LTC3219 Datasheet, PDF (8/20 Pages) Linear Technology – 250mA Universal Nine Channel LED Driver

LTC3219
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
efficiency 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 flying capacitors are
charged on alternate clock phases from V_BAT 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_BAT 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_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).
R_OL
+
–+ ^{1.5VBAT OR 2VBAT}
CPO
–
3219 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 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_OUT
=
1.5V_BAT –
R_OL
V_CPO
(1)
For 2x mode, the available current is given by:
I_OUT
=
2V_BAT – V_CPO
R_OL
(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-
nificant overall increase in available current is achieved.
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