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LTC3210-1
OPERATION
When the LTC3210-1 operates in either 1.5x mode or 2x
mode, the charge pump can be modeled as a Thevenin-
equivalent circuit to determine the amount of current
available from the effective input voltage and effective
open-loop output resistance, R OL (Figure 2).
a 3.1V supply. If the LED forward voltage is 3.8V and the
current sources require 100mV, the advantage voltage for
1.5x mode is 3.1V ? 1.5 – 3.8V – 0.1V or 750mV. Notice
that if the input voltage is raised to 3.2V, the advantage
voltage jumps to 900mV— a 20% improvement in avail-
able strength.
–
I OUT =
+
R OL
1.5V BAT OR 2V BAT
+
CPO
–
32101 F02
From Figure 2, for 1.5x mode the available current is
given by:
(1. 5 V BAT – V CPO )
R OL
I OUT =
V BAT = 3V
V BAT = 3V
Figure2.ChargePumpThevenin
Equivalent Open-Loop Circuit
R OL is dependent on a number of factors including the
switching term, 1/(2f OSC ? C FLY ), internal switch resis-
tances and the nonoverlap period of the switching circuit.
However, for a given R OL , the amount of current available
will be directly proportional to the advantage voltage of
1.5V BAT – CPO for 1.5x mode and 2V BAT – CPO for 2x
mode. Consider the example of driving white LEDs from
3.8
V CPO = 4.2V
3.6 C2 = C3 = C4 = 2.2μF
3.4
3.2
3.0
2.8
2.6
For 2x mode, the available current is given by:
(2 V BAT – V CPO )
R OL
Notice that the advantage voltage in this case is 3.1V ? 2
– 3.8V – 0.1V = 2.3V. R OL is higher in 2x mode but a sig-
nificant overall increase in available current is achieved.
Typical values of R OL as a function of temperature are
shown in Figure 3 and Figure 4.
4.6
V CPO = 4.8V
4.4 C2 = C3 = C4 = 2.2μF
4.2
4.0
3.8
3.6
3.4
2.4
–40
–15
10
35
60
85
3.2
–40
–15
10
35
60
85
TEMPERATURE (?C)
32101 F03
TEMPERATURE (?C)
32101 F04
Figure 3. Typical 1.5x R OL vs Temperature
Figure 4. Typical 2x R OL vs Temperature
32101fd
0
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