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LTC3209-1/LTC3209-2
OPERATIO
The LTC3209-1 has 6 MAIN outputs, 1 CAM output and 1
AUX output. The LTC3209-2 has 5 MAIN outputs, 2 CAM
outputs and 1 AUX output.
Power Management
The LTC3209-1/LTC3209-2 use a switched capacitor
charge pump to boost CPO to as much as 2 times the input
voltage up to 5.1V. The part starts up in 1x mode. In this
mode, V BAT is connected directly to CPO. This mode
provides maximum efficiency and minimum noise. The
LTC3209-1/LTC3209-2 will remain in 1x mode until a
MAIN or CAM LED current source drops out. Dropout
occurs when a current source voltage becomes too low for
the programmed current to be supplied. When dropout is
detected, the LTC3209-1/LTC3209-2 will switch into 1.5x
mode. The CPO voltage will then start to increase and will
attempt to reach 1.5x V BAT up to 4.6V. Any subsequent
dropout will cause the part to enter the 2x mode. The CPO
voltage will attempt to reach 2x V BAT up to 5.1V. The part
will be reset to 1x mode whenever a DAC data bit is updated
via the I 2 C port or on the falling edge of the CAMHL signal.
A 2-phase nonoverlapping clock activates the charge
pump switches. In the 2x mode the flying capacitors are
charged on alternate clock phases from V BAT to minimize
input current ripple and CPO voltage ripple. In 1.5x mode
the flying capacitors are charged in series during the first
Soft-Start
Initially, when the part is in shutdown, a weak switch
connects V BAT1 to CPO. This allows V BAT1 to slowly charge
the CPO output capacitor and prevent large charging
currents to occur.
The LTC3209-1/LTC3209-2 also employs 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 125 μ s. Soft-start occurs at
the start of both 1.5x and 2x mode changes.
Charge Pump Strength
When the LTC3209-1/LTC3209-2 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
effective open-loop output resistance, R OL (Figure 1).
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.
clock phase and stacked in parallel on V BAT during the
second phase. This sequence of charging and discharging
R OL
+
–
the flying capacitors continues at a constant frequency of
850kHz.
The currents delivered by the LED current sources are
+
1.5V BAT OR 2V BAT
320912 F01
CPO
–
controlled by an associated DAC. Each DAC is programmed
via the I 2 C port.
Figure 1. Charge Pump Thevenin Equivalent Open-Loop Circuit
320912fa
9
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