参数资料
| 型号： | NCP5318FTR2G |
| 厂商： | ON Semiconductor |
| 文件页数： | 30/32页 |
| 文件大小： | 0K |
| 描述： | IC CTLR CPU 2/3/4 PHASE 32-LQFP |
| 产品变化通告： | Product Obsolescence 08/Apr/2011 |
| 标准包装： | 1 |
| 应用： | 控制器，CPU |
| 输入电压： | 9.5 V ~ 13.2 V |
| 输出数： | 4 |
| 工作温度： | 0°C ~ 70°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-LQFP |
| 供应商设备封装： | 32-LQFP（7x7） |
| 包装： | 剪切带 (CT) |
| 其它名称： | NCP5318FTR2GOSCT |
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NCP5318
10. Current Limit Setting
When the output of the current sense amplifier (COx in the
block diagram) exceeds the voltage on the I LIM pin, the part
will latch off. For inductive sensing, the I LIM pin voltage
should be set based on the inductor ’s maximum resistance
(R LMAX ). The design must consider the inductor ’s
resistance increase due to current heating and ambient
temperature rise. Also, depending on the current sense
points, the circuit board may add additional resistance. In
general, the temperature coefficient of copper is +0.39% per
° C. If using a current sense resistor (R SENSE ), the I LIM pin
voltage should be set based on the maximum value of the
sense resistor.
For the overcurrent protection to avoid false tripping, the
voltage at the I LIM pin should be set even higher if the
R CSx x C CSx time constant is set faster than the L O / R L time
constant. A step load change may cause the current signal to
appear larger than the actual inductor current and trip the
current limit at a lower level than desired. The waveforms in
Figure 36 show a simulation of the current sense signal and
the actual inductor current during a positive step in load
current with values of L = 500 nH, R L = 1.6 m W , R CSx =
20 k W , and C CSx = 0.01 m F. In this case, ideal current signal
compensation would require V CSx to be 31 k. Due to the
faster than ideal RC time constant, there is an overshoot of
50% and the overshoot decays with a 200 m s time constant.
With this compensation, the I LIM pin threshold must be set
more than 50% above the full load current to avoid
triggering current limit during a large output load step.
The proper I LIM pin voltage can be calculated by:
VILIM + (IRIPP ? P (2 #PH) ) IL) RL (1 ) 0.004
(TL * 25)) g ) OSILIM
where:
I L = maximum converter current (A)
R L = maximum 25 ° C sense element
resistance ( W )
g = maximum current sense to I LIM gain
(see tabulated specs)
I RIPP ? P = peak ? to ? peak phase ripple current (A)
#PH = number of phases
T L = inductor temperature at overload ( ° C)
OS ILIM = maximum I LIM offset
(see tabulated specs) (V)
This voltage can be programmed by a resistor divider
from the R OSC pin, as shown in Figure 37.
R OSC
R1
I LIM
R2
Figure 37. Programming the Current Limit
When the NCP5318 is powered up, the R OSC pin will be
1.0 V. This allows the user to determine the resistor divider
above by:
R2 = R TOTAL x V LIM / 1.0 V
R1 = R TOTAL ? R2
Where R TOTAL is determined as in Section 1 above.
Figure 36. Inductive sensing waveform during a load
step with fast RC time constant (50 m s/div)
http://onsemi.com
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