LTC3204-5_15 Datasheet, PDF (8/12 Pages) Linear Technology – Low Noise Regulated Charge Pump in 2 2 DFN

LTC3204-3.3/LTC3204-5/
LTC3204B-3.3/LTC3204B-5
APPLICATIO S I FOR ATIO
Power Efficiency
The power efficiency (η) of the LTC3204-3.3/LTC3204-5/
LTC3204B-3.3/LTC3204B-5 is similar to that of a linear
regulator with an effective input voltage of twice the actual
input voltage. 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 would be 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 LTC3204-3.3/LTC3204-5/
LTC3204B-3.3/LTC3204B-5 are negligible and the expres-
sion above is valid. For example, with V_IN = 3V, I_OUT =
100mA and V_OUT regulating to 5V, the measured efficiency
is 81.8% which is in close agreement with the theoretical
83.3% calculation.
(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_OL
≅ 2∑RS
S=1 TO 4
+
1
f _OSC• C_FLY
Typical R_OL values as a function of temperature are shown
in Figure 2.
8
V_IN = 2.7V
V_OUT = 4.5V
7
6
5
Maximum Available Output Current
For the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/
LTC3204B-5,the maximum available output current and
voltage can be calculated from the effective open-loop
output resistance, R_OL, and the effective input voltage,
2V_IN(MIN).
R_OL
+
–+ ^2VIN
I_OUT
V_OUT
–
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Figure 1. Equivalent Open-Loop Circuit
From Fig. 1, the available current is given by:
I_OUT
=
2V_IN – V_OUT
R_OL
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
8
4
–50
0
50
TEMPERATURE (°C)
100
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Figure 2. Typical R_OL vs Temperature
V_IN, V_OUT Capacitor Selection
The style and value of capacitors used with the LTC3204-3.3/
LTC3204-5/LTC3204B-3.3/LTC3204B-5 determine several
important parameters such as regulator control loop sta-
bility, 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. These capacitors should be 1µF or greater. Tantalum
and aluminum capacitors are not recommended because
of their high ESR.
The value of C_OUT directly controls the amount of output
ripple for a given load current. Increasing the size of C_OUT
will reduce the output ripple at the expense of higher
minimum turn-on time. The peak-to-peak output ripple
is approximately given by the expression:
V_{RIPPLE(P−P)}
≅
I_OUT
2f _OSC• C_OUT
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