参数资料
| 型号： | SC470ITSTRT |
| 厂商： | Semtech |
| 文件页数： | 14/31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
This calculation assumes the absolute worst-case
condition of a full-load to no-load step transient occurring
when the inductor current is at its highest. The
capacitance required for smaller transient steps may be
calculated by substituting the desired current for the I OUT
term.
For our example:
C OUT(MIN) = 595μF.
We will select 440μF, using two 220μF, 25m ?
capacitors in parallel. For smaller load release overshoot,
660μF may be used.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
For our example:
I VALLEY = 5.13A, R DS(ON) = 9m ? and R ILIM = 7.76k ?
We select the next lowest 1% resistor value: 7.68k ?
Adding an Additional Output Voltage For Dynamic
Voltage Switching
If we design this output to be capable of dynamically
switching between 1.2V and 1.0V, then we would repeat
these calculations to determine if any components need
changing. The 1.0V output suggests a value for C TOP of
82pF, but the value of 56pF required by the 1.2V design
should work fine, and can always be increased if neces-
sary. Also, the current limit resistor required is slightly
higher: R ILIM = 7.87k ? . The higher value should be used.
I IN ( RMS ) = V OUT ? ( V BAT ( MIN ) ? V OUT ) ?
I OUT
V BAT _ MIN
A RMS
Lastly, the bottom feedback resistor, R BOT will need to
change to 20.0k ? . The schematic in Figure 8 on Page
17 shows the complete design.
For our example:
I IN(RMS) = 2.14A RMS
Input capacitors should be selected with sufficient ripple
current rating for this RMS current, for example a 10μF,
1210 size, 25V ceramic capacitor can handle a little more
than 2A RMS (Refer to manufacturer’s data sheets).
Finally, we calculate the current limit resistor value. As
described in the current limit section, the current limit
looks at the “valley current”, which is the average output
current minus half the ripple current. We use the
maximum room temperature specification for MOSFET
R DS(ON) at V GS = 4.5V for purposes of this calculation:
Dynamically Switching Output Voltages
It is important to note that in order for dynamic output
voltage switching to work, the SC470 must be in
Continuous Conduction Mode (EN/PSV = floating) when
transitioning from V OUT(HIGH) to V OUT(LOW) . Otherwise the
SC470 has no means to discharge the output voltage
and may OVP and latch off when this transition is initiated
(depending upon the difference between the two
voltages). If CCM is on, the SC470 will actively discharge
the output down to the correct voltage.
Dynamically switching output voltages is very easy,
requiring one switch to add or remove an additional
resistor in parallel to the bottom feedback resistor. Ideally,
I VALLEY = I OUT ?
I RIPPLE _ VBAT ( MIN )
2
A
the resistor will be switched using an open drain output
from another IC, as shown in Figure 4.
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under
normal operating conditions.
R ILIM = ( I VALLEY ? 1 . 2 ) ?
R DS ( ON ) ? 1. 4
10 ? 10 ? 6
Ohms
? 2005 Semtech Corp.
14
www.semtech.com
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