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© 2022 ROHM Co., Ltd.
65UG028E Rev.001
2022.7
User's Guide
BD7F105EFJ-EVK-001
Figure 35. Application Block Diagram
Application Design Examples-continued
2.
Output voltage
When the built-in switching MOSFET is turned OFF, the SW pin voltage V
SW
becomes higher than the VIN pin voltage.
Since the difference between the SW pin voltage and the VIN pin voltage is equal to the primary flyback voltage, the
secondary output voltage is calculated from this voltage. The SW pin voltage V
SW
at turn-off is calculated by the
following equation.
𝑉
𝑆𝑊
= 𝑉
𝐼𝑁
+
𝑁
𝑃
𝑁
𝑆
× (𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
)
𝑉
𝑆𝑊
: SW pin voltage
𝑉
𝐼𝑁
: VIN pin voltage
𝑁
𝑃
: No. of primary transformer turns
𝑁
𝑆
: Secondary transformer turns
𝑉
𝑂𝑈𝑇
: Output voltage
𝑉
𝐹
: Forward voltage of the output diode on the secondary side
The primary flyback voltage is converted to the FB-pin inrush current I
FB
by the external resistor R
FB
between FB-SW
terminals. Since the FB pin voltage becomes almost equal to the VIN pin voltage by the IC's internal circuit, the FB
pin inrush current I
RFB
is calculated by the following equation.
𝐼
𝐹𝐵
=
𝑉
𝑆𝑊
− 𝑉
𝐹𝐵
𝑅
𝐹𝐵
=
𝑉
𝐼𝑁
+
𝑁
𝑃
𝑁
𝑆
× (𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
) − 𝑉
𝐹𝐵
𝑅
𝐹𝐵
=
𝑁
𝑃
𝑁
𝑆
× (𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
)
𝑅
𝐹𝐵
𝐼
𝐹𝐵
: FB pin inrush current
𝑉
𝐹𝐵
: FB terminal voltage
𝑅
𝐹𝐵
: External resistor between FB and SW pins
FB
SW
VIN
V
OUT+
V
OUT-
SDX/EN
L_COMP
REF
GND
V
IN
V
F
R
FB
R
REF
N
P
N
S
[V]
[A]
