MB39C007 Datasheet, PDF (14/52 Pages) Fujitsu Component Limited. – 2 ch DC/DC Converter IC Built-in Switching FET & voltage detection function, PFM/PWM Synchronous Rectification, and Down Conversion Support

MB39C007
■ APPLICATION NOTES
[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 μH external
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.
VIN − VOUT
D
1
(VIN − VOUT) × VOUT
IPK = IOUT +
×
×
= IOUT +
L
fosc
2
2 × L × fosc × VIN
L
IOUT
VIN
VOUT
D
fosc
: External inductor value
: Load current
: Power supply voltage
: Output setting voltage
: ON-duty to be switched ( = VOUT/VIN)
: Switching frequency (2 MHz)
ex) When 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 is obtained by the following formula.
IPK = IOUT +
(VIN − VOUT) × VOUT
(3.7 V − 2.5 V) × 2.5 V
= 0.8 A +
=: 0.89 A
2 × L × fosc × VIN
2 × 2.2 μH × 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 MICROELEC-
TRONICS 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%) . FUJITSU MICROELECTRONICS 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, FUJITSU MICROELECTRONICS recommends the type
with an internal fuse.
14
DS04-27246-2E