Datasheet

LTC3219
8
3219fa
OPERATION
Power Management
The LTC3219 uses a switched capacitor charge pump to
boost CPO to as much as 2 times the input voltage up to
5.04V. The part starts up in 1x mode. In this mode V
BAT
is
connected directly to CPO. This mode provides maximum
effi ciency and minimum noise. The LTC3219 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 drop-
out is detected, the LTC3219 will switch into 1.5x mode.
The CPO voltage will then start to increase and attempt
to reach 1.5x V
BAT
, up to 4.53V. Any subsequent dropout
will cause the part to enter the 2x mode. The CPO voltage
will attempt to reach 2x V
BAT
, up to 5.04V.
A 2-phase non-overlapping clock activates the charge
pump switches. In the 2x mode, the fl ying capacitors are
charged on alternate clock phases from V
BAT
to minimize
CPO voltage ripple. In 1.5x mode, the fl ying capacitors are
charged in series during the fi rst clock phase and stacked
in parallel on V
BAT
during the second phase. This sequence
of charging and discharging the fl 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
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.
The LTC3219 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 125μs. Soft-start occurs at the start of
both 1.5x and 2x mode changes.
Charge Pump Strength
When the LTC3219 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 1).
–
+
CPO
1.5V
BAT
OR 2V
BAT
+
–
3219 F01
R
OL
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 resistances
and the non-overlap period of the switching circuit. How-
ever, for a given R
OL
, the amount of current available is
directly proportional to the advantage voltage of 1.5V
BAT
– CPO for 1.5x mode and 2V
BAT
– CPO for 2x mode. Con-
sider 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
VV
R
OUT
BAT CPO
OL
=
15.–
(1)
For 2x mode, the available current is given by:
I
VV
R
OUT
BAT CPO
OL
=
2–
(2)
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-
nifi cant overall increase in available current is achieved.