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R7986ATR データシート(PDF) 16 Page - STMicroelectronics

部品番号 R7986ATR
部品情報  3 A step-down switching regulator for aerospace applications
PDF  37 Pages
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メーカー  STMICROELECTRONICS [STMicroelectronics]
ホームページ  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

R7986ATR データシート(HTML) 16 Page - STMicroelectronics

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Application information
R7986A
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DocID026997 Rev 1
and:
Equation 8
where VF is the forward voltage on the freewheeling diode and VSW is voltage drop across
the internal PDMOS.
The peak-to-peak voltage across the input capacitor can be calculated as:
Equation 9
where ESR is the equivalent series resistance of the capacitor.
Given the physical dimension, ceramic capacitors can well meet the requirements of the
input filter sustaining a higher input RMS current than electrolytic/tantalum types. In this
case the equation of CIN as a function of the target VPP can be written as follows:
Equation 10
neglecting the small ESR of ceramic capacitors.
Considering
= 1, this function has its maximum in D = 0.5, therefore, given the maximum
peak-to-peak input voltage (VPP_MAX), the minimum input capacitor (CIN_MIN) value is:
Equation 11
Typically, CIN is dimensioned to keep the maximum peak-to-peak voltage in the order of 1%
of VINMAX.
In Table 6 some multi-layer ceramic capacitors suitable for this device are reported:
A ceramic bypass capacitor, as close to the VCC and GND pins as possible, so that
additional parasitic ESR and ESL are minimized, is recommended in order to prevent
instability on the output voltage due to noise. The value of the bypass capacitor can go from
100 nF to 1 µF.
Table 6. Input MLCC capacitors
Manufacture
Series
Cap value (
F)
Rated voltage (V)
Taiyo Yuden
UMK325BJ106MM-T
10
50
GMK325BJ106MN-T
10
35
Murata
GRM32ER71H475K
4.7
50
DMIN
VOUT VF
+
VINMAX VSW
–
--------------------------------------
=
VPP
IO
CIN FSW
-------------------------
1
D
----
–

 D D
----
1D
–

+
ESR IO
+
=
CIN
IO
VPP FSW
---------------------------
1
D
----
–

 D D
----
1D
–

+
=
CIN_MIN
IO
2VPP_MAX FSW

------------------------------------------------
=



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