ADP3020 Просмотр технического описания (PDF) - Analog Devices

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ADP3020

High-Efficiency Notebook Computer Power Supply Controller

Analog Devices 

ADP3020

The Design

The details of designing the power inductor are covered in many

reference texts, and will not be covered here. Examples of soft-

ware and reference books that can be used for quick design of

the power inductor are given below:

Software—Magnetic Designer from Intusoft, www.intusoft.com

“Designing Magnetic Components for High Frequency

DC-DC Converters,” McLyman, Kg Magnetics Inc.,

ISBN 1-883107-00-08 (for advanced users)

“Power Supply Cookbook,” Marty Brown, EDN Series for

Design Engineers, ISBN 0-7506-9442-4 (for beginners and

intermediate users)

CIN and COUT Selection

In continuous conduction mode, the source current of the upper

MOSFET is approximately a square wave of duty cycle VOUT/VIN.

To prevent large voltage transients, a low ESR input capacitor

sized for the maximum rms current must be used. The maximum

rms capacitor current is given by:

( ) IRMS ≈

VOUT × VIN – VOUT

× IMAX

VIN

(6)

This formula has a maximum at VIN = 2 × VOUT, where IRMS =

IOUT/2. Note that the capacitor manufacturer’s ripple current

ratings are often based on only 2000 hours of life. This makes it

advisable to further derate the capacitor, or to choose a capacitor

rated at a higher temperature than required. Several capacitors

may also be paralleled to meet size or height requirements in the

design. If electrolytic or tantalum capacitors are used, an addi-

tional 0.1 µF–1 µF ceramic bypass capacitor should be placed in

parallel with CIN.

The selection of COUT is driven by the required effective series

resistance (ESR) and the desired output ripple. A good rule of

thumb is to limit the ripple voltage to 1% of the nominal output

voltage. It is assumed that the total ripple is caused by two factors:

25% comes from the COUT bulk capacitance value, and 75%

comes from the capacitor ESR. The value of COUT can be deter-

mined by:

COUT

=

2×

I RIPPLE

f × VRIPPLE

(7)

where IRIPPLE = 0.3 × IOUT and VRIPPLE = 0.01 × VOUT. The

maximum acceptable ESR of COUT can then be found using:

ESR ≤ 0.75 × VRIPPLE

I RIPPLE

(8)

Manufacturers such as Vishay, AVX, Elna, WIMA and Sanyo

provide good high-performance capacitors. Sanyo’s OSCON

semiconductor dielectric capacitors have lower ESR for a given

size, at a somewhat higher price. Choosing sufficient capacitors

to meet the ESR requirement for COUT will normally exceed

the amount of capacitance needed to meet the ripple current

requirement.

In surface-mount applications, multiple capacitors may have to

be paralleled to meet the capacitance, ESR, or RMS current

handling requirements. Aluminum electrolytic and dry tantalum

capacitors are available in surface-mount configurations. In the

case of tantalum, it is critical that capacitors are surge tested for

use in switching power supplies. Recommendations for output

capacitors are shown in Table VI.

Power MOSFET Selection

N-channel power MOSFETs must be selected for use with the

ADP3020 for both the main and synchronous switch. The main

selection parameters for the power MOSFETs are the threshold

voltage (VGS(TH)) and ON-resistance (RDS(ON)). An internal LDO

generates a 5 V supply that is boosted above the input voltage

using a bootstrap circuit. This floating 5 V supply is used for the

upper MOSFET gate drive. Logic-level threshold MOSFETs

must be used for both the main and synchronous switches.

Maximum output current (IMAX) determines the RDS(ON) require-

ment for the two power MOSFETs. When the ADP3020 is

operating in continuous mode, the simplifying assumption can

be made that one of the two MOSFETs is always conducting the

load current. The duty cycles for the MOSFETs are given by:

Upper MOSFET Duty Cycle = VOUT

(9)

VIN

Lower MOSFET Duty Cycle = VIN – VOUT

(10)

VIN

Maximum Output

Current

Input Capacitors

Output Capacitors

+3.3 V Output

Output Capacitors

+5 V Output

Table VI. Recommended Capacitor Manufacturers

2A

TOKIN Multilayer

Ceramic Caps, 22 µF/25 V

P/N: C55Y5U1E226Z

TAIYO YUDEN INC.

Ceramic Caps, Y5V Series

10 µF/25 V

P/N: TMK432BJ106KM

SANYO POSCAP TPC

Series, 68 µF/10 V

SANYO POSCAP TPC

Series, 68 µF/10 V

4A

TOKIN Multilayer

Ceramic Caps, 2 × 22 µF/25 V

P/N: C55Y5U1E226Z

TAIYO YUDEN INC.

Ceramic Caps, Y5V Series

2 ×10 µF/25 V

P/N: TMK432BJ106KM

SANYO POSCAP TPC

Series, 2 × 68 µF/10 V

SANYO POSCAP TPC

Series, 2 × 68 µF/10 V

10 A

TOKIN Multilayer

Ceramic Caps, 2 × 22 µF/25 V

P/N: C55Y5U1E226Z

VISHEY Ceramic Caps,

Z5U Series, 2 × 15 µF/25 V

SANYO POSCAP TPB

Series, 2 × 220 µF/4.0 V

SANYO POSCAP TPB

Series, 2 × 330 µF/6.3 V

REV. 0

–15–

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