NCP1578MNR2G【PDF 18/22 页】IC REG DL BCK/LINEAR SYNC 20-QFN

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NCP1578

http://onsemi.com

stable and high bandwidth overall loop. Worst case

component variation should be considered when selecting

the components of impedance network such that the control

loop phase margin should be greater than 45?

FREQUENCY

0dB

Modulator & Filter Gain

Open Loop Error Amp Gain

Compensation Gain

Converter Gain

Figure 36. Bode Plot of the Converter Gain

= F

ESR

= F

= 0.5 F

ESR

= 0.5 F

Input Capacitor Selection

It is used to minimize the input voltage ripple from the

power supply source. The input capacitors should be placed

as close as possible to the drain of the high-side MOSFET

and source of the low-side MOSFET. The PCB trace style

should be in form of short and wide ones. The voltage rating

and the RMS current rating are the important parameters for

the bulk input capacitor. In typical application, the bulk

capacitor should be selected such that the voltage and

current ratings must above the maximum input voltage and

largest RMS current required by system. As a safety

guideline, the capacitor voltage rating should be at least 1.5

times greater than the maximum input voltage. And the

RMS current rating requirement is approximately half of the

DC load current. The required input capacitor RMS ripple

current rating may be estimated by the following equation:-

Cin(RMS)

out

in(min)

out

in(min)

(eq. 8)

Ceramic capacitor is the good choice of the input

capacitor for notebook application due to its low ESR and

good ripple current rating and high voltage rating.

Aluminum electrolytic capacitors are also good choice.

They are relatively low cost but they should be used in

parallel connection to lower the ESR which is intrinsically

high compared with ceramic capacitors.

Output Capacitor Selection

The output capacitor's ESR will determine the output

ripple voltage and the initial voltage drop after a high

slew-rate transient. So, only specialized low-ESR

capacitors should be used for switching regulator

applications. For steady state ripple, both ESR and

capacitance of the output capacitor contribute output ripple

voltage. Normally, ESR is the dominant factor for output

ripple voltage. The output ripple voltage DV

can be

estimated by the following equation:

+DI

ESR)

8 C

out

(eq. 9)

and

) V

L F

(eq. 10)

= DI

X ESR if C

out

is large enough.

Where:

= Inductor ripple current

ESR = Effective Series Resistance of the output capacitor

L = Inductance

out

= Output capacitance

= Switching frequency

= Input Voltage

= Regulated output voltage

From the above equations, it can be seen that the output

ripple voltage can be reduced by either using the inductor

with larger inductance or the output capacitor with smaller

ESR value. In general rule of thumb, the inductor ripple

current is typically 30% of the maximum load current and

the ripple voltage is typically 2% of the output voltage.

The output capacitor also plays an important role in

response of load step up or release transients. The voltage

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