ltc3220 Linear Technology Corporation, ltc3220 Datasheet - Page 16

• Manufacturer Part Number: ltc3220
• Description: 360ma Universal 18-channel Led Driver
• Manufacturer: Linear Technology Corporation

LTC3220/LTC3220-1
APPLICATIONS INFORMATION
V
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
capacitors are not recommended due to high ESR.
The value of C
ripple for a given load current. Increasing the size of C
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:
where f
quency or typically 850kHz and C
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 signifi cantly
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
16
IN
V
, CPO Capacitor Selection
RIPPLEP-P
OSC
is the LTC3220/LTC3220-1 oscillator fre-
CPO
=
3f
VIN
OSC
directly controls the amount of output
I
and C
OUT
• C
CPO
CPO
. Tantalum and aluminum
CPO
Figure 4. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Board Space)
is the output storage
CPO
(3)
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
of C
pin (V
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
GND
BAT
LTC3220-1
LTC3220
VIN
CPO
IN
controls the amount of ripple present at the input
). The LTC3220/LTC3220-1 input current will be
controls the amount of output ripple, the value
3220 F04
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