output

voltage ripple can be estimated by:

∆VOUT

=

8

×

VOUT

fS2 × L

×

C2

×

⎜⎜⎝⎛1

−

VOUT

VIN

⎟⎟⎠⎞

In the case of tantalum or electrolytic

capacitors, the ESR dominates the impedance

at the switching frequency. For simplification,

the output ripple can be approximated to:

∆VOUT

=

VOUT

fS × L

×

⎜⎜⎝⎛1 −

VOUT

VIN

⎟⎟⎠⎞ × RESR

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP2309 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP2309 employs current mode control for easy

compensation and fast transient response. The

system stability and transient response are

controlled through the COMP pin. COMP pin is

the output of the internal transconductance

error amplifier. A series capacitor-resistor

combination sets a pole-zero combination to

control the characteristics of the control system.

The DC gain of the voltage feedback loop is

given by:

A VDC

= RLOAD

× GCS

× A VEA

×

VFB

VOUT

Where VFB is the feedback voltage, 0.923V;

AVEA is the error amplifier voltage gain; GCS is

the current sense transconductance and RLOAD

is the load resistor value.

The system has 2 poles of importance. One is

due to the compensation capacitor (C3) and the

output resistor of 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.

MP2309 Rev. 1.4

www.MonolithicPower.com

7

3/13/2006

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© 2006 MPS. All Rights Reserved.

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