参数资料
| 型号： | MC34703PNB |
| 厂商： | Freescale Semiconductor |
| 文件页数： | 31/46页 |
| 文件大小： | 0K |
| 描述： | IC POWER SUPPLY QUICC 33-QFN |
| 标准包装： | 1 |
| 应用： | 电源 |
| 电流 - 电源： | 60mA |
| 电源电压： | 2.8 V ~ 13.5 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 33-PowerQFN |
| 供应商设备封装： | 33-PQFN（10x10） |
| 包装： | 管件 |
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TYPICAL APPLICATIONS
Where D is minimum switching duty cycle, F s is the
operating frequency in Hertz, L is output inductance in
Henries, and ? V pp is output ripple voltage in Volts.
For example, if the output voltage is 1.8V, the minimum
duty cycle is 33%, the operating frequency is 300 kHz, the
inductance is 10H and ? V pp is 50 mV, the minimum
capacitance of output capacitor will be calculated as 3.3 μ F,
as start value. However, this value is only concerned with the
ripple voltage. In the system, one needs to consider load
changes from overload to minimum load and keep the output
voltage within spec, so that the actual output capacitor value
varies with the system requirements.
The relationship between the ripple voltage and the ESR
of output capacitor is shown as follows:
? V pp = ? I ripple × R ESR
Output Current Limit Setting
The default setting of the limit is 11 A. When the output
current exceeds 11 A, the current limit timer starts to time out
while the control circuit limits the output current. If the over
current condition lasts for more than 10 ms, the buck
regulator is shut off and tuned on again after 100 ms. This
type of operation provides equivalent protection to the analog
V RESET
V OUT
V LDO
I OUT
Figure 30. Buck is in the Over Current Condition
When the buck output current (Channel 3) reaches and
exceeds limit, Reset Pin (Channel 1) is pulled down
immediately. Channel 4 is Buck output voltage (1.8V);
Channel 2 is LDO output voltage (3.3V).
LINEAR REGULATOR
Output Voltage Setting
The output voltage (V LDO ) of the linear regulator (LDO)
can be set by the following equation:
For example, if R 10 is chosen as 15 k ? (recommended),
R 11 will be calculated as 18.75 k ? for the output voltage of
1.8 V; in a same manner, R 11 is 46.87 k ? for the output
voltage of 3.3 V.
We recommend using 1% tolerance resistors (R 10 and
R 11 ) for the precise output voltage.
The following table shows the recommended values of
and referred to different buck output voltage:
Table 12. Recommendation of and Value for Different
Voltage
V ldo = V ref × ? 1 + -------- ?
R 10 ?
R 11
?
Where V REF is the linear regulator reference voltage,
typically is 0.8V at the LFB Pin.
Figure 31 shows the MC34703 linear regulator circuit with
its compensation circuit.
Vo
0.8
1.8
2.0
3.3
5.0
R10 (K ? )
15
15
15
15
15
R11 (K) ?
?
18.7
22.6
47
78.7
34703
Analog Integrated Circuit Device Data
Freescale Semiconductor
31
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