CX8508 Ver la hoja de datos (PDF) - Unspecified

Número de pieza

componentes Descripción

Fabricante

CX8508

Monolithic Synchronous Buck Regulator / 2A Output Current

Unspecified 

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The worst-case condition

occurs at VIN = 2VOUT, where ICIN = 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 for low ESR

capacitors can be estimated by:

Where CIN is the input capacitance value.

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 voltageripple can be estimated

by:

Compensation Components

CX8508 employs current mode control for easy

compensation and fast transient response. The sys-

tem 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 com-

bination to control the characteristics of the control

system.

The DC gain of the voltage feedback loop is given by:

Where VFB is the feedback voltage, 0.925V;

AVEA is the error amplifier voltage gain; GCS is the cur-

rent 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:

Where CO 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:

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:

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:

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 ca-

pacitance of the output capacitor, is located at:

In this case (as shown in Figure 2), a third pole set by

the compensation capacitor (C6) and the compensa-

tion resistor (R3) is used to compensate the effect of

the ESR zero on the loop gain. This pole is located at:

The characteristics of the output capacitor also affect

the stability of the regulation system. The CX8507

optimized for a wide range of capacitance and ESR

values.

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