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AMS1505CT 参数 Datasheet PDF下载

AMS1505CT图片预览
型号: AMS1505CT
PDF下载: 下载PDF文件 查看货源
内容描述: 5A低压差稳压器 [5A LOW DROPOUT VOLTAGE REGULATORS]
分类和应用: 稳压器调节器输出元件
文件页数/大小: 9 页 / 80 K
品牌: ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
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AMS1505  
APPLICATION HINTS  
to allow this capability. To ensure good transient response with  
heavy load current changes capacitor values on the order of  
100mF are used in the output of many regulators. To further  
improve stability and transient response of these devices larger  
values of output capacitor can be used.  
5V  
CONTROL  
POWER  
SENSE  
3.3V  
The modern processors generate large high frequency current  
transients. The load current step contains higher order frequency  
components than the output coupling network must handle until  
the regulator throttles to the load current level. Because they  
contain parasitic resistance and inductance, capacitors are not  
ideal elements. These parasitic elements dominate the change in  
output voltage at the beginning of a transient load step change.  
The ESR of the output capacitors produces an instantaneous step  
in output voltage (DV=DI)(ESR). The ESL of the output  
capacitors produces a droop proportional to the rate of change of  
the output current (V= L)(DI/Dt). The output capacitance  
produces a change in output voltage proportional to the time until  
the regulator can respond (DV=Dt) (DI/C). Figure 4 illustrates  
these transient effects.  
AMS1505  
OUTPUT  
+
VOUT  
-
RP  
ADJ  
LOAD  
R1  
R2  
RP  
Figure 2. Remote Load Sensing  
(DIOUT)(RP)  
VOUT  
FIGURE 1  
ESR  
VOUT  
EFFECTS  
FIGURE 2  
ESL  
EFFECTS  
CAPACITANCE  
EFFECTS  
IOUT  
SLOPE, V/t = DI/C  
POINT AT WHICH REGULATOR  
TAKES CONTROL  
TIME  
Figure 4.  
Figure 3. Remote Sensing Improves Load Regulation  
Output Voltage  
Voltage drops due to RP are not eliminated; they will add to the  
dropout voltage of the regulator regardless of whether they are  
inside or outside the regulation loop. The AMS1505 can control  
the voltage at the load as long as the input-output voltage is  
greater than the total of the dropout voltage of the device plus the  
voltage drop across RP.  
The AMS1505 series develops a 1.25V reference voltage  
between the Sense pin and the Adjust pin (Figure5). Placing a  
resistor between these two terminals causes a constant current to  
flow through R1 and down through R2 to set the overall output  
voltage. In general R1 is chosen so that this current is the  
specified minimum load current of 10mA.The current out of the  
Adjust pin is small, typically 50mA and it adds to the current  
from R1. Because IADJ is very small it needs to be considered  
only when very precise output voltage setting is required. For  
best regulation the top of the resistor divider should be connected  
directly to the Sense pin.  
Stability  
The circuit design used in the AMS1505 series requires the use of  
an output capacitor as part of the device frequency compensation.  
The addition of 150mF aluminum electrolytic or a 22mF solid  
tantalum on the output will ensure stability for all operating  
conditions. For best frequency response use capacitors with an  
ESR of less than 1W.  
VCONTROL  
+
In order to meet the transient requirements of the processor larger  
value capacitors are needed. Tight voltage tolerances are required  
in the power supply. To limit the high frequency noise generated  
by the processor high quality bypass capacitors must be used. In  
order to limit parasitic inductance (ESL) and resistance (ESR) in  
the capacitors to acceptable limits, multiple small ceramic  
capacitors in addition to high quality solid tantalum capacitors are  
required.  
CONTROL  
POWER  
OUTPUT  
VOUT  
VPOWER  
+
+
AMS1505  
SENSE  
ADJ  
VREF  
R1  
R2  
IADJ  
50mA  
When the adjustment terminal is bypassed to improve the ripple  
rejection, the requirement for an output capacitor increases. The  
Adjust pin is brought out on the fixed voltage device specifically  
VOUT = VREF (1+ R2/R1)+IADJR2  
Figure 5. Setting Output Voltage  
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140