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A6985F 数据表(PDF) 53 Page - STMicroelectronics

部件名 A6985F
功能描述  38 V 500 mA synchronous step-down switching regulator
PDF  77 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

A6985F 数据表(HTML) 53 Page - STMicroelectronics

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A6985F
Application notes
77
The peak-to-peak voltage across the input filter can be calculated as:
Equation 47
In case of negligible ESR (MLCC capacitor) the equation of CIN as a function of the target
VPP can be written as follows:
Equation 48
Considering
 = 1 this function has its maximum in D = 0.5:
Equation 49
Typically CIN is dimensioned to keep the maximum peak-peak voltage across the input filter
in the order of 5% VIN_MAX.
7.6.2
Inductor selection
The inductor current ripple flowing into the output capacitor determines the output voltage
ripple (please refer to Section 7.6.3). Usually the inductor value is selected in order to keep
the current ripple lower than 20% - 40% of the output current over the input voltage range.
The inductance value can be calculated by Equation 50:
Equation 50
Where TON and TOFF are the on and off time of the internal power switch. The maximum
current ripple, at fixed VOUT, is obtained at maximum TOFF that is at minimum duty cycle
(see Section 7.6.1: Input capacitor selection to calculate minimum duty).
Table 13. Input capacitors
Manufacturer
Series
Size
Cap value (
F)
Rated voltage (V)
TDK
C3225X7S1H106M
1210
10
50
C3216X5R1H106M
1206
Taiyo Yuden
UMK325BJ106MM-T
1210
VPP
IOUT
CIN fSW
-------------------------
1
D
----
–

 D
----
ESR
IOUT IL
+

+
=
CIN
IOUT
VPP fSW
--------------------------
1
D
----
–

 D
----
=
CINMIN
IOUT
4VPPMAX fSW
----------------------------------------------
=
I
L
VIN VOUT
–
L
------------------------------ TON
VOUT
L
-------------- TOFF
==



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