MB39C014 datasheet(14/32 Pages) CYPRESS

14 / 32 page

Document Number: 002-08361 Rev. *C

Page 14 of 32

MB39C014

10. Application Notes

10.1 Selection of Components

10.1.1 Selection of an External Inductor

Basically it dose not need to design inductor. This IC is designed to operate efficiently with a 2.2 µH (2.0 MHz operation) or 1.5 µH

(3.2 MHz operation) inductor.

The inductor should be rated for a saturation current higher than the LX peak current value during normal operating conditions, and

should have a minimal DC resistance. (100 mΩ or less is recommended.)

LX peak current value IPK is obtained by the following formula.

: External inductor value

IOUT

: Load current

VIN

: Power supply voltage

VOUT : Output setting voltage

: ON- duty to be switched(

= VOUT/VIN)

fosc : Switching frequency (2.0 MHz or 3.2 MHz)

ex) At VIN

= 3.7 V, VOUT = 2.5 V, IOUT = 0.8 A, L = 2.2 µH, fosc = 2.0 MHz

The maximum peak current value IPK;

10.1.2 I/O Capacitor Selection

■ Select a low equivalent series resistance (ESR) for the VDD input capacitor to suppress dissipation from ripple currents.

■ Also select a low equivalent series resistance (ESR) for the output capacitor. The variation in the inductor current causes ripple

currents on the output capacitor which, in turn, causes ripple voltages an output equal to the amount of variation multiplied by the

ESR value. The output capacitor value has a significant impact on the operating stability of the device when used as a DC/DC

converter. Therefore, Cypress generally recommends a 4.7 µF capacitor, or a larger capacitor value can be used if ripple voltages

are not suitable. If the VIN/VOUT voltage difference is within 0.6 V, the use of a 10 µF output capacitor value is recommended.

■ Types of capacitors

Ceramic capacitors are effective for reducing the ESR and afford smaller DC/DC converter circuit. However, power supply functions

as a heat generator, therefore avoid to use capacitor with the F-temperature rating (

− 80% to + 20%). Cypress recommends

capacitors with the B-temperature rating (

± 10% to ± 20%).

Normal electrolytic capacitors are not recommended due to their high ESR.

Tantalum capacitor will reduce ESR, however, it is dangerous to use because it turns into short mode when damaged. If you insist

on using a tantalum capacitor, Cypress recommends the type with an internal fuse.

IPK

= IOUT +

VIN

− VOUT

= IOUT +

(VIN

− VOUT) × VOUT

fosc

2 × L × fosc × VIN

IPK

= IOUT +

(VIN

− VOUT) × VOUT = 0.8A+ (3.7 V − 2.5 V) × 2.5 V := 0.89A

2 × L × fosc × VIN

2 × 2.2 µH × 2 MHz × 3.7 V

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