EV2351DQ-00A Просмотр технического описания (PDF) - Monolithic Power Systems

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EV2351DQ-00A

1.5A, 23V, 1.4MHz Step-Down Converter

Monolithic Power Systems 

TM

MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER

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.

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

External Bootstrap Diode

It is recommended that an external bootstrap

diode be added when the system has a 5V

fixed input or the power supply generates a 5V

output. This helps improve the efficiency of the

regulator. The bootstrap diode can be a low

cost one such as IN4148 or BAT54.

5V

BS

MP2351

SW

10nF

MP2351_F02

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty

cycle operation (when VOUT >65%) and high

VIN

output voltage (VOUT>12V) applications.

MP2351 Rev. 1.5

www.MonolithicPower.com

7

1/6/2006

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