LTC3218 Datasheet, PDF (8/12 Pages) Linear Technology – 400mA Single Wire Camera LED Charge Pump

LTC3218
OPERATION
Charge Pump Strength
Mode Switching
When the LTC3218 operates in 2x mode, the charge pump
can be modeled as a Thevenin-equivalent circuit to deter-
mine the amount of current available from the effective
input voltage and effective open-loop output resistance,
R_OL (Figure 1).
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:
2V_IN − CPO
R_OL
(3)
The LTC3218 will automatically switch from 1x mode to
2x mode whenever the LED forward voltage approaches
the maximum CPO voltage for that mode. The part will
wait approximately 500µs before switching to the next
mode. This delay allows the LED to warm up and reduce
its forward voltage which may remove the dropout condi-
tion. The part may be reset to 1x mode by bringing the part
into shutdown by setting the ENF and ENT pins low. Once
these pins are low, either one or both may be immediately
brought high to re-enable the part.
in 2x mode.
R_OL
+
–+ ^2VIN
CPO
–
3218 F01
Figure 1. Charge Pump Open-Loop Thevenin-Equivalent Circuit
APPLICATIONS INFORMATION
V_IN, CPO Capacitor Selection
The value and type of capacitors used with the LTC3218
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 be
used for both C_VIN and C_CPO. Tantalum and aluminum ca-
pacitors are not recommended because of their 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 the output ripple at the expense of higher start-
up current. The peak-to-peak output ripple for 2x mode is
approximately given by the expression:
V_{RIPPLE(P−P)}
=
I_OUT
2f_OSC • C_CPO
8
Where f_OSC is the LTC3218’s oscillator frequency (typically
1MHz) and C_CPO is the output storage capacitor.
Both the style and value of the output capacitor can sig-
nificantly affect the stability of the LTC3218. As shown in
the Block Diagram, the LTC3218 uses a control loop to
adjust the strength of the charge pump to match the cur-
rent required at the output. The error signal of this loop
is stored directly on the output charge storage capacitor.
The charge storage 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
3µF of actual capacitance over all conditions.
Likewise, excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC3218. To prevent
poor load transient response and instability, the ESR of the
output capacitor should be kept below 80mΩ. Multilayer
ceramic chip capacitors typically have exceptional ESR
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