ADP3025 Просмотр технического описания (PDF) - Analog Devices

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ADP3025

High Efficiency Dual Output Power Supply Controller

Analog Devices 

ADP3025

CIN AND COUT SELECTION

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

In continuous conduction mode, the source current of the

provide good high performance capacitors. Sanyo’s OSCON

upper MOSFET is approximately a square wave of duty cycle

semiconductor dielectric capacitors have lower ESR for a given

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

size, at a somewhat higher price. Choosing sufficient capacitors

to meet the ESR requirement for COUT normally exceeds the

amount of capacitance needed to meet the ripple current

requirement.

( ) IRMS = VOUT × VIN − VOUT × IOUT(MAX)

VIN

(8)

In surface-mount applications, multiple capacitors may have to

be paralleled to meet the capacitance, ESR, or rms current han-

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

dling requirements. Aluminum electrolytic and dry tantalum

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

ratings are often based on only 2,000 hours of life. Therefore,

the user should further derate the capacitor, or choose one rated

at a higher temperature than required. Several capacitors may

be paralleled to meet size or height requirements in the design.

E If electrolytic or tantalum capacitors are used, an additional

0.1 µF to 1 µF ceramic bypass capacitor should be placed in

parallel with CIN.

T The selection of COUT is driven by the required effective series

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

is to limit the ripple voltage to 1% of the nominal output vol-

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

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

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

determined by

L COUT =

IRIPPLE

(9)

2 × f ×VRIPPLE

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

O acceptable ESR of COUT can then be found using

ESR ≤ 0.75 × VRIPPLE

S IRIPPLE

(10)

B Table 8. Recommended Capacitor Manufacturers

Maximum Output Current

2A

Input Capacitors

TOKIN Multilayer

OCeramic Caps, 22 µF/25 V

capacitors are available in surface-mount configurations. In the

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

use in switching power supplies. Recommendations for output

capacitors are shown in Table 8.

POWER MOSFET SELECTION

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

ADP3025 for the main and synchronous switches. 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 by

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 ADP3025 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 DutyCycle = VOUT

(11)

VIN

Lower MOSFET DutyCycle = VIN −VOUT

(12)

VIN

4A

TOKIN Multilayer

Ceramic Caps, 2 × 22 µF/25 V

P/N: C55Y5U1E226Z

P/N: C55Y5U1E226Z

TAIYO YUDEN INC.

Ceramic Caps, Y5V Series 10 µF/25 V

P/N: TMK432BJ106KM

TAIYO YUDEN INC.

Ceramic Caps, Y5V Series 2 × 10 µF/25 V

P/N: TMK432BJ106KM

Output Capacitors

SANYO POSCAP TPC

SANYO POSCAP TPC

3.3 V Output

Series, 68 µF/10 V

Series, 2 × 68 µF/10 V

Output Capacitors

SANYO POSCAP TPC

SANYO POSCAP TPC

5 V Output

Series, 68 µF/10 V

Series, 2 × 68 µF/10 V

Rev. A | Page 16 of 24

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