Technical Note
9/13
BD8301MUV
www.rohm.com
2009.09 - Rev.C
○
c
2009 ROHM Co., Ltd. All rights reserved.
(2) Output capacitor
A ceramic capacitor with low ESR is recommended for output in order to reduce output ripple.
There must be an adequate margin between the maximum rating and output voltage of the capacitor, taking the DC bias
property into consideration.
Output ripple voltage when ceramic capacitor is used is obtained by the following equation.
Setting must be performed so that output ripple is within the allowable ripple voltage.
(3) Setting of oscillation frequency
Oscillation frequency can be set using a resistance value connected to the RT pin (1 pin).
Oscillation frequency is set at 1 MHz when RT = 47 kΩ, and frequency is inversely proportional to RT value.
See Fig. 23 for the relationship between RT and frequency.
Soft-start time changes along with oscillation frequency.
See Fig. 24 for the relationship between RT and soft-start time.
* Note that the above example of frequency setting is just a design target value, and may differ from the actual equipment.
(4) Output voltage setting
The internal reference voltage of the ERROR AMP is 0.8 V. Output voltage should be obtained by referring to Equation
(8) of Fig. 25.
VREF
0.8V
VOUT
ERROR AMP
R1
R2
INV
Vo= ×0.8 [V] ・・・ (8)
R2
(R1+R2)
Vpp=⊿IL× + ⊿IL×R
ESR
[V] ・・・ (5)
2π×f×Co
1
100
1000
10000
1 10 100 1000
RT PIN RESISTANCE [k
Ω
]
SWITCHNG FREQUENCY [kHz]
Fig. 23 Oscillation frequency – RT pin resistance
0.1
1
10
1 10 100 1000
RT PIN RESISTANCE [k
Ω
]
SOFT START TIME [msec]
Fig. 24 Soft-start time – RT pin resistance
Fig. 25 Setting of feedback resistance
