CM3406 データシートの表示（PDF） - Monolithic Power Systems

部品番号

コンポーネント説明

メーカー

CM3406

1.5A, 210KHz Step-Down Converter

Monolithic Power Systems 

TM

The DC gain of the voltage feedback loop is:

A VDC

= RLOAD

× GCS

× A VEA

×

VFB

VOUT

Where RLOAD is the load resistor value, GCS is

the current sense transconductance and AVEA is

the error amplifier voltage gain.

The system has two 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.

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). This zero is located

at:

fZ1

=

1

2π × C3 × R3

The system may have another zero of

importance, if the output capacitor has a large

capacitance and/or a high ESR value. The zero,

due to the ESR and capacitance of the output

capacitor, is located at:

fESR

=

1

2π × C2 × RESR

In this case, a third pole set by the

compensation capacitor (C6) and the

compensation resistor (R3) is used to

compensate the effect of the ESR zero on the

loop gain. This pole is located at:

fP3

=

1

2π × C6 × R3

The goal of compensation design is to shape

the converter transfer function to get a desired

loop gain. The system crossover frequency

where the feedback loop has the unity gain is

important.

Lower crossover frequencies result in slower

line and load transient responses, while higher

crossover frequencies could cause system

unstable. A good rule of thumb is to set the

crossover frequency to below one-tenth of the

switching frequency.

To optimize the compensation components, the

following procedure can be used:

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. Determine the

R3 value by the following equation:

R3 = 2π × C2 × fC × VOUT

GEA × GCS VFB

Where fC is the desired crossover frequency,

which is typically less than one tenth of the

switching frequency.

2. Choose the compensation capacitor (C3) to

achieve the desired phase margin. For

applications with typical inductor values, setting

the compensation zero, fZ1, to below one forth

of the crossover frequency provides sufficient

phase margin. Determine the C3 value by the

following equation:

C3 >

4

2π × R3 × fC

Where, R3 is the compensation resistor value.

3. Determine if the second compensation

capacitor (C6) is required. It is required if the

ESR zero of the output capacitor is located at

less than half of the switching frequency, or the

following relationship is valid:

1

< fS

2π × C2 × RESR 2

If this is the case, then add the second

compensation capacitor (C6) to set the pole fP3

at the location of the ESR zero. Determine the

C6 value by the equation:

C6 = C2 × RESR

R3

Share Link: