Design guide IDP2303(A)
General features and system design considerations
Application Note
37
Revision 2.0
2017-05-03
5.1.2
Selection of VCC capacitor
According to the start-up procedure described above, when setting the value of the VCC capacitor, the
following conditions must be considered:
The requirement for the start-up time of the IC (
𝑡
𝑠𝑡𝑎𝑟𝑡𝑢𝑝
<1 second) is determined by the customer. With
the same charging current, a larger capacitor value gives a longer start up time, and vice-versa.
After the VCC voltage reaches VCC_ON, the energy stored in the VCC capacitor should be enough to
supply the IC after it becomes active and before the LLC starts to supply the VCC voltage.
To meet the first criteria,
𝐶
𝑉𝐶𝐶
<
𝐼
𝐻𝑉
∗ 𝑡
𝑠𝑡𝑎𝑟𝑡𝑢𝑝
𝑉
𝑐𝑐_𝑜𝑛
=
𝑉
90_𝑎𝑣𝑔
𝑅
𝑡𝑜𝑡𝑎𝑙
∗ 1
20.5
=
90 ∗
2√2
𝜋
51.7
∗ 1
20.5
=
1.57 𝑚𝐴 ∗ 1𝑠
20.5𝑉
= 76 µ𝐹
𝑅
𝑡𝑜𝑡𝑎𝑙
= 𝑅
𝑒𝑥𝑡
+ 𝑅
𝑖𝑛𝑡
= 51 + 0.7 = 51.7 𝑘𝛺
The value of VCC_ON can be found in the datasheet.
𝑅
𝑖𝑛𝑡
is the effective resistance considering the IC internal current consumption.
To meet the second criteria,
𝐶
𝑉𝐶𝐶
>
𝐼
𝑡𝑜𝑡𝑎𝑙∗
𝑡
𝑉𝐶𝐶_𝑂𝑁 − 𝑉𝐶𝐶_𝑂𝐹𝐹
=
22.94 𝑚𝐴 ∗ 20 𝑚𝑠
(20.5 − 7.5)𝑉
= 35 𝑢𝐹
𝐼
𝑡𝑜𝑡𝑎𝑙
= 𝐼
𝑎𝑐𝑡𝑖𝑣𝑒
+ 𝐼
𝑉𝐷𝐷𝑃
− 𝐼
𝐻𝑉
= 22.5 + 2 − 1.56 = 22.94 𝑚𝐴
The value of
𝐼
𝑎𝑐𝑡𝑖𝑣𝑒
, 𝐼
𝑉𝐷𝐷𝑃
and
𝑉𝐶𝐶_𝑂𝐹𝐹
can be found in the datasheet.
Combining the results above,
35 µ𝐹 < 𝐶
𝑉𝐶𝐶
< 76 µ𝐹
5.1.3
VCC supply circuit linear regulator
It is recommended to regulate the voltage supplied to the VCC pin to not be higher than 15 V, so as to reduce
the power consumed by the IC and avoid over temperature protection being triggered. A reference circuit
using a linear regulator in the VCC supply circuit is shown below.
Figure 24
Linear regulator in the VCC supply circuit
