LTC3240-3.3_15 Datasheet, PDF (8/12 Pages) Linear Technology – 3.3V/2.5V Step-Up/ Step-Down Charge Pump DC/DC Converter

LTC3240-3.3/LTC3240-2.5
APPLICATIO S I FOR ATIO
Power Efficiency
During LDO operation, the power efficiency (η) of the
LTC3240 is given by:
η = ^POUT = ^{VOUT •IOUT} = ^VOUT
P_IN
V_IN •I_OUT
V_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 V_IN = 3.7V, I_OUT = 100mA and V_OUT
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 = ^POUT = ^{VOUT • IOUT} = ^VOUT
P_IN V_IN • 2I_OUT 2V_IN
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 V_IN = 2.5V, I_OUT =
100mA and V_OUT regulating to 3.3V is 64% which is in
close agreement with the theoretical value of 66%.
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
depends on many factors such as the oscillator frequency
(f_OSC), value of the flying capacitor (C_FLY), the nonoverlap
time, the internal switch resistances (R_S), and the ESR of
the external capacitors. A first order approximation for
R_OL is given below:
R_0L
≅
2∑
S=1 TO
R_S
4
+
f_OSC
1
• C_FLY
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_OL, and
the effective output voltage, 2V_IN(MIN).
From Figure 1, the available current is given by:
I_OUT
=
2V_IN – V_OUT
R_OL
Typical R_OL values as a function of temperature are shown
in Figure 2.
R_OL
–+ ^2VIN
+
I_OUT
V_OUT
–
3240 F01
Figure 1. Equivalent Open-Loop Circuit
10
V_IN = 1.8V
V_OUT = 3V
9
8
7
6
5
–40 –15 10
35
60
85
TEMPERATURE (°C)
3240 G07
Figure 2. Typical R_OL vs Temperature
V_IN, V_OUT 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 (<0.1Ω) ceramic capacitors be used for both C_IN and
C_OUT. C_IN should be 1µF or greater while C_OUT should be
4.7µF or greater. Tantalum and aluminum capacitors are
not recommended because of their high ESR.
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