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参数资料
| 型号： | AS2700AT-3.3 |
| 元件分类： | 固定正电压单路输出LDO稳压器 |
| 英文描述： | 3.3 V FIXED POSITIVE LDO REGULATOR, 0.5 V DROPOUT, PSSO2 |
| 封装： | TO-263, 3 PIN |
| 文件页数： | 4/9页 |
| 文件大小： | 145K |
| 代理商： | AS2700AT-3.3 |
Rev. 10/11/00
AS2700/01/02
APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the AS2700/01/02 requires a 10
F or greater
capacitor between output and ground. Oscillation could occur
without this capacitor.
Most types of
tantalum or aluminum
electrolytic works fine here.
For operations below -25
°C solid
tantalum is recommended since the many aluminum types have
electrolytes that freeze at about -30
°C. The ESR of about 5 or less
and resonant frequency above 500 kHz are the most important
parameters in the value of the capacitor. The capacitor value may be
increased without limit.
At lower values of output current, less output capacitance is required
for stability. For the currents below 10mA the value of the capacitor
can be reduced to 22
F and 0.15F for 1mA. More output
capacitance is needed for the 8-pin version at voltages below 5V
since it runs the error amplifier at lower gain. At worst case 22
F or
greater must be used for the condition of 750mA load at 1.23V
output.
The AS2700/01/02, unlike other low dropout regulators will remain
stable and in regulation with no load in addition to the internal
voltage divider. This feature is especially important in applications
like CMOS RAM keep-alive. When setting the output voltage of the
AS2700/01/02 version with external resistors, a minimum load of
5mA is recommended. 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 then a 1
A tantalum or aluminum electrolytic capacitor
should be placed from the input to the ground.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the
AS2700/01/02 output falls out of regulation by more than around
5%. This is around 75mV offset divided by the 1.235 reference
voltage. This trip level remains 5% below normal regardless of the
programmed output voltage of the regulator. Figure 1 shows the
timing diagram depicting the ERROR signal and the regulator output
voltage as the AS2700/01/02 input is ramped up and down. The
ERROR signal becomes low at around 1.3V input, and goes
high around 5V input (input voltage at which Vout = 4.75 ). Since
the AS2700/01/02’s dropout voltage is load dependent, the input
voltage trip point (around 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. Depending on the system requirements the
resistor may be returned to 5V output or other supply voltage. In
determining the value of this resistor, note that the output is rated to
sink 250
A, this value adds to battery drain in a low battery
condition. Suggested values range from 100K to 1M
. If the output
is unused this resistor is not required.
PROGRAMMING THE OUTPUT VOLTAGE OF AS2700/01/02
The AS2700/01/02 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. Refer to the
below equation for the programming of the output voltage:
VOUT = VREF
× (1 + R1\ R2)+ IFBR1
The VREF is 1.235 and IFB is the feedback bias current, nominally -20
nA. The minimum recommended load current of 1
A forces an
upper limit of 1.2 M
on value of R2. If no load is presented the IFB
produces an error of typically 2% in VOUT, which may be eliminated
at room temperature by trimming R1. To improve the accuracy
choose the value of R2 = 100k this reduces the error by 0.17% and
increases the resistor program current by 12
A. Since the
AS2700/01/02 typically draws 100
A at no load with SHUTDOWN
open-circuited this is a small price to pay
REDUCING OUTPUT NOISE
It may be an advantage to reduce the AC noise present at the output.
One way is to reduce the regulator bandwidth by increasing the size
of the output capacitor. This is the only way that noise can be
reduced on the 3 lead AS2700/01/02 but is relatively inefficient, as
increasing the capacitor from 1
F to 220F only decreases the noise
from 430
V to 160VRMS for a 100 kHz bandwidth at 5V output.
Noise could also be reduced fourfold by a bypass capacitor across R1,
since it reduces the high frequency gain from 4 to unity. Pick
CBYPASS
1 / 2πR1 × 200 Hz
or choose 0.01
F. When doing this, the output capacitor must be
increased to 3.3
F to maintain stability. These changes reduce the
output noise from 430
V to 100VRMS for a 100 kHz bandwidth at
5V output. With the bypass capacitor added, noise no longer scales
with output voltage so that improvements are more dramatic at higher
output voltages.
+
4.75V
OUTPUT
VOLTAGE
ERROR*
_______
INPUT
VOLTAGE
1.3V
5.0V
Figure 1. ERROR Output Timing
_______
* See Application Info.
AS2702
Fig.2 Adjustable Voltage Regulator
V
IN
V
OUT
GND
ADJ
SHUTDOWN
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相关代理商/技术参数 |
参数描述 |
|---|---|
| AS2701 | 制造商:ACC SILICONES 功能描述:ADHESIVE, SILICONE, TC FAST CURE, 264ML |
| AS2701A | 制造商:AMSCO 制造商全称:austriamicrosystems AG 功能描述:AS-Interface Slave IC |
| AS2701AT | 制造商:AMSCO 制造商全称:austriamicrosystems AG 功能描述:AS-Interface Slave IC |
| AS2702 | 制造商:AMSCO 制造商全称:austriamicrosystems AG 功能描述:AS-Interface Slave IC |
| AS2702/SOIC16-SAMPLE | 制造商:AMS 功能描述:AS Series 22.5 to 34 V Surface Mount AS-Interface Slave SOIC-16 |
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