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参数资料
| 型号： | LTC3867EUF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 18/36页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 24-QFN |
| 产品培训模块： | LTC3867 Synchronous Step-Down DC/DC Controller |
| 标准包装： | 91 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1.2MHz |
| 占空比： | 98% |
| 电源电压： | 4 V ~ 38 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： | 24-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3867
APPLICATIONS INFORMATION
R = R O ? exp ? B ? ?
?
T O + 273 ? ? ?
TheNTCresistorhasanegativetemperaturecoefficient,
meaning its value decreases as temperature rises. The
V ITEMP voltage, therefore, decreases as temperature in-
creases and in turn, the V SENSEMAX(ADJ) will increase to
compensate the DCR temperature coefficient. The NTC
resistor, however, is nonlinear and the user can linear-
ize its value by building a resistor network with regular
resistors. Consult the NTC manufacturer’s data sheets for
detailed information.
Another use for the ITEMP pins, in addition to NTC com-
pensated DCR sensing, is adjusting V SENSE(MAX) to values
between the nominal values of 30mV, 40mV, 50mV, 60mV
and 75mV for a more precise current limit. This is done
by applying a voltage less than 1.4V to the ITEMP pin.
V SENSE(MAX) will be varied per the previous equation and
the same duty cycle limitations will apply. The current limit
can be adjusted using this method either with a sense
resistor or DCR sensing.
NTC Compensated DCR Sensing
For DCR sensing applications where a more accurate
current limit is required, a network consisting of an NTC
thermistor placed from the ITEMP pin to ground will
provide correction of the current limit over temperature.
Figure 3b shows this network. Resistors R S and R P will
linearize the impedance the ITEMP pin sees. To implement
NTC compensated DCR sensing, design the DCR sense
filter network per the same procedure mentioned in the
previous selection, except calculate the divider components
using the room temperature value of the DCR.
1. Set the ITEMP pin resistance to 46.7k at 25°C. With
30μA flowing out of the ITEMP pin, the voltage on the
ITEMP pin will be 1.4V at room temperature. Current
limit correction will occur for inductor temperatures
greater than 25°C.
2. Calculate the ITEMP pin resistance and the maximum
inductor temperature which is typically 100°C. Use the
equations:
Calculate the values for R P and R S . A simple method is to
graph the following R S versus R P equations with R S on
the y-axis and R P on the x-axis.
R S = R ITEMP25C – R NTC25C || R P
R S = R ITEMP100C – R NTC100C || R P
Next, find the value of R P that satisfies both equations
which will be the point where the curves intersect. Once
R P is known, solve for R S .
The resistance of the NTC thermistor can be obtained
from the vendor’s data sheet either in the form of graphs,
tabulated data or formulas. The approximate value for the
NTC thermistor for a given temperature can be calculated
from the following equation:
? ? 1 1 ? ?
–
? ? T + 273
where:
R = resistance at temperature T, which is in degrees C
R O = resistance at temperature T O , typically 25°C
B = B-constant of the thermistor.
Figure 6 shows a typical resistance curve for a 100k
thermistor and the ITEMP pin network over temperature.
Starting values for the NTC compensation network are
listed below:
? NTC R O = 100k
? R S = 20k
? R P = 50k
But, the final values should be calculated using the above
equations and checked at 25°C and 100°C.
R ITEMP100C =
V ITEMP100C
30μA
V ITEMP100C = 1.4V – 3.64
18
I MAX ? DCR(MAX) ? R2 / ( R1 + R2 ) ? ( 100 ° C – 25 ° C ) ? 0.4 / 100
V SENSE(MAX)
3867f
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