LTC3230_15 Datasheet, PDF (10/16 Pages) Linear Technology – 5-LED Main/Sub Display Driver with Dual LDO

LTC3230
OPERATION
Charge Pump Soft-Start
In shutdown, CPO is disconnected from V_IN and is pulled
down through a 14.3k resistor. When enabled, a weak switch
connects V_IN to CPO. This allows V_IN to slowly charge the
CPO output to prevent large charging currents.
The LTC3230 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 50μs. Soft-start occurs at the start of
both 1.5x and 2x modes.
Charge Pump Strength and Regulation
Regulation is achieved by sensing the voltage at the CPO
pin and modulating the charge pump strength based
on the error signal. The CPO regulation voltages are set
internally, and are dependent on the charge pump modes
as shown in Table 1.
Table 1. Charge Pump Output Regulation Voltages
CHARGE PUMP MODE
REGULATED V_CPO
1.5x
4.5V
2x
5V
When the LTC3230 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 2).
R_OL
+
–+ ^{1.5VIN OR 2VIN}
CPO
–
3230 F02
Figure 2. Charge Pump Thevenin Equivalent Open-Loop Circuit
R_OL is dependent on a number of factors including the
switching term, 1/(2 • f_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
1.5 • V_IN – CPO for 1.5x mode and 2 • V_IN – 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 avail-
able strength.
From Figure 2, for 1.5x mode the available current is
given by:
I_OUT
=
1.5
•
V_IN –
R_OL
V_CPO
For 2x mode, the available current is given by:
I_OUT
=
2
•
V_IN – V_CPO
R_OL
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
significant increase in available current is achieved.
Typical values of R_OL as a function of temperature are
shown in Figures 3 and 4.
Mode Switching
The LTC3230 will automatically switch from 1x mode
to 1.5x mode and subsequently to 2x mode whenever
a dropout condition is detected at any LED pin. Dropout
occurs when a current source voltage becomes too low
for the programmed current to be supplied. The time from
dropout detection to mode switching is typically 0.5ms.
The charge pump mode is reset back to 1x when the LED
drivers are shut down (ENM = ENS = Low) or on the falling
edge of either ENM or ENS. An internal comparator will
not allow the main switches to connect V_IN and CPO in
1x mode until the voltage at the CPO pin has decayed to
less than or equal to the voltage at the V_IN pin.
LDO Operation
Two independent low drop-out linear regulators are in the
LTC3230. Each regulator may be independently enabled
(ENLDO1 and ENLDO2) from each other and from the
3230fa
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