参数资料
| 型号： | BD9778HFP-TR |
| 厂商： | Rohm Semiconductor |
| 文件页数： | 9/19页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 2A HRP7 |
| 标准包装： | 1 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1 V ~ 35 V |
| 输入电压： | 7 V ~ 35 V |
| 频率 - 开关： | 50kHz ~ 500kHz |
| 电流 - 输出： | 2A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | TO-263-8，D²Pak（7 引线+接片），TO-263CA |
| 包装： | 标准包装 |
| 供应商设备封装： | HRP7 |
| 其它名称： | BD9778HFP-DKR BD9778HFP-DKR-ND BD9778HFPDKR |
BD9778F, BD9778HFP, BD9001F, BD9781HFP
Description of external components
Technical Note
V IN
V IN
SW
L
V O
+
C
C in
Di
C O
R1
RT
INV
R T
C T
SS
GND
FB
C C
R C
R2
C SS
Fig.30
Design procedure
Vo = Output voltage, Vin (Max.) = Maximum input voltage
Io (Max.) = Maximum load current, f = Oscillation frequency
1. Setting or output voltage
Output voltage can be obtained by the formula shown below.
V O =1 x (1+R1/R2)
Use the formula to select the R1 and R2.
Furthermore, set the R2 to 30 k? or less.
Select the current passing through the R1 and R2 to be small
enough for the output current.
2. Selection of coil (L)
The value of the coil can be obtained by the formula shown
below:
L=(V IN -V O ) x V O / (V IN x f x ?I O )
?I O: Output ripple current
f = Operating frequency
?Io should typically be approximately 20 to 30% of Io.
If this coil is not set to the optimum value, normal (continuous)
oscillation may not be achieved. Furthermore, set the value of
the coil with an adequate margin so that the peak current passing
through the coil will not exceed the rated current of the coil.
Calculation example
When Vo = 5 V and R2 = 10 k? ,
5=1 x (1+R1/10k?)
R1=40k?
When V IN = 13.2 V, Vo = 5 V, Io = 2 A, and f = 100 kHz,
L=(13.2-5) x 5/13.2 x 1/100k x 1/(2 x 0.3)
=51.8μH 47μ
L=47 μ H
3. Selection of output capacitor (Co)
The output capacitor can be determined according to the
output ripple voltage ?Vo (p-p) required.
Obtain the required ESR value by the formula shown below
and then select the capacitance.
?IL=(V IN -V O ) x V O /(L x f x V IN )
?Vpp=?IL x ESR+(?IL x Vo)/(2 x Co x f x V IN )
Set the rating of the capacitor with an adequate margin to the
output voltage. Also, set the maximum allowable ripple current
with an adequate margin to ?IL. Furthermore, the output rise
time should be shorter than the soft start time. Select the output
capacitor having a value smaller than that obtained by the
formula shown below.
3.5m x (I Limit -Io(Max))
C Max =
Vo
I L imit:2A(BD9778F/HFP,BD9001F), 4A(BD9781HFP)
If this capacitance is not optimum, faulty startup may result.
( 3.5m is soft start time(min.))
www.rohm.com
? 2010 ROHM Co., Ltd. All rights reserved.
V IN =13.2V, Vo=5V, L=100μH, f=100kHz
?IL=(13.2-5) x 5/(100 x 10 -6 x 100 x 10 3 x 13.2)
0.31
When I Limit : 2 A, Io (Max) = 1 A, and Vo = 5V,
C Max =3.5m x (2-1)/5
=700μ
9/16
?IL=0.31A
C Max= 700μF
2010.02 - Rev. B
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