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© 2019 ROHM Co., Ltd.
No. 61UG024E Rev.002
Jul. 2021
User’s Guide
BM2P141X-EVK-001
2.1 Determining Coil Inductance
– Continued
Also, calculate L value so that the overcurrent detection becomes maximum load current I
OUT
: 715 mA or more. Overcurrent
detection is calculated by the current flowing through the MOSFET when operating in continuous mode at the minimum
switching frequency f
SW
(Min) = 60 kHz. When the current flowing through the MOSFET (≠ the coil current at switching ON)
exceeds the minimum value Ipeak (Min): 1.8 A of the overcurrent detection current, the MOSFET is turned OFF. Since a delay
of approximately tdly = 0.1 μsec occurs, in reality, the peak current exceeds the Ipeak value and the peak current becomes Ip.
The peak current Ip is obtained by setting the current slope at switching ON to ΔI
L
,
𝐼
𝑃
= 𝐼𝑝𝑒𝑎𝑘 + ∆𝐼
𝐿
× 𝑡𝑑𝑙𝑦
𝐼
𝑃
= 𝐼𝑝𝑒𝑎𝑘 +
𝑉
𝐼𝑁
− 𝑉𝑜
𝐿
× 𝑡𝑑𝑙𝑦
Calculate the output current Io (LIM) at overcurrent detection by securing a margin of 10% from the maximum load current of
715 mA, and setting it as 787 mA.
𝐼
𝑂𝑈𝑇
(𝐿𝐼𝑀) = 𝐼𝑝 −
𝐼𝑟𝑖𝑝𝑝𝑙𝑒
2
> 𝐼
𝑂𝑈𝑇
(𝑀𝑎𝑥)
Calculate the minimum value of the L value of the coil. From the above formula,
L >
{𝑉
𝐼𝑁
(𝑀𝑖𝑛)×𝑡𝑑𝑙𝑦×𝑓
𝑆𝑊
(𝑀𝑖𝑛)−(𝑉
𝑂𝑈𝑇
+𝑉
𝐹
)}×(𝑉
𝐼𝑁
(𝑀𝑖𝑛)−𝑉
𝑂𝑈𝑇
)
2×𝑓
𝑆𝑊
(𝑀𝑖𝑛)×(𝐼
𝑂𝑈𝑇
(𝑀𝑎𝑥)−𝐼𝑝𝑒𝑎𝑘(𝑀𝑖𝑛))×𝑉
𝐼
(𝑀𝑖𝑛)
= 95.0
[μH]
Therefore, the inductance value of the coil is discontinuous mode when the rated current Io (Typ) is 0.5 A, and in order to detect
the overcurrent of the maximum load current Io (Max): 0.7
15 A or more, the condition of 95.0 μH to 206.3 μH , A coil of 150 μH
is selected.
2.2 Inductor Current Calculation
Calculate the maximum peak current of the inductor. The condition where the peak current is maximized is when the input
voltage is the maximum voltage VIN (Max): 380 V, the maximum load current Io (Max): 0.715 A, and the switching frequency is
60 kHz at the minimum.The ripple current Iripple of the coil is given by the following formula.
𝐼𝑟𝑖𝑝𝑝𝑙𝑒 =
𝑑𝑖
𝑑𝑡
× 𝑡
𝑂𝑁
=
{𝑉
𝐼𝑁
(𝑀𝑎𝑥) − (𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
)}
𝐿
×
(𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
)
𝑉
𝐼𝑁
(𝑀𝑎𝑥) × 𝑓
𝑆𝑊
(𝑀𝑖𝑛)
