LTC3220-1_15 Datasheet, PDF (16/20 Pages) Linear Technology – 360mA Universal 18-Channel LED Driver

LTC3220/LTC3220-1
APPLICATIONS INFORMATION
V_IN, CPO Capacitor Selection
The style and value of the capacitors used with the LTC3220/
LTC3220-1 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 C_VIN 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_RIPPLEP-P
=
I_OUT
3f_OSC • C_CPO
(3)
where f_OSC is the LTC3220/LTC3220-1 oscillator fre-
quency or typically 850kHz and C_CPO is the output storage
capacitor.
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 type and value of the output capacitor can significantly
affect the stability of the LTC3220/LTC3220-1. As shown in
the Block Diagram, the LTC3220/LTC3220-1 use 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 3.2μF of capacitance over all
conditions and the ESR should be less than 80mΩ.
Multilayer ceramic chip capacitors typically have excep-
tional ESR performance. MLCCs 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 C_VIN controls the amount of ripple present at the input
pin (V_IN). The LTC3220/LTC3220-1 input current will be
relatively constant while the charge pump is either in the
input charging phase or the output charging phase but will
drop to zero during the clock nonoverlap times. Since the
nonoverlap time is small (~25ns), these missing “notches”
will result in only a small perturbation on the input power
supply line. Note that a higher ESR capacitor such as tan-
talum will have higher input noise due to the higher ESR.
Therefore, ceramic capacitors are recommended for low
ESR. Input noise can be further reduced by powering the
LTC3220/LTC3220-1 through a very small series inductor
as shown in Figure 4. A 10nH inductor will reject the fast
current notches, thereby presenting a nearly constant
current load to the input power supply. For economy, the
10nH inductor can be fabricated on the PC board with
about 1cm (0.4") of PC board trace.
V_BAT
LTC3220
LTC3220-1
GND
3220 F04
Figure 4. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Board Space)
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