BD9011EKN 查看數據表（PDF） - ROHM Semiconductor
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BD9011EKN
ROHM Semiconductor
BD9011EKN Datasheet PDF : 29 Pages
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(4) Feedback resistor design
Please refer to the following equation in determining the proper feedback resistance. The recommended setting is in a range
between 10kΩ and 330kΩ. Resistance less than 10kΩ risks decreased power efficiency, while setting the resistance value
higher than 330kΩ will result in an internal error amp input bias current of 0.2uA increasing the offset voltage.
Vo
Internal ref. 0.8V
R8
R8 +R9
Vo =
× 0.8 [V] ・・・（11）
R9
FB
R9
Fig-20
(5) Setting switching frequency
The triangular wave switching frequency can be set by connecting a resistor to the RT 15(33) pin. The RT sets the frequency
by adjusting the charge/discharge current in relation to the internal capacitor. Refer to the figure below in determining proper
RT resistance, noting that the recommended resistance setting is between 50kΩ and 130kΩ. Settings outside this range
may render the switching function inoperable, and proper operation of the controller overall cannot be guaranteed when
unsupported resistance values are used.
550
500
450
400
350
300
250
50
60
70
80
90
100
110
120
130
RT [ kΩ]
Fig-21 RT vs. switching frequency
(6) Setting the soft start delay
The soft start function is necessary to prevent an inrush of coil current and output voltage overshoot at startup. The figure
below shows the relation between soft start delay time and capacitance, which can be calculated using equation (12) at right.
10
1
0.8V(typ.)×CSS
TSS =
[sec]・・・(12)
ISS(10μA Typ.)
0.1
0.01
0.001
0.01
0.1
SS CAPACITANCE[uF]
Fig-22 SS capacitance vs. delay time
Recommended capacitance values are between 0.01uF and 0.1uF. Capacitance lower than 0.01uF may generate output
overshoots. Please use high accuracy components (such as X5R) when implementing sequential startups involving other
power sources. Be sure to test the actual devices and applications to be used, since the soft start time varies, depending on
input voltage, output voltage and capacitance, coils and other characteristics.
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