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LTC3207/LTC3207-1
OPERATION
Power Management
The LTC3207/LTC3207-1 use a switched capacitor charge
pump to boost CPO to as much as two times the input
voltage up to 5.05V. The part starts up in 1x mode. In
this mode, V BAT is connected directly to CPO. This mode
provides maximum ef?ciency and minimum noise. The
LTC3207/LTC3207-1 will remain in 1x mode until an LED
Charge Pump Strength
When the LTC3207/LTC3207-1 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 1).
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
+
–
R OL
1.5V BAT OR 2V BAT
+
CPO
LTC3207/LTC3207-1 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 4.55V. Any subsequent dropout will cause
the part to enter the 2x mode. The CPO voltage will attempt
to reach 2x V BAT up to 5.05V.
A 2-phase non-overlapping clock activates the charge
pump switches. In the 2x mode, the ?ying capacitors are
charged on alternate clock phases from V BAT to minimize
CPO voltage ripple. In 1.5x mode the ?ying capacitors are
charged in series during the ?rst clock phase and stacked
in parallel on V BAT during the second phase. This sequence
of charging and discharging the ?ying capacitors continues
at a constant-frequency of 850kHz.
The current delivered by each LED current source is con-
trolled by an associated DAC. Each DAC is programmed
via the I 2 C port.
Soft-Start
–
3207 F01
Figure 1. Equivalent Open-Loop
R OL is dependent on a number of factors, including the
switching term, 1/(2f OSC ? C FLY ), internal switch resist-
ances and the non-overlap 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 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 available strength.
From Figure 1, for 1.5x mode the available current is
given by:
I OUT =
Initially, when the part is in shutdown, a weak switch
connects V BAT to CPO. This allows V BAT to slowly charge
the CPO output capacitor and to prevent large charging
currents from occurring.
1 . 5 V BAT ? V CPO
R OL
For 2x mode, the available current is given by:
(1)
The LTC3207/LTC3207-1 also employ a soft-start feature
on its charge pump to prevent excessive inrush current
I OUT =
2 V BAT ? V CPO
R OL
(2)
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 125μs. Soft-start occurs at the
start of both 1.5x and 2x mode changes.
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-
ni?cant overall increase in available current is achieved.
3207fc
8
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