LTC3208 Datasheet, PDF (10/24 Pages) Linear Technology – High Current Software Confi gurable Multidisplay LED Controller

LTC3208
U
OPERATIO
Soft-Start
Initially, when the part is in shutdown, a weak switch con-
nects V_BAT to CPO. This allows V_BAT1,2 to slowly charge
the CPO output capacitor and prevent large charging
currents to occur.
The LTC3208 also employs a soft-start feature on its charge
pump to prevent excessive inrush current and supply volt-
age droop when switching into the step-up modes. The
current available to the CPO pin is increased linearly over
a typical period of 150µs. Soft start occurs at the start of
both 1.5x and 2x mode changes.
Charge Pump Strength
When the LTC3208 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 is dependent on a number of factors including the
switching term, 1/(2f_OSC• C_FLY), internal switch resistances
and the nonoverlap 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 white 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)
R_OL
+
–+ ^{1.5VBAT OR 2VBAT CPO}
–
3208 F01
Figure 1. Charge Pump Thevenin–Equivalent Open-Loop Circuit
10
For 2X mode, the available current is given by:
I_OUT
=
2V_BAT – V_CPO
R_OL
(2)
Notice that the advantage voltage in the 2x case is
3.1V • 2 – 3.8V – 0.1V = 2.3V. R_OL is higher in 2x mode, but a
significant overall increase in available current is
achieved.
Typical values of R_OL as a function of temperature are
shown in Figure 2 and Figure 3.
2.5
V_BAT = 3V
V_CPO = 4.2V
2.3
C2 = C3 = 2.2µF
C4 = 4.7µF
2.1
1.9
1.7
1.5
–40 –15 10
35
60
85
TEMPERATURE (°C)
3208 F02
Figure 2. Typical 1.5x R_OL vs Temperature
2.8
V_BAT = 3V
V_CPO = 4.8V
2.6 C2 = C3 = 2.2µF
C4 = 4.7µF
2.4
2.2
2.0
1.8
1.6
–40 –15 10
35
60
85
TEMPERATURE (°C)
3208 F03
Figure 3. Typical 2x R_OL vs Temperature
3208fa