数据搜索系统,热门电子元器件搜索 |
|
UC1914 数据表(PDF) 10 Page - Texas Instruments |
|
UC1914 数据表(HTML) 10 Page - Texas Instruments |
10 / 14 page 10 UC1914 UC2914 UC3914 Overload Comparator The linear amplifier in the UC3914 ensures that the ex- ternal NMOS does not source more than the current IMAX, defined above as: I VCC V R MAX IMAX SENSE = − In the event that output current exceeds the programmed IMAX by more than 200mV/RSENSE, the output of the lin- ear amplifier will immediately be pulled low (with respect to VOUTS) providing no gate drive to the NMOS, and preventing current from being delivered to the load. This situation could occur if the external NMOS is not re- sponding to a command from the IC or output load condi- tions change quickly to cause an overload condition before the linear amplifier can respond. For example, if the NMOS is sourcing current into a load and the load suddenly becomes short circuited, an overload condition may occur. The short circuit will cause the VGS of the NMOS to immediately increase, resulting in increased load current and voltage drop across RSENSE. If this drop exceeds the overload comparator threshold, the amplifier output will be quickly pulled low. It will also cause the CT pin to begin charging with I3, a 3mA current source (refer to Fig. 2) and continue to charge until approximately one volt below VCC, where it is clamped. This allows a con- stant fault to show up on FAULT and since the voltage on CT will only charge past 2.5V in an overload fault condi- tion, it can be used for detection of output NMOS failure or to build redundancy into the system. Estimating Minimum Timing Capacitance The startup time of the IC may not exceed the fault time for the application. Since the timing capacitor, CT, deter- mines the fault time, its minimum value can be deter- mined by calculating the startup time of the IC. The startup time is dependent upon several external compo- nents. A load capacitor, CLOAD, should be tied between VOUTS and GND. Its value should be greater than that of CPUMP, the reservoir capacitor tied from VPUMP to VOUTS (see Fig. 4). Given values of CLOAD, Load, RSENSE, VCC and the resistors determining the voltage on IMAX, the user can calculate the approximate startup time of the node VOUT. This time must be less than the time it takes for CT to charge to 2.5V. Assuming the user has determined the fault current, RSENSE can be calcu- lated by: R mV I SENSE FAULT = 50 IMAX is the maximum current the UC3914 will allow through the transistor M1. During startup with an output capacitor, M1 can be modeled as a constant current source of value IMAX where: I VCC V R MAX IMAX SENSE = − Given this information, calculation of startup time is now possible via the following: Current Source Load: T CVCC II START LOAD MAX LOAD = • – Resistive Load: TR C n VCC IR START LOAD LOAD MAX LOAD =− • • − • l 1 The only remaining external component which may af- fect the minimum timing capacitor is the optional power limiting resistor, RPL. If the addition of RPL is desirable, its value can be determined from the “Fault Timing” sec- tion above. The minimum timing capacitor values are now given by Current Source Load: CT T R VCC R START PL PL min =• •+ • − 10 2 2 4 APPLICATION INFORMATION (cont.) Figure 4. Estimating minimum timing capacitor. UDG-97056 |
类似零件编号 - UC1914 |
|
类似说明 - UC1914 |
|
|
链接网址 |
隐私政策 |
ALLDATASHEETCN.COM |
ALLDATASHEET是否为您带来帮助? [ DONATE ] |
关于 Alldatasheet | 广告服务 | 联系我们 | 隐私政策 | 链接交换 | 制造商名单 All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |