LTC3210-1_15 Datasheet, PDF (11/16 Pages) Linear Technology – MAIN/CAM LED Controller with 64-Step Brightness Control in 3mm 3mm QFN

LTC3210-1
Operation
Shutdown Current
In shutdown mode all the circuitry is turned off and the
LTC3210-1 draws a very low current from the V_BAT sup-
ply. Furthermore, CPO is weakly connected to V_BAT. The
LTC3210-1 enters shutdown mode when both the ENM
and ENC pins are brought low at 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.
Thermal Protection
The LTC3210-1 has built-in overtemperature protection.
At internal die temperatures of around 150°C thermal shut
down will occur. This will disable all of the current sources
and charge pump until the die has cooled by about 15°C.
This thermal cycling will continue until the fault has been
corrected.
Mode Switching
The LTC3210-1 will automatically switch from 1x mode
to 1.5x mode and subsequently to 2x mode whenever a
dropout condition is detected at an LED pin. Dropout occurs
when a current source voltage becomes too low for the
programmed current to be supplied. The time from drop-
out detection to mode switching is typically 0.4ms.
The part is reset back to 1x mode when the part is shut
down (ENM = ENC = Low) or on the falling edge of ENC.
An internal comparator will not allow the main switches to
connect V_BAT and CPO in 1x mode until the voltage at the
CPO pin has decayed to less than or equal to the voltage
at the V_BAT pin.
Applications Information
V_BAT, CPO Capacitor Selection
The style and value of the capacitors used with the
LTC3210-1 determine several important parameters such
as regulator control loop stability, output ripple, charge
pump strength and minimum start-up time.
To reduce noise and ripple, it is recommended that low
equivalent series resistance (ESR) ceramic capacitors are
used for both CV_BAT and C_CPO. Tantalum and aluminum
capacitors are not recommended due to high ESR.
The value of C_CPO directly controls the amount of output
ripple for a given load current. Increasing the size of C_CPO
will reduce output ripple at the expense of higher start-up
current. The peak-to-peak output ripple of the 1.5x mode
is approximately given by the expression:
V_{RIPPLE(P−P)}
=
I_OUT
(3f_OSC • C_CPO
)
(3)
Where f_OSC is the LTC3210-1 oscillator frequency or
typically 800kHz and CCPO is the output storage capa-
citor.
The output ripple in 2x mode is very small due to the fact
that load current is supplied on both cycles of the clock.
Both style and value of the output capacitor can signifi-
cantly affect the stability of the LTC3210-1. As shown in
the Block Diagram, the LTC3210-1 uses a control loop
to adjust the strength of the charge pump to match the
required output current. The error signal of the loop is
stored directly on the output capacitor. The output capacitor
also serves as the dominant pole for the control loop. To
prevent ringing or instability, it is important for the output
capacitor to maintain at least 1.3µF of capacitance over
all conditions.
In addition, excessive output capacitor ESR >100mW will
tend to degrade the loop stability. Multilayer ceramic chip
capacitors typically have exceptional ESR performance
and when combined with a tight board layout will result
in very good stability. As the value of C_CPO controls the
amount of output ripple, the value of CV_BAT controls the
amount of ripple present at the input pin (_VBAT). The
LTC3210-1’s input current will be relatively constant while
the charge pump is either in the input charging phase or
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