Datasheet

LTC3215
7
3215fc
OPERATION
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 mode
as shown in Table 1.
Table 1. Charge Pump Output Regulation Voltages
CHARGE PUMP MODE V
CPO
1.5x 4.6V
2x 5.1V
In shutdown mode all circuitry is turned off and the
LTC3215 draws a very low current from the V
IN
supply.
Furthermore, CPO is weakly connected to V
IN
. The LTC3215
enters shutdown mode when the EN pin is brought low.
Since EN is a high impedance CMOS input it should never
be allowed
to float. To ensure that its state is defined, it
must always be driven with valid logic levels.
Thermal Protection
The LTC3215 has built-in overtemperature protection.
Thermal shutdown circuitry will shutdown the I
LED
output
when the junction temperature exceeds approximately
150°C. It will re-enable the I
LED
output once the junc-
tion temperature drops back to approximately 135°C.
The LTC3215 will cycle in and
out of thermal shutdown
indefinitely without latch up or damage until the heat
source is removed.
Soft-Start
To prevent excessive inrush current during start-up and
mode switching, the LTC3215 employs built-in soft-start
circuitry. Soft-start is achieved by increasing the amount
of current available to the output charge storage capacitor
linearly over a period of approximately 250µs.
Charge Pump Strength
When the LTC3215
operates in either the 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 1).
Figure 1. Charge Pump Open-Loop Thevenin-Equivalent Circuit
–
+
+
–
CPO
3215 F01
R
OL
1.5V
IN
OR
2V
IN
R
OL
is dependent on a number of factors including the
oscillator frequency, flying capacitor values and switch
resistances.
From Figure 1, we can see that the output current is
proportional to:
(1.5V
IN
– CPO)/R
OL
or (2V
IN
– CPO)/R
OL
(2)
in the 1.5x mode or 2x mode respectively.
LED Current Programming
The LTC3215 includes an accurate, programmable cur-
rent source that is capable of driving LED currents up to
350mA continuously and up to 700mA for pulsed opera-
tion. Pulsed operation may be achieved by toggling the
EN pin. In either continuous or pulsed operation, proper
board layout is required for effective heat sinking.
The current may
be programmed using a single external
resistor. Equation 1, used to calculate the resistor value
from the desired current level is repeated below:
R
SET
= 3990/I
LED
(1)
For applications requiring multiple current levels, several
schemes may be used to change the resistance for the R
SET
resistor. Figure 2 shows two such schemes. The circuit
in Figure 2a uses the I/O output of a microcontroller to
switch a second resistor (R2) in parallel or series with R1,
changing the effective I
SET
current. Alternatively, the circuit
in Figure 2b uses a pulse-width modulator (PWM)
to vary
the current through R
SET
, which changes the LED current.