CX8508 Ver la hoja de datos (PDF) - Unspecified

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CX8508

Monolithic Synchronous Buck Regulator / 2A Output Current

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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 insta-

bility. A good rule of thumb is to set the cross-

over frequency below one-tenth of the switching

frequency.

To optimize the compensation components, the

following procedure can be used.

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 rela-

tionship is valid:

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:

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. Determine the

R3 value by the following equation:

Where fC is the desired crossover frequency

which is typically below one tenth of the switch-

ing frequency.

2. Choose the compensation capacitor (C3) to

achieve the desired phase margin. For applica-

tions with typical inductor values, setting the

compensation zero, fZ1, below one-forth of the

crossover frequency provides sufficient phase

margin.

Determine the C3 value by the following equa-

tion:

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 im-

prove the efficiency of the regulator. The bootstrap

diode can be a low cost one such as IN4148 or

BAT54.

CX8508

Where R3 is the compensation resistor.

3. Determine if the second compensation

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty cycle

operation when output voltage (VOUT>12V)

applications

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