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参数资料
型号： AS1335-BTDT-AD
厂商： AUSTRIAMICROSYSTEMS AG
元件分类： DC/DC变换器
英文描述： 1.5A Buck Converter
文件页数： 13/18页
文件大小： 1656K
代理商： AS1335-BTDT-AD
www.austriamcrosystems.com/DC-DC_Step-Down/AS1335
Revision 1.03
13 - 18
AS1335
Datasheet - Application Information
Higher value, low cost ceramic capacitors are available in very small case sizes, and their high ripple current, high volt-
age rating, and low ESR make them ideal for switching regulator applications. Because the AS1335 control loop is not
dependant on the output capacitor ESR for stable operation, ceramic capacitors can be used to achieve very low out-
put ripple and accommodate small circuit size.
At light loads, the converter operates in powersave mode and the output voltage ripple is in direct relation to the output
capacitor and inductor value used. Larger output capacitor and inductor values minimize the voltage ripple in power-
save mode and tighten DC output accuracy in powersave mode.
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 minimizing the interference with other circuits caused by high input voltage spikes, a low ESR
input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given as:
where the maximum average output current I
MAX
equals the peak current minus half the peak-to-peak ripple current,
I
MAX
= I
LIM
-
Δ
I
L
/2
This formula has a maximum at 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 limit for better input voltage filtering. Take care when using small
ceramic input capacitors. When a small ceramic capacitor is used at the input, and the power is being supplied through
long wires, such as from a 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 mistaken as loop instability, or could even damage the part by
exceeding the maximum ratings.
Ceramic Input and Output Capacitors
When choosing ceramic 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 from their wide variation in capacitance over temperature, become resistive at high frequen-
cies and therefore should not be used.
Because ceramic capacitors lose a lot of their initial capacitance at their maximum rated voltage, it is recommended
that either a higher input capacity or a capacitance with a higher rated voltage is used.
Table 4. Recommended External Components
Name
Part Number
Value
Rating
Type
Size
B
Manufacturer
Kemet
www.kemet.com
Murata
www.murata.com
Coilcraft
C
OUT
T520B107M006ATE040
100μF
6.3V
Tantal
(3.5x2.8x1.9mm)
C
IN
, C
OUT
GRM21BR60J226ME39
22μF
6.3V
X5R
0805
L
MOS6020-222ML
MOS6020-472ML
2.2μH
4.7μH
3.26A
1.82A
35m
Ω
50m
Ω
6.8x6.0x2.4mm
6.8x6.0x2.4mm
(EQ 5)
Δ
V
OUT
V
OUT
1
----------------------
V
L
-V
–
f
8
C
OUT
f
×
--------------1
ESR
+
×
×
=
(EQ 6)
I
RMS
I
MAX
V
------------------------------------–
V
V
IN
OUT
)
×
=
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