ltc3210-1 Linear Technology Corporation, ltc3210-1 Datasheet - Page 10

• Manufacturer Part Number: ltc3210-1
• Description: Main/cam Led Controller With 64-step Brightness Control In 3mm X 3mm Qfn
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC3210-1
Soft-Start
Initially, when the part is in shutdown, a weak switch
connects V
the CPO output capacitor to prevent large charging
currents.
The LTC3210-1 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 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
1.5x
2x
to CPO. This allows V
U
Regulated V
BAT
4.55V
5.05V
to slowly charge
CPO
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.
When the LTC3210-1 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
BAT
is dependent on a number of factors including the
– CPO for 1.5x mode and 2V
Figure 2. Charge Pump Thevenin
Equivalent Open-Loop Circuit
+
–
OSC
OL
1.5V
, the amount of current available
BAT
• C
R
OL
OR 2V
FLY
OL
BAT
), internal switch resis-
(Figure 2).
32101 F02
CPO
+
–
BAT
– CPO for 2x
32101fb