MB39C015 Datasheet, PDF (15/43 Pages) Cypress Semiconductor – High efficiency : 96% (Max)

MB39C015
10. Application Notes
10.1 Selection of components
■ Selection of an external inductor
Basically it dose not need to design inductor. This IC is designed to operate efficiently with a 2.2 H 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.
  ^ IPK IOUT
VIN VOUT
×
D
×
L
fosc
1 _{ IOUT }  (VIN VOUT) × VOUT
2
2 × L × fosc × VIN
L
: External inductor value
IOUT : Load current
VIN : Power supply voltage
VOUT : Output setting voltage
D
: ON-duty to be switched ( = VOUT/VIN)
fosc : Switching frequency (2.0 MHz)
ex) When VIN  3.7 V, VOUT  2.5 V, IOUT  0.8 A, L  2.2 H, fosc  2.0 MHz
The maximum peak current value IPK;
 (VIN VOUT) × VOUT
(3.7 V  2.5 V) × 2.5 V
IPK  IOUT 
2 × L × fosc × VIN
 0.8 A 
=: 0.89 A
2 × 2.2 H × 2.0 MHz × 3.7 V
■ 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, FUJITSU SEMICONDUCTOR generally recommends a 4.7 F 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 F 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.
Document Number: 002-08364 Rev. *A
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