LTC3220-1_15 Datasheet, PDF (8/20 Pages) Linear Technology – 360mA Universal 18-Channel LED Driver

LTC3220/LTC3220-1
OPERATION
Power Management
The LTC3220/LTC3220-1 use a switched capacitor charge
pump to boost CPO as much as 2 times the input voltage
up to 5.1V. The part starts up in 1x mode. In this mode,
V_IN is connected directly to CPO. This mode provides
maximum efficiency and minimum noise. The LTC3220/
LTC3220-1 will remain in 1x mode until an 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 LTC3220/
LTC3220-1 will switch into 1.5x mode. The CPO voltage
will then start to increase and will attempt to reach 1.5×
V_IN up to 4.6V. Any subsequent dropout will cause the
part to enter the 2x mode. The CPO voltage will attempt
to reach 2× V_IN up to 5.1V.
A 2-phase non-overlapping clock activates the charge
pump switches. In the 2x mode the flying capacitors are
charged on alternate clock phases from V_IN to minimize
CPO voltage ripple. In 1.5x mode the flying capacitors are
charged in series during the first clock phase and stacked
in parallel on V_IN during the second phase. This sequence
of charging and discharging the flying 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 I²C port.
Soft-Start
Initially, when the part is in shutdown, a weak switch
connects V_IN to CPO. This allows V_IN to slowly charge the
CPO output capacitor and prevent large charging currents
from occurring.
The LTC3220/LTC3220-1 also employ a soft-start feature
on the 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 125μs. Soft-start occurs at the
start of both 1.5x and 2x mode changes.
Charge Pump Strength
When the LTC3220/LTC3220-1 operate in either 1.5x mode
or 2x mode, the charge pump can be modeled as a Theve-
nin-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 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_IN – CPO for 1.5x mode and 2V_IN – CPO for 2x mode.
Consider the example of driving LEDs from a 3.1V supply.
R_OL
+
+–
1.5V_IN OR
2V_IN
CPO
–
3220 F01
Figure 1. Charge Pump Open-Loop Thevenin Equivalent Circuit
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