LTC3210_15 Datasheet, PDF (10/16 Pages) Linear Technology – MAIN/CAM LED Controller in 3mm 3mm QFN

LTC3210
OPERATION
Soft-Start
Initially, when the part is in shutdown, a weak switch
connects V_BAT to CPO. This allows V_BAT to slowly charge
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
CHARGE PUMP MODE
REGULATED V_CPO
1.5x
4.55V
2x
5.05V
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_OL (Figure 2).
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
R_OL
+
–+ ^{1.5VBAT OR 2VBAT CPO}
–
Figure 2. Charge Pump Thevenin-Equivalent Circuit
10
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 2, for 1.5x mode the available current is given by:
I_OUT
=
(1.5V_BAT –
R_OL
V_CPO )
For 2x mode, the available current is given by:
I_OUT
=
(2V_BAT – 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 sig-
nificant overall increase in available current is achieved.
Typical values of R_OL as a function of temperature are
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_BAT supply.
Furthermore, CPO is weakly connected to V_BAT. The
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 define
the shutdown state when the drivers are in a high imped-
ance state.
3.8
V_BAT = 3V
3.6
V_CPO = 4.2V
C2 = C3 = C4 = 2.2µF
3.4
3.2
3.0
2.8
2.6
2.4
–40
–15 10
35
60
TEMPERATURE (˚C)
85
3210 F03
Figure 3. Typical 1.5x R_OL vs Temperature
3210fb