APW7077 Просмотр технического описания (PDF) - Anpec Electronics

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APW7077

PWM Step-Up DC-DC Converter

Anpec Electronics 

APW7077/A

Function Description (Cont.)

Design Example(Cont.)

Determine the average inductor current and peak in-

ductor current:

I =1.38A

L

∆IL=0.218A

Ipk=1.45A

Therefore, a 10 uH inductor with saturation current

larger than 1.73 A can be selected as the initial trial.

Determine the output capacitance value for the desired

output ripple voltage:

COUT=33uF

The ESR of the output capacitor is 0.05Ω. Therefore,

a Tantalum capacitor with value of 33 uF to 47uF and

ESR of 0.05Ω can be used as the output capacitor.

However, according to experimental result, 220uF

output capacitor gives better overall operational stability

and smaller ripple voltage.

External Component Selection

Diode Selection

External Switch Transistor

The output diode for a boost regulator must be cho-

sen correctly depending on the output voltage and the

output current. The diode must be rated for a reverse

voltage equal to or greater than the output voltage used.

The average current rating must be greater than the

maximum load current expected, and the peak cur-

rent rating must be greater than the peak inductor

current. During short circuit testing, or if short circuit

conditions are possible in the application, the diode

current rating must exceed the switch current limit.

The diode is the largest source of loss in DC–DC

converters. The most importance parameters which

affect their efficiency are the forward voltage drop, VF,

and the reverse recovery time, trr. The forward voltage

drop creates a loss just by having a voltage across

the device while a current flowing through it. The re-

verse recovery time generates a loss when the diode

is reverse biased, and the current appears to actually

flow backwards through the diode due to the minority

carriers being swept from the P–N junction. Using

Schottky diodes with lower forward voltage drop will

decrease power dissipation and increase efficiency.

An enhancement N–channel MOSFET or a bipolar NPN

transistor can be used as the external switch transistor.

Since enhancement MOSFET is a voltage driven

device, it is a more efficient switch than a BJT

transistor. However, the MOSFET requires a higher

voltage to turn on as compared with BJT transistors.

An enhancement N–channel MOSFET can be selected

by the following guidelines:

• Low ON–resistance, RDS(on).

• Low gate threshold voltage, VGS(th), typically

<1.5V, it is especially important for the low VOUT

device, like VOUT = 2.4V.

• Rated continuous drain current, ID, should be

larger than the peak inductor current, i.e. ID > IPK.

• Gate capacitance should be 1200 pF or less.

For bipolar NPN transistor, medium power transistor

with continuous collector current typically 1A to 5A

and VCE(sat) < 0.2 V should be employed. The driv-

ing capability is determined by the DC current gain,

HFE, of the transistor and the base resistor, Rb; and

Copyright © ANPEC Electronics Corp.

15

Rev. A.4 - Sep, 2005

www.anpec.com.tw

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