LTC3210-2 Datasheet, PDF (10/16 Pages) Linear Technology – MAIN/CAM LED Controllers with 32-Step Brightness Control in 3mm × 3mm QFN

LTC3210-2/LTC3210-3
OPERATION
Soft-Start
Initially, when the part is in shutdown, a weak switch con-
nects V_BAT to CPO. This allows V_BAT to slowly charge the
CPO output capacitor to prevent large charging currents.
The LTC3210-2/LTC3210-3 also employ a soft-start feature
on its charge pump to prevent excessive inrush current
and supply droop when switching into the step-up modes.
The current available to the CPO pin is increased linearly
over a typical period of 150μs. Soft-start occurs at the
start of both 1.5x and 2x mode changes.
Charge Pump Strength and Regulation
Regulation is achieved by sensing the voltage at the CPO
pin and modulating the charge pump strength based
on the error signal. The CPO regulation voltages are set
internally, and are dependent on the charge pump modes
as shown in Table 1.
Table 1. Charge Pump Output Regulation Voltages
Charge Pump Mode
Regulated V_CPO
1.5x
4.55V
2x
5.05V
When the LTC3210-2/LTC3210-3 operate 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 ef-
fective open-loop output resistance, R_OL (Figure 2).
R_OL
+
–+ ^{1.5VBAT OR 2VBAT CPO}
–
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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.
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