LTC3221-5 Linear Technology, LTC3221-5 Datasheet - Page 8

• Manufacturer Part Number: LTC3221-5
• Description: Micropower
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
Output Ripple
Low frequency regulation mode ripple exists due to the
hysteresis in the comparator CMP and propagation delay
in the charge pump control circuit. The amplitude and
frequency of this ripple are heavily dependent on the load
current, the input voltage and the output capacitor size.
The LTC3221 family uses a controlled current, I
current to the output. This helps to keep the output ripple
fairly constant over the full input voltage range. Typical
combined output ripple for the LTC3221-3.3 with V
2V under maximum load is 35mV
X5R case size 0603 output capacitor.
A high frequency ripple component may also be present
on the output capacitor due to the charge transfer action
of the charge pump. In this case the output can display
a voltage pulse during the charging phase. This pulse
results from the product of the charging current and the
LTC3221/
LTC3221-3.3/LTC3221-5
8
depends on many factors such as the oscillator frequency
(f
time, the internal switch resistances (R
the external capacitors. A fi rst order approximation for
R
Typical R
in Figure 2.
Figure 2. Effective Open-Loop Output Resistance vs Temperature
OSC
OL
R
is given below:
OL
), value of the fl ying capacitor (C
≅
OL
2
S
values as a function of temperature are shown
=
15
14
13
12
11
10
9
8
7
6
5
1
–50
∑
TO
V
V
IN
OUT
4
= 1.8V
–25
U
R
= 3V
S
+
TEMPERATURE (°C)
0
f
OSC
U
25
•
1
C
FLY
P-P
50
W
FLY
using a 4.7µF 6.3V
S
75
), the nonoverlap
) and the ESR of
3221 F02
100
SW
U
to deliver
IN
=
ESR of the output capacitor. It is proportional to the input
voltage, the value of the fl ying capacitor and the ESR of
the output capacitor.
A smaller output capacitor and/ or larger output current
load will result in higher ripple due to higher output volt-
age slew rates.
There are several ways to reduce output voltage ripple.
For applications requiring lower peak-to-peak ripple, a
larger C
A larger capacitor will reduce both the low and high fre-
quency ripple due to the lower charging and discharging
slew rates, as well as the lower ESR typically found with
higher value (larger case size) capacitors. A low ESR ce-
ramic output capacitor will minimize the high frequency
ripple, but will not reduce the low frequency ripple unless
a high capacitance value is used.
V
The style and value of capacitors used with the LTC3221
family determine several important parameters such as
output ripple, charge pump strength and minimum start-
up time.
To reduce noise and ripple, it is recommended that low
ESR (< 0.1Ω) capacitors be used for both C
These capacitors should be either ceramic or tantalum
and should be 2.2µF or greater. Aluminum capacitors are
not recommended because of their high ESR.
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or alumi-
num should never be used for the fl ying capacitor since
its voltage can reverse upon start-up of the LTC3221.
Low ESR ceramic capacitors should always be used for
the fl ying capacitor.
The fl ying capacitor controls the strength of the charge
pump. In order to achieve the rated output current, it is
necessary to have at least 0.6µF of capacitance for the
fl ying capacitor. For very light load applications, the fl ying
capacitor may be reduced to save space or cost.From the
fi rst order approximation of R
Open-Loop Output Resistance,” the theoretical minimum
output resistance of a voltage doubling charge pump can
IN
, V
OUT
OUT
Capacitor Selection
capacitor (4.7µF or greater) is recommended.
OL
in the section “Effective
IN
and C
OUT
3221f
.