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AMS2954CD-33 参数 Datasheet PDF下载

AMS2954CD-33图片预览
型号: AMS2954CD-33
PDF下载: 下载PDF文件 查看货源
内容描述: 250毫安低压差稳压器 [250mA LOW DROPOUT VOLTAGE REGULATOR]
分类和应用: 稳压器
文件页数/大小: 15 页 / 172 K
品牌: ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
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AMS2954  
APPLICATION HINTS  
External Capacitors  
Since the AMS2954’s dropout voltage is load dependent (see  
curve in typical performance characteristics), the input voltage trip  
point (about 5V) will vary with the load current. The output  
voltage trip point (approx. 4.75V) does not vary with load.  
The error comparator has an open-collector output which requires  
an external pull-up resistor. This resistor may be returned to the  
output or some other supply voltage depending on system  
requirements. In determining a value for this resistor, note that the  
output is rated to sink 400mA, this sink current adds to battery  
drain in a low battery condition. Suggested values range from  
100K to 1MW. The resistor is not required if this output is unused.  
A 1.0 mF or greater capacitor is required between output and  
ground for stability at output voltages of 5V or more. At lower  
output voltages, more capacitance is required (2.2m or more is  
recommended for 2.5V, 3.0V and 3.3V versions). Without this  
capacitor the part will oscillate. Most types of tantalum or  
aluminum electrolytic works fine here; even film types work but  
are not recommended for reasons of cost. Many aluminum types  
have electrolytes that freeze at about -30°C, so solid tantalums are  
recommended for operation below -25°C. The important  
parameters of the capacitor are an ESR of about 5 W or less and  
resonant frequency above 500 kHz parameters in the value of the  
capacitor. The value of this capacitor may be increased without  
limit.  
4.75V  
OUTPUT  
VOLTAGE  
At lower values of output current, less output capacitance is  
required for stability. The capacitor can be reduced to 0.33 mF for  
currents below 10 mA or 0.1 mF for currents below 1 mA. Using  
the adjustable versions at voltages below 5V runs the error  
amplifier at lower gains so that more output capacitance is needed.  
For the worst-case situation of a 300mA load at 1.23V output  
(Output shorted to Feedback) a 3.3mF (or greater) capacitor should  
be used.  
ERROR*  
5V  
INPUT  
VOLTAGE  
1.3V  
Unlike many other regulators, the AMS2954, will remain stable  
and in regulation with no load in addition to the internal voltage  
divider. This is especially important in CMOS RAM keep-alive  
applications. When setting the output voltage of the AMS2954  
version with external resistors, a minimum load of 1mA is  
recommended.  
A 1mF tantalum or aluminum electrolytic capacitor should be  
placed from the AMS2954/AMS2954 input to the ground if there  
is more than 10 inches of wire between the input and the AC filter  
capacitor or if a battery is used as the input.  
FIGURE 1. ERROR Output Timing  
*When VIN 1.3V the error flag pin becomes a high impedance,  
and the error flag voltage rises to its pull-up voltage. Using Vout as  
the pull-up voltage (see Figure 2), rather than an external 5V  
source, will keep the error flag voltage under 1.2V (typ.) in this  
condition. The user may wish to drive down the error flag voltage  
using equal value resistors (10 k suggested), to ensure a low-  
level logic signal during any fault condition, while still allowing a  
valid high logic level during normal operation.  
Stray capacitance to the AMS2954 Feedback terminal can cause  
instability. This may especially be a problem when using a higher  
value of external resistors to set the output voltage. Adding a 100  
pF capacitor between Output and Feedback and increasing the  
output capacitor to at least 3.3 mF will fix this problem.  
Programming the Output Voltage  
The AMS2954 may be pin-strapped for the nominal fixed output  
voltage using its internal voltage divider by tying the output and  
sense pins together, and also tying the feedback and VTAP pins  
together. Alternatively, it may be programmed for any output  
voltage between its 1.235V reference and its 30V maximum  
rating. As seen in Figure 2, an external pair of resistors is  
required.  
Error Detection Comparator Output  
The comparator produces a logic low output whenever the  
AMS2954 output falls out of regulation by more than  
approximately 5%. This figure is the comparator’s built-in offset  
of about 60 mV divided by the 1.235 reference voltage (Refer to  
the block diagram). This trip level remains “5% below normal”  
regardless of the programmed output voltage of the 2951. For  
example, the error flag trip level is typically 4.75V for a 5V output  
or 11.4V for a 12V output. The out of regulation condition may be  
due either to low input voltage, current limiting, or thermal  
limiting.  
Figure 1 gives a timing diagram depicting the ERROR signal and  
the regulator output voltage as the AMS2954 input is ramped up  
and down. For 5V versions the ERROR signal becomes valid  
(low) at about 1.3V input. It goes high at about 5V input (the input  
voltage at which Vout = 4.75 ).  
The complete equation for the output voltage is:  
Vout = VREF ´ (1 + R1/ R2)+ IFBR1  
where VREF is the nominal 1.235 reference voltage and IFB is the  
feedback pin bias current, nominally -20 nA. The minimum  
recommended load current of 1 mA forces an upper limit of 1.2  
MW on value of R2, if the regulator must work with no load (a  
condition often found in CMOS in standby) IFB will produce a 2%  
typical error in VOUT which may be eliminated at room  
temperature by trimming R1. For better accuracy, choosing R2 =  
100k reduces this error to 0.17% while increasing the resistor  
program current by 12 mA. Since the AMS2954 typically draws 60  
mA at no load with Pin 2 open-circuited, this is a small price to  
pay.  
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140