ltc3210 Linear Technology Corporation, ltc3210 Datasheet - Page 10

• Manufacturer Part Number: ltc3210
• Description: Main/cam Led Controller In 3mm ? 3mm Qfn
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC3210
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 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
When the LTC3210 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
R
switching term, 1/(2f
tances and the nonoverlap period of the switching circuit.
10
OL
is dependent on a number of factors including the
Charge Pump Mode
BAT
1.5x
2x
OL
to CPO. This allows V
(Figure 2).
U
OSC
• C
FLY
), internal switch resis-
Regulated V
BAT
Figure 2. Charge Pump Thevenin-Equivalent Circuit
4.55V
5.05V
to slowly charge
CPO
+
–
1.5V
BAT
R
OL
OR 2V
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.
From Figure 2, for 1.5x mode the available current is given by:
For 2x mode, the available current is given by:
Notice that the advantage voltage in this case is 3.1V • 2
– 3.8V – 0.1V = 2.3V. R
nifi cant overall increase in available current is achieved.
Typical values of R
shown in Figure 3 and Figure 4.
Shutdown Current
In shutdown mode all the circuitry is turned off and the
LTC3210 draws a very low current from the V
Furthermore, CPO is weakly connected to V
LTC3210 enters shutdown mode when both the ENM
and ENC pins are brought low for 150μs (typ). ENM and
ENC have 250k internal pull down resistors to defi ne
the shutdown state when the drivers are in a high imped-
ance state.
BAT
I
I
OUT
OUT
BAT
CPO
+
–
=
=
– CPO for 1.5x mode and 2V
( .
(
1 5
2
V
BAT
V
BAT
R
R
OL
–
OL
OL
–
V
CPO
V
OL
as a function of temperature are
CPO
OL
, the amount of current available
)
is higher in 2x mode but a sig-
)
BAT
– CPO for 2x
BAT
BAT
supply.
. The
3210fa