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LTC3240-3.3 数据表(PDF) 8 Page - Linear Technology

部件名 LTC3240-3.3
功能描述  3.3V/2.5V Step-Up/ Step-Down Charge Pump DC/DC Converter
Download  12 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC3240-3.3 数据表(HTML) 8 Page - Linear Technology

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LTC3240-3.3/LTC3240-2.5
8
3240fb
Power Efficiency
During LDO operation, the power efficiency (
η) of the
LTC3240 is given by:
η=
=
=
P
P
VI
VI
V
V
OUT
IN
OUT
OUT
IN
OUT
OUT
IN
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 efficiency of
LTC3240-3.3, with VIN = 3.7V, IOUT = 100mA and VOUT
regulating to 3.3V is 87% which is in close agreement
with the theoretical value of 89%.
During charge pump operation, the power efficiency (
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
efficiency is 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 the quiescent current of the LTC3240 are negligible
and the expression above is valid. For example, the mea-
sured efficiency of LTC3240-3.3 with VIN = 2.5V, IOUT =
100mA and VOUT regulating to 3.3V is 64% which is in
close agreement with the theoretical value of 66%.
Effective Open-Loop Output Resistance (ROL)
The effective open-loop output resistance (ROL) of a charge
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
(fOSC), value of the flying capacitor (CFLY), the nonoverlap
time, the internal switch resistances (RS), and the ESR of
the external capacitors. A first order approximation for
ROL is given below:
RR
fC
LS
STO
OSC
FLY
0
14
2
1
+
=
For the LTC3240 in charge pump mode, the maximum
available output current and voltage can be calculated
from the effective open-loop output resistance, ROL, and
the effective output voltage, 2VIN(MIN).
From Figure 1, the available current is given by:
I
VV
R
OUT
IN
OUT
OL
=
2–
Typical ROL values as a function of temperature are shown
in Figure 2.
APPLICATIO S I FOR ATIO
Figure 1. Equivalent Open-Loop Circuit
+
ROL
IOUT
VOUT
2VIN
3240 F01
+
Figure 2. Typical ROL vs Temperature
VIN, VOUT Capacitor Selection
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 (<0.1
Ω) ceramic capacitors be used for both CIN and
COUT. CIN should be 1µF or greater while COUT should be
4.7µF or greater. Tantalum and aluminum capacitors are
not recommended because of their high ESR.
TEMPERATURE (
°C)
–40
8
9
10
60
3240 G07
7
6
5
–15
10
35
85
VIN = 1.8V
VOUT = 3V


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