LTC3210-3 LINER [Linear Technology], LTC3210-3 Datasheet - Page 10

• Manufacturer Part Number: LTC3210-3
• Manufacturer: LINER [Linear Technology]

LTC3210-2/LTC3210-3
OPERATION
Soft-Start
Initially, when the part is in shutdown, a weak switch con-
nects V
CPO output capacitor to prevent large charging currents.
The LTC3210-2/LTC3210-3 also employ 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 150μs. Soft-start occurs at the
start of both 1.5x and 2x mode changes.
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
10
Charge Pump Mode
BAT
to CPO. This allows V
1.5x
2x
BAT
Regulated V
to slowly charge the
4.55V
5.05V
CPO
Figure 2. Charge Pump Thevenin Equivalent Open-Loop Circuit
When the LTC3210-2/LTC3210-3 operate 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 ef-
fective open-loop output resistance, R
R
switching term, 1/(2f
tances and the nonoverlap period of the switching circuit.
However, for a given R
will be directly proportional to the advantage voltage of
1.5V
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.
OL
is dependent on a number of factors including the
BAT
– CPO for 1.5x mode and 2V
+
–
1.5V
OSC
OL
BAT
, the amount of current available
R
OL
• C
OR 2V
FLY
BAT
), internal switch resis-
321023 F02
CPO
–
+
OL
BAT
(Figure 2).
– CPO for 2x
321023fa