LTC3221/
LTC3221-3.3/LTC3221-5
7
3221f
Power Effi ciency
The input current of a doubling charge pump like the LTC3221
family is always twice that of the output current. This is
true regardless of whether the output voltage is unregulated
or regulated or of the regulation method used. In an ideal
unregulated doubling charge pump, conservation of energy
implies that the input current has to be twice that of the
output current in order to obtain an output voltage twice
that of the input voltage. In a regulated charge pump like
the LTC3221, the regulation of V
OUT
is similar to that of a
linear regulator, with the voltage difference between 2 • V
IN
(Input voltage plus the voltage across a fully charged fl ying
capacitor) and V
OUT
being absorbed in an internal pass
transistor. In the LTC3221, the controlled current I
SW
acts as
a pass transistor. So the input current of an ideal regulated
doubling charge pump is the same as an unregulated one,
which is equal to twice the output current. The effi ciency
(n) of an ideal regulated doubler is therefore given by:
η= = =
P
P
VI
VI
V
V
OUT
IN
OUT OUT
IN OUT
OUT
IN
•
•2 2
At moderate to high output power, the switching losses and
quiescent current of the LTC3221 family are negligible and
the expression is valid. For example, an LTC3221-5 with V
IN
= 3V, I
OUT
= 60mA and V
OUT
regulating to 5V, has a mea-
sured effi ciency of 82% which is in close agreement with
the theoretical 83.3% calculation. The LTC3221 product
family continues to maintain good effi ciency even at fairly
light loads because of its inherently low power design.
Maximum Available Output Current
For the adjustable LTC3221, the maximum available output
current and voltage can be calculated from the effective
open-loop output resistance, R
OL
, and effective output
voltage, 2V
IN(MIN)
.
From Figure 1 the available current is given by:
I
VV
R
OUT
IN OUT
OL
=
2–
Effective Open-Loop Output Resistance (R
OL
)
The effective open-loop output resistance(R
OL
) of a charge
pump is a very important parameter which determines the
strength of the charge pump. The value of this parameter
APPLICATIO S I FOR ATIO
WUU
U
OPERATIO
U
(Refer to Block Diagrams)
The LTC3221 has a FB pin in place of the
⎯
S
⎯
H
⎯
D
⎯
N pin. This
allows the output voltage to be programmed using an
external resistive divider.
Burst Mode Operation
The LTC3221 family regulates the output voltage throughout
the full 60mA load range using Burst Mode control. This
keeps the quiescent current low at light load and improves
the effi ciency at full load by reducing the switching losses.
All the internal circuitry except the comparator is kept off
if the output voltage is high and the fl ying capacitor has
been fully charged. These circuits are turned on only if V
OUT
drops below the comparator lower threshold. At light load,
V
OUT
stays above this lower threshold for a long period of
time, this result in a very low average input current.
Soft-Start and Short-Circuit Protection
The LTC3221 family uses a controlled current, I
SW
to
deliver current to the output. This helps to limit the input
and output current during start-up and output short-circuit
condition. During start up I
SW
is used to charge up the fl ying
capacitor and output capacitor, this limits the input current
to approximately 240mA. During short-circuit condition,
the output current is delivered through I
SW
and this limits
the output current to approximately 120mA. This prevents
excessive self-heating that causes damage to the part.
Figure 1. Equivalent Open-Loop Circuit
+
–
+
–
2V
IN
I
OUT
V
OUT
3221 F01
R
OL
