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参数资料
型号： AS1324-BTTT-18
厂商： AUSTRIAMICROSYSTEMS AG
元件分类： DC/DC变换器
英文描述： 600mA Buck Converter; Package Type: TSOT23-5; Output Range: 1.8V
封装： TSOT23-5
文件页数： 13/20页
文件大小： 970K
代理商： AS1324-BTTT-18
wwwaustriamcrosystems.com/DC-DC_Step-Down/AS1324
Revision 1.05
13 - 20
AS1324
Datasheet - Application Information
9.4 Input Capacitor Selection
In continuous mode, the source current of the PMOS is a square wave of the duty cycle V
OUT
/V
IN
. To prevent large voltage transients while
mnimzing the interference with other circuits caused by high input voltage spikes, a low ESR input capacitor sized for the maximumRMS
current must be used. The maximumRMS capacitor current is given as:
where the maximumaverage output current I
MAX
equals the peak current mnus half the peak-to-peak ripple current, I
MAX
= I
LIM
-
Δ
I
L
/2
This formula has a maximumat V
IN
= 2V
OUT
where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly used for design because
even significant deviations only provide negligible affects.
The input capacitor can be increased without any limt for better input voltage filtering. Take care when using small ceramc input capacitors.
When a small ceramc capacitor is used at the input, and the power is being supplied through long wires, such as froma wall adapter, a load step
at the output, or V
IN
step on the input, can induce ringing at the VIN pin. This ringing can then couple to the output and be mstaken as loop
instability, or could even damage the part by exceeding the maximumratings.
9.4.1 Ceramic Input and Output Capacitors
When choosing ceramc capacitors for C
IN
and C
OUT
, the X5R or X7R dielectric formulations are recommended. These dielectrics have the
best temperature and voltage characteristics for a given value and size. Y5V and Z5U dielectric capacitors, aside fromtheir wide variation in
capacitance over temperature, become resistive at high frequencies and therefore should not be used.
Because ceramc capacitors lose a lot of their initial capacitance at their maximumrated voltage, it is recommended that either a higher input
capacity or a capacitance with a higher rated voltage is used.
9.5 Feedback Resistor Selection
In the AS1324-AD, the output voltage is set by an external resistor divider connected to V
FB
(see Figure 27)
. This circuitry allows for remote
voltage sensing and adjustment.
Figure 27. Setting the AS1324 Output Voltage
Resistor values for the circuit shown in
Figure27
can be calculated as:
The output voltage can be adjusted by selecting different values for R
1
and R
2
. For R
1
a value between 10k
Ω
and 500k
Ω
is recommended. A
higher resistance of R
1
and R
2
will result in a lower leakage current at the output. It is recommended to keep V
IN
500mV higher than V
OUT
.
Table 6. Recommended Input and Output Capacitor
Part Number
C
TC Code
Rated Voltage
Dimensions (L/W/T)
Manufacturer
Taiyo Yuden
Murata
wwwmurata.com
JMK212BJ226MG-T
22μF
X5R
6.3V
0805
GRM188R60J106ME47
10μF
X5R
6.3V
0603
GRM21BR71A475KA73
4.7μF
X7R
10V
0805
(EQ 6)
I
RMS
I
MAX
V
-------------------------------------–
V
V
IN
)
×
=
R
2
0.6V
≤
V
OUT
≤
5.5V
R
1
AS1324
5
V
FB
2
GND
R
1
<<R
2
(EQ 7)
V
OUT
0,6
1
R
2
R
1
------
+
×
=
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