LTC3205_15 Datasheet, PDF (7/16 Pages) Linear Technology – Multidisplay LED Controller

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OPERATIO
Power Management
To optimize efficiency, the power management section of
the LTC3205 provides two methods of supplying power to
the CPO pin: 1:1 direct connect mode or 2:3 boost mode.
When either the main or sub displays of the LTC3205 are
enabled, the power management system connects the CPO
pin directly to V_IN with a low impedance switch. If the voltage
supplied at V_IN is high enough to power all of the LEDs with
the programmed current, the system will remain in this
“direct connect” mode providing maximum efficiency.
Internal circuits monitor all MAIN and SUB current sources
for the onset of “dropout,” the point at which the current
sources can no longer supply programmed current. As the
battery voltage falls, the LED with the largest forward volt-
age will reach the “drop out” threshold first. When any of
the four main or two sub display LEDs reach the dropout
threshold, the LTC3205 will switch to boost mode and au-
tomatically soft-start the 2:3 boost charge pump. The con-
stant frequency charge pump is designed to minimize the
amount of noise generated at the V_IN supply.
The 2:3 step-up charge pump uses a patented constant
frequency architecture to combine the best efficiency with
the maximum available power at the lowest noise level.
If the red, green or blue LEDs are programmed to be on at
any duty cycle, the charge pump runs continuously.
Soft-Start
To prevent excessive inrush current and supply droop
when switching into step-up mode, the LTC3205 employs
a soft-start feature on its charge pump. The current
available to the CPO pin is increased linearly over a period
of 1.2ms.
Charge Pump Strength
When the LTC3205 operates in 2:3 boost mode, the
charge pump can be modeled as a Thevenin-equivalent
circuit to determine the amount of current available from
the effective input voltage, 1.5V_IN and the 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.
LTC3205
However, for a given R_OL, the amount of current available
will be directly proportional to the advantage voltage 1.5V_IN
– V_CPO. Consider the example of driving white LEDs from
a 3.1V supply. If the LED forward voltage is 3.8V and the
current sources require 100mV, the advantage voltage is
3.1V • 1.5V – 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, the available current is given by:
I_OUT
=
1.5V_IN – V_CPO
R_OL
Typical values of R_OL as a function of temperature are
shown in Figure 2.
R_OL
+– _1.5VIN
+
CPO
–
3205 F01
Figure 1. Equivalent Open-Loop Circuit
3.2
V_IN = 3V
V_CPO = 4.2V
3.0 C_IN = C_CPO = C_FLY1 = C_FLY2 = 1µF
2.8
2.6
2.4
2.2
2.0
–40
–15 10
35
60
85
TEMPERATURE (°C)
3205 F02
Figure 2. Typical R_OL vs Temperature
Zero Shutdown Current
Although the LTC3205 is designed to have very low shut-
down current, it will draw about 400nA on V_IN when in
shutdown. For applications that require zero shutdown
current, the DV_CC pin can be grounded. This will reduce the
V_IN current to very near zero. Internal logic ensures that the
3205f
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