AAT2820 Datasheet, PDF (13/18 Pages) Advanced Analogic Technologies – Triple-Output Charge Pump Regulator

AAT2820
Triple-Output Charge Pump Regulator
V_POS (V)
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
R4, Closest Value (kΩ)
59.0
69.8
78.7
88.7
100.0
110.0
120.0
130.0
140.0
150.0
160.0
169.0
180.0
191.0
200.0
210.0
220.0
232.0
240.0
Table 4: Closest Value for R4 if Using 12.1kΩ
as R3.
Capacitor Selection
Careful selection of the three external capacitors
C_IN, C_FLY, and C_OUT is important because they will
affect turn-on time, output ripple, efficiency, and
load transient response. Optimum performance will
be obtained when low equivalent series resistance
(ESR) ceramic capacitors are used. In general, low
ESR may be defined as less than 100mΩ. A value
of 1µF for input and flying capacitors is a good
starting point when designing with the AAT2820.
This not only provides for a very small printed cir-
cuit board area, but cost is further reduced by the
minimized bill of materials.
Input Capacitor
A 1µF multilayer ceramic chip capacitor is suggest-
ed for the input. This capacitor should be connect-
ed between the IN pin and ground; 1µF should be
suitable for most applications. Even though the
AAT2820 switching ripple and noise are very low,
back-injected line noise may be further reduced by
increasing the value of C_IN. A low equivalent series
inductance (ESL) ceramic capacitor is ideal for this
function. The size required will vary depending on
the load, output voltage, and input voltage charac-
teristics. Other types of capacitors may be used for
C_IN at the cost of compromised circuit performance.
Output Capacitor
The output capacitor (C_OUT) should be connected
between the OUT pin and ground. Switching noise
and ripple seen on the charge pump output
increases with load current. Typically, the output
capacitor should be 5 to 10 times greater than the
flying capacitor. To minimize stray inductance, the
capacitor should be placed as closely as possible
to the IC. This keeps the high frequency content of
the input current localized, minimizing radiated and
conducted EMI.
A 1µF ceramic capacitor is recommended for most
applications for optimum transient response.
However, if the application has a larger load from
the main and multiplier stage charge pump outputs,
a 4.7µF ceramic capacitor is suggested to reduce
the feedback injection noise from the multiplier
stage and lower switching ripple. Capacitor types
other than ceramic capacitors can be used for C_OUT.
However, capacitors composed of non-ceramic
material will typically have a greater value of ESR,
resulting in increased output switching ripple.
Charge Pump Capacitor (C_FLY)
Due to the switching operation of the voltage dou-
bling circuit topology, current flow through the flying
capacitor is bi-directional. The flying capacitor
selected must be a non-polarized type. A 1µF low
ESR ceramic capacitor is ideal for most applications.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT2820. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic counterparts. A ceramic capacitor typically
has very low ESR, is lowest cost, has a smaller
2820.2006.04.1.4
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