MB39C308 Datasheet, PDF (31/60 Pages) Fujitsu Component Limited. – 6ch DC/DC Converter IC for LPIA Platform VR

MB39C308
• FET Selection (CH1, CH2)
This IC operation requires the voltage which is generated between drain and source on the high-side FET. Set
the on resistance of high-side FET within the below range for reference.
CH1 high-side FET on resistance : 24 mΩ to 40 mΩ
CH2 high-side FET on resistance : 12 mΩ to 20 mΩ
The current limit value for over current protection (OCP) is determined by the high-side FET on resistance in
use. The current limit value is obtained by the following formula.
0.141
IO1_OCP =
−
RON1
0.5
L × fOSC
(VDD − VO1) × VO1
×
VDD
0.133
IO2_OCP =
−
RON2
0.5
L × fOSC
(VDD − VO2) × VO2
×
VDD
VDD : Switching system power supply voltage [V]
VO : Output setting voltage [V]
RON : High-side FET on resistance [Ω]
L : Inductor value [H]
fOSC : Switching frequency [Hz]
Also, 2.5 V drive products are recommended for the high-side FET. A bootstrap diode is recommended to connect
to the high-side FET for the use of 4 V drive products (see “• Bootstrap Diode Selection” for the detail).
In order to judge whether the electrical current flowing through the FET is within the rated value, the maximum
value of the current flowing through the FET needs to be found. The maximum current flowing through the FET
can be found from the following formula.
ΔIL
IDMAX ≥ IoMAX +
2
IDMAX : Maximum value of FET drain current [A]
IoMAX : Maximum load current [A]
ΔIL : Inductor ripple current peak-to-peak value [A]
Furthermore, in order to judge whether the power dissipation of the FET is within the rated value, the power
dissipation of the FET needs to be found. The power dissipation of the high-side FET can be found from the
following formula.
PHisideFET = PRON + PSW
PHisideFET : High-side FET power dissipation [W]
PRON : High-side FET conducting power dissipation [W]
PSW
: High-side FET SW power dissipation [W]
DS04-27261-6E
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