LTC3210-1_15 Datasheet, PDF (10/16 Pages) Linear Technology – MAIN/CAM LED Controller with 64-Step Brightness Control in 3mm 3mm QFN

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).
R_OL
+
–+ ^{1.5VBAT OR 2VBAT CPO}
–
32101 F02
Figure 2. Charge Pump Thevenin
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
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.
From Figure 2, for 1.5x mode the available current is
given by:
I_OUT
=
(1.5V_BAT –
R_OL
V_CPO)
For 2x mode, the available current is given by:
I_OUT
=
(2V_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.
3.8 _{VBAT = 3V}
V_CPO = 4.2V
3.6 _{C2 = C3 = C4 = 2.2µF}
3.4
3.2
3.0
2.8
2.6
2.4
–40
–15 10
35
60
85
TEMPERATURE (˚C)
32101 F03
Figure 3. Typical 1.5x R_OL vs Temperature
4.6 _{VBAT = 3V}
4.4
V_CPO = 4.8V
C2 = C3 = C4 = 2.2µF
4.2
4.0
3.8
3.6
3.4
3.2
–40
–15 10
35
60
85
TEMPERATURE (˚C)
32101 F04
Figure 4. Typical 2x R_OL vs Temperature
32101fd
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