参数资料
| 型号： | SC1485ITSTRT |
| 厂商： | Semtech |
| 文件页数： | 9/27页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM 28-TSSOP |
| 标准包装： | 1 |
| PWM 型： | 控制器 |
| 输出数： | 2 |
| 占空比： | 90% |
| 电源电压： | 1.8 V ~ 25 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： | 28-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： | SC1485ITSDKR |
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POWER MANAGEMENT
NOT RECOMMENDED FOR NEW DESIGN
SC1485
Output Voltage Selection
The output voltage (OUT2 shown) is set by the feedback
resistors R9 & R13 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:
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 Undervoltage
Protection).
The current sensing circuit actually regulates the
inductor valley current (see Figure 3). This means that if
the current limit is set to 10A, the peak current through
V OUT = ? 1 +
? ? 0 . 5
?
?
R 9 ?
R 13 ?
the inductor would be 10A plus the peak-to-peak ripple
current, and the average current through the inductor
would be 10A plus 1/2 the peak-to-peak ripple current.
The equations for setting the valley current and
8
9
EN/PSV2
TON2
BST2
DH2
21
20
calculating the average current through the inductor are
shown below:
VOUT2
10
VOUT2
LX2
19
C15
56p
0402
R9
20k0
0402
R13
20k0
11
12
13
14
VCCA2
FB2
PGD2
VSSA2
ILIM2
VDDP2
DL2
PGND2
18
17
16
15
I PEAK
I LOAD
I LIMIT
0402
U1
SC1485 OUT2
VSSA2
TIME
I LIMIT = 10e - 6 ?
R ILIM
Figure 2: Setting The Output Voltage
Current Limit Circuit
Current limiting of the SC1485 can be accomplished in
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
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 sets the over
current threshold. This resistor R ILIM is connected to a
10 μ A current source within the SC1485 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
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.
Valley Current-Limit Threshold Point
Figure 3: Valley Current Limiting
The equation for the current limit threshold is as follows:
A
R SENSE
Where (referring to Figure 8 on Page 16) R ILIM is R10 and
R SENSE is the R DS(ON) of Q4.
For resistor sensing, a sense resistor is placed between
the source of Q4 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
drop across R ILIM , a positive over-current exists and the
high side MOSFET will not be allowed to turn on. When
using an external sense resistor R SENSE is the resistance
of the sense resistor.
? 2005 Semtech Corp.
9
www.semtech.com
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