参数资料
| 型号： | SC470ITSTRT |
| 厂商： | Semtech |
| 文件页数： | 13/31页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM 14-TSSOP |
| 标准包装： | 1 |
| PWM 型： | 控制器 |
| 输出数： | 1 |
| 电源电压： | 5 V ~ 5.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 14-TSSOP（0.173"，4.40mm 宽） |
| 包装： | 标准包装 |
| 产品目录页面： | 1358 (CN2011-ZH PDF) |
| 其它名称： | SC470ITSDKR |
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NOT RECOMMENDED FOR NEW DESIGN
POWER MANAGEMENT
Application Information (Cont.)
SC470
? ?
? 1 1 ?
R TOP ? ?
R ESR_TR(MAX) = 10.4m ? for a full 6A load transient
Wewill select a value of 12.5m ? maximum for our
design, which would be achieved by using two 25m ?
output capacitors in parallel.
C TOP
=
? ?
? Z TOP
2 ? π ? f SW _ VBAT ( MIN )
F
Note that for constant-on converters there is a mini-
mum ESR requirement for stability which can be
calculated as follows:
For our example we will use R TOP = 20.0k ? and
R BOT = 14.3k ? , therefore:
Z TOP = 6.67k ? and C TOP = 60pF
R ESR ( MIN ) =
3
2 ? π ? C OUT ? f SW
We will select a value of C TOP = 56pF. Calculating the
value of V FB based upon the selected C TOP :
?
? R BOT +
?
?
?
This criteria should be checked once the output
capacitance has been determined.
Now that we know the output ESR we can calculate
the output ripple voltage:
?
?
?
V FB _ VBAT ( MIN ) = V RIPPLE _ VBAT ( MIN ) ? ?
?
?
For our example:
1
R TOp
R BOT
1
+ 2 ? π ? f SW _ VBAT ( MIN ) ? C TOP
?
?
?
? V P ? P
?
?
?
V RIPPLE _ VBAT ( MAX ) = R ESR ? I RIPPLE _ VBAT ( MAX ) V P ? P
and,
V RIPPLE _ VBAT ( MIN ) = R ESR ? I RIPPLE _ VBAT ( MIN ) V P ? P
For our example:
V RIPPLE_VBAT(MAX) = 27mV P-P and V RIPPLE_VBAT(MIN) = 22mV P-P
Note that in order for the device to regulate in a
controlled manner, the ripple content at the feedback
pin, V FB , should be approximately 15mV P-P at minimum
V BAT , and worst-case no smaller than 10mV P-P . If
V RIPPLE_VBAT(MIN) is less than 15mV P-P the above component
values should be revisited in order to improve this. Quite
often a small capacitor, C TOP , is required in parallel with
the top feedback resistor, R TOP , in order to ensure that
V FB is large enough. C TOP should not be greater than
100pF. The value of C TOP can be calculated as follows,
where R BOT is the bottom feedback resistor. Firstly
calculating the value of Z TOP required:
V FB_VBAT(MIN) = 14.8mV P-P - good
Next we need to calculate the minimum output
capacitance required to ensure that the output voltage
does not exceed the transient maximum limit, POSLIM TR ,
starting from the actual static maximum, V OUT_ST_POS , when
a load release occurs:
V OUT _ ST _ POS = V OUT + ERR DC V
For our example:
V OUT_ST_POS = 1.222V
POSLIM TR = V OUT ? TOL TR V
Where TOL TR is the transient tolerance. For our
example:
POSLIM TR = 1.296V
The minimum output capacitance is calculated as
follows:
= ? ( V RIPPLE _ VBAT ( MIN ) ? 0 . 015 ) Ohms
Z TOP
?
?
I
? I OUT + RIPPLE _ VBAT ( MAX ) ?
(
POSLIM TR ? V OUT _ ST _ POS
R BOT
0 . 015
Secondly calculating the value of C TOP required to achieve
this:
C OUT ( MIN )
= L ?
? ?
2
2
2
)
F
? 2005 Semtech Corp.
13
www.semtech.com
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