ltc3240-3.3 Linear Technology Corporation, ltc3240-3.3 Datasheet - Page 8

• Manufacturer Part Number: ltc3240-3.3
• Description: 3.3v/2.5v Step-up/ Step-down Charge Pump Dc/dc Converter
• Manufacturer: Linear Technology Corporation

LTC3240-3.3/LTC3240-2.5
APPLICATIO S I FOR ATIO
Power Effi ciency
During LDO operation, the power effi ciency (η) of the
LTC3240 is given by:
At moderate to high output power, the quiescent cur-
rent of the LTC3240 is negligible and the expression
above is valid. For example, the measured effi ciency of
LTC3240-3.3, with V
regulating to 3.3V is 87% which is in close agreement
with the theoretical value of 89%.
During charge pump operation, the power effi ciency (η)
of the LTC3240 is similar to that of a linear regulator with
an effective input voltage of twice the actual input volt-
age. This occurs because the input current for a voltage
doubling charge pump is approximately twice the output
current. In an ideal regulating voltage doubler the power
effi ciency is given by:
At moderate to high output power, the switching losses
and the quiescent current of the LTC3240 are negligible
and the expression above is valid. For example, the mea-
sured effi ciency of LTC3240-3.3 with V
100mA and V
close agreement with the theoretical value of 66%.
Effective Open-Loop Output Resistance (R
The effective open-loop output resistance (R
pump is a very important parameter which determines the
strength of the charge pump. The value of this parameter
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
8
OSC
OL
R
η =
η =
is given below:
0
), value of the fl ying capacitor (C
L
P
P
≅
P
P
OUT
OUT
2
IN
IN
S
=
=
=
1
OUT
∑
TO
V
V
V
V
OUT
OUT
IN
4
IN
regulating to 3.3V is 64% which is in
R
U
• 2
•
S
IN
•
•
I
OUT
I
I
+
I
OUT
OUT
OUT
= 3.7V, I
f
OSC
U
=
=
•
1
V
V
2
C
OUT
V
OUT
V
OUT
FLY
IN
IN
W
= 100mA and V
FLY
S
IN
), the nonoverlap
), and the ESR of
= 2.5V, I
OL
OL
) of a charge
)
U
OUT
OUT
=
V
The style and value of capacitors used with the LTC3240
determine several important parameters such as regulator
control loop stability, output ripple, charge pump strength
and minimum start-up time.
To reduce noise and ripple, it is recommended that low
ESR (<0.1Ω) ceramic capacitors be used for both C
C
4.7µF or greater. Tantalum and aluminum capacitors are
not recommended because of their high ESR.
For the LTC3240 in charge pump mode, the maximum
available output current and voltage can be calculated
from the effective open-loop output resistance, R
the effective output voltage, 2V
From Figure 1, the available current is given by:
Typical R
in Figure 2.
IN
OUT
I
, V
OUT
. C
OUT
IN
=
OL
should be 1µF or greater while C
Capacitor Selection
2
10
V
values as a function of temperature are shown
Figure 1. Equivalent Open-Loop Circuit
9
7
6
5
8
Figure 2. Typical R
–40
IN
V
V
R
IN
OUT
–
OL
= 1.8V
V
= 3V
–15
OUT
+
–
2V
TEMPERATURE (°C)
R
OL
IN
10
OL
35
vs Temperature
IN(MIN)
I
OUT
3240 F01
60
+
–
.
3240 G07
V
OUT
OUT
85
should be
OL
IN
, and
and
3240fb