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© 2022 ROHM Co., Ltd.
65UG028E Rev.001
2022.7
User's Guide
BD7F105EFJ-EVK-001
V
OUT
Np
Ns
V
F
Figure 36. Snubber Circuit
Application Design Examples-continued
7.
Snubber circuit
Excessive on the SW pin at turn-off when the degree of coupling of the transformer is low or the large current line of
the board is long, etc.
Voltage may be applied.
To suppress this, use the snubber circuitry indicated by Figure 34.
This snubber circuit clamps the voltage when the flyback v surge voltage exceeds this snubber voltage.
The clamping voltage is determined by the following equation.
𝑉
𝐶𝐿𝐴𝑀𝑃
= 𝑉
𝐹2
+ 𝑉
𝑧
𝑉
𝐶𝐿𝐴𝑀𝑃
: Clamp setting voltage of snubber
𝑉
𝐹2
: Forward voltage of the Schottky diode
𝑉
𝑧
: Zener voltage of the zener diode
At turn-off if the clamp setting voltage is lower than the flyback voltage
𝑁
𝑃
𝑁
𝑆
× (𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
)
A current flows through the zener. Therefore, be sure to set a voltage higher than the flyback voltage.
In addition, the snubber circuit may not be clamped at the set clamping voltage because of its operational
responsiveness.
Therefore, be sure to check the clamp voltage in actual operation.
8.
SDX/EN terminal resistor
8.1
Setting Enable Voltage
After releasing VIN UVLO, Enable voltage V
IN_ENABLE
can be set by the following equation.
𝑉
𝐼𝑁_𝐸𝑁𝐴𝐵𝐿𝐸
= 𝑉
𝐸𝑁1
×
𝑅
1
+ (𝑅
2
//𝑅
𝑆𝐷𝑋/𝐸𝑁
)
𝑅
2
//𝑅
𝑆𝐷𝑋/𝐸𝑁
𝑉
𝐼𝑁_𝐸𝑁𝐴𝐵𝐿𝐸
: Target operations start VIN voltage
𝑉
𝐸𝑁1
: Enable volt1
𝑅
2
//𝑅
𝑆𝐷𝑋/𝐸𝑁
: Partial pressure resistance between R
2
and R
SDX/EN
inside the ICs
[V]
[V]
