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© 2018 ROHM Co., Ltd.
No. 60UG092E Rev.001
JULY.2018
User’s Guide
BM2P0161-EVK-003
2-2
．
Current sensing resistor
；
R4
The current-sensing resistor and current limiter is set maximum output power.
R4 =
W + 20= × X* 5/ 7
9
=
0.4 + 0.02 × 0.212/65000 × 10
Y
0.9828
= 0.473
R4 is selected 0.43
Ω
.
Calculation of R4 power loss
P
Z[ \]^_
=
9
6
× 4 = 0.982
6
8 × 0.43Ω = 0.415a
P
Z[ bcd
= Ipav × R4 = 0.212A × 0.43Ω = 0.0912W
Set the value 1W or above in consideration of pulse resistance.
The structure of the resistance may vary the pulse resistance even with the same power rating.
Check with the resistor manufacturers for details.
2-3
．
VCC diode
；
D2
A high-speed diode (Small size rectifier or fast recovery diode) is recommended as the VCC-diode.
Diode current is only IC current, it maximum value is around 15mA.
When D2_Vf=1V, reverse voltage applied to the VCC-diode:
Vdr = VCC max + Vf + VIN max ×
X
This IC has VCC OVP function, VCC OVP (max) =29.0V.
Reverse voltage of the diode is set so as not to exceed the Vr of diode in conditions of VCC OVP (max).
Vdr = 29V + 1.0V + 400V ×
14 *O
77 *O = 103
With a design-margin taken into account, selected RRE02VSM4S
0.2A/400V
2-4
．
VCC winding surge-voltage limiting resistor
；
R5
Based on the transformer’s leakage inductance (Lleak), a large surge-voltage (spike noise) may occur during the instant when
the MOSFET is switched from ON to OFF. This surge-voltage is induced in the VCC winding, and as the VCC voltage increases
the IC’s VCC overvoltage protection may be triggered.
A limiting resistor R5 (approximately 5
Ω
to 22
Ω
) is inserted to reduce the surge-voltage that is induced in the VCC winding.
Confirm the rise in VCC voltage while the resistor is assembled in the product.
