LTC3212 Datasheet, PDF (7/12 Pages) Linear Technology – RGB LED Driver and Charge Pump

LTC3212
Operation
An overcurrent shutdown mode on the I_SET pins will pre-
vent damage to the part and the LED by shutting down
the LED drivers. Choosing an R_SET value of 5.9k or greater
will ensure that the part stays out of this mode. When in
normal operating mode current, regulation is achieved by
controlling an active current source.
In shutdown mode all internal circuitry is turned off and
the LTC3212 draws very little current from the V_IN supply.
The LTC3212 enters shutdown mode after the LEDEN pin
is brought low for 350µs.
Short-Circuit Protection
When LEDEN is brought high, the part will connect V_IN
to CPO through a weak pull-up until CPO has charged to
near V_IN. After the LTC3212 detects that the CPO voltage
is near the V_IN voltage, it then enables 1x mode. If the
CPO is shorted or falls below approximately 1V, then the
LTC3212 is disabled. After falling below 1V the LTC3212
will use the weak pull-up to charge CPO to near V_IN before
re-enabling the chip.
Soft-Start
To prevent excessive inrush during start-up and mode
switching, the LTC3212 employs built-in soft-start circuitry.
Soft-start is achieved by increasing the current available
to the CPO capacitor over a period of approximately
100µs.
Charge Pump Strength
When the LTC3212 operates in 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 the effective open-loop output
resistance, R_OL.
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 maximum
output current in 2x mode is proportional to:
2V_IN – CPO
R_OL
LED Current Programming
The LTC3212 includes three accurate, programmable cur-
rent sources that are capable of driving LED currents up
to 25mA continuously. The current is programmed using
an external resistor for each channel. The equation for
each external resistance is:
R_SETG
=
177.6
I_LEDG
R_SETB
=
177.6
I_LEDB
R_SETR
=
177.6
I_LEDR
Alternatively, if either the I_SETR or I_SETB pins are connected
to V_IN, the respective LEDR and/or LEDB current will au-
tomatically use the R_SETG resistor and be set to:
I_LEDR
=
177.6
R_SETG
I_LEDB
=
177.6
R_SETG
R_OL
+
+– _2VIN
CPO
–
3212 F01
Figure 1. CPO Equivalent Open-Loop
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