参数资料
| 型号： | SC470ITSTRT |
| 厂商： | Semtech |
| 文件页数： | 9/31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
Output Voltage Selection
The output voltage is set by the feedback resistors R3 &
R7 of Figure 2 below. The internal reference is 1.5V, so
the voltage at the feedback pin is multiplied by three to
match the 1.5V reference. Therefore, the
output can be set to a minimum of 0.5V. The equation
for setting the output voltage is:
will not be turned on until the voltage drop across the
sense element (resistor or MOSFET) falls below the
voltage across the R ILIM resistor. In an extreme over-
current situation, the top MOSFET will never turn back
on and eventually the part will latch off due to output
undervoltage (see Output Under-voltage Protection).
The current sensing circuit actually regulates the
V OUT = ? 1 +
? ? 0 . 5
?
?
R3 ?
R 7 ?
inductor valley current (see Figure 3). This means that if
the current limit is set to 10A, the peak current through
the inductor would be 10A plus the peak ripple current,
and the average current through the inductor would be
10A plus 1/2 the peak-to-peak ripple current. The
U1
EN/PSV
TON
SC470
BST
DH
equations for setting the valley current and calculating
the average current through the inductor are shown
below:
VOUT
VOUT
LX
C5
56p
R3
20k0
0402
VCCA
ILIM
I PEAK
0402
FB
PGD
VDDP
DL
I LOAD
I LIMIT
R7
20k0
VSSA
PGND
0402
TIME
Figure 2: Setting The Output Voltage
Current Limit Circuit
Current limiting of the SC470 can be accomplished in
Valley Current-Limit Threshold Point
Figure 3: Valley Current Limiting
The equation for the current limit threshold is as follows:
two ways. The on-state resistance of the low-side MOSFET
can be used as the current sensing element or sense
resistors in series with the low-side source can be used
I LIMIT = 10e - 6 ?
R ILIM
R SENSE
A
if greater accuracy is desired. R DS(ON) sensing is more
efficient and less expensive. In both cases, the R ILIM
resistor between the ILIM pin and LX pin set the over
current threshold. This resistor R ILIM is connected to a
10 μ A current source within the SC470 which is turned
on when the low side MOSFET turns on. When the
voltage drop across the sense resistor or low side
MOSFET equals the voltage across the RILIM resistor,
positive current limit will activate. The high-side MOSFET
Where (referring to Figure 8 on Page 17) R ILIM is R4 and
R SENSE is the R DS(ON) of Q2.
For resistor sensing, a sense resistor is placed between
the source of Q2 and PGND. The current through the
source sense resistor develops a voltage that opposes
the voltage developed across R ILIM . When the voltage
developed across the R SENSE resistor reaches the voltage
? 2005 Semtech Corp.
9
www.semtech.com
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