LTC3206 Datasheet, PDF (12/16 Pages) Linear Technology – I2C Multidisplay LED Controller

LTC3206
APPLICATIO S I FOR ATIO
where f_OSC is the LTC3206’s oscillator frequency (typically
960kHz) and C_CPO is the output charge storage capacitor
on CPO. Both the style and value of the output capacitor
can significantly affect the stability of the LTC3206. The
LTC3206 uses a linear control loop to adjust the strength
of the charge pump to match the current 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 to form the dominant pole for the
control loop. To prevent ringing or instability, it is impor-
tant for the output capacitor to maintain at least 0.6µF of
capacitance over all conditions. Likewise, excessive ESR
on the output capacitor will tend to degrade the loop
stability of the LTC3206. The closed-loop output resis-
tance of the LTC3206 is designed to be 0.4Ω. For a 100mA
load current change, the error signal will change by about
40mV. If the output capacitor has 0.4Ω or more of ESR,
the closed-loop frequency response will cease to roll off in
a simple one-pole fashion and poor load transient re-
sponse or instability could result. Multilayer ceramic chip
capacitors typically have exceptional ESR performance.
MLCC capacitors combined with a tight board layout, will
yield very good stability. As the value of C_CPO controls the
amount of output ripple, the value of C_IN controls the
amount of ripple present at the input pin (V_IN). The input
current to the LTC3206 will be relatively constant while the
charge pump is on either the input charging phase or the
output charging phase but will drop to zero during the
clock nonoverlap times. Since the non-overlap 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 tantalum will have
higher input noise due to the input current change times
the ESR. Therefore, ceramic capacitors are again recom-
mended for their exceptional ESR performance. Input
noise can be further reduced by powering the LTC3206
through a very small series inductor as shown in Figure 6.
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.
Flying Capacitor Selection
10nH
V_IN
V_IN
0.1µF
2.2µF LTC3206
GND
3206 F06
Figure 6. 10nH Inductor Used for Input Noise
Reduction (Approximately 1cm of Wire)
Warning: A polarized capacitor such as tantalum or alumi-
num should never be used for the flying capacitors since
their voltage can reverse upon start-up of the LTC3206.
Ceramic capacitors should always be used for the flying
capacitors.
The flying capacitor controls the strength of the charge
pump. In order to achieve the rated output current it is
necessary to have at least 1µF of capacitance for each of
the flying capacitors. Capacitors of different materials lose
their capacitance with higher temperature and voltage at
different rates. For example, a ceramic capacitor made of
X7R material will retain most of its capacitance from
–40°C to 85°C whereas a Z5U or Y5V style capacitor will
lose considerable capacitance over that range. Z5U and
Y5V capacitors may also have a very poor voltage coeffi-
cient causing them to lose 60% or more of their capaci-
tance when the rated voltage is applied. Therefore, when
comparing different capacitors, it is often more appropri-
ate to compare the amount of achievable capacitance for
a given case size rather than comparing the specified
capacitance value. For example, over rated voltage and
temperature conditions, a 1µF, 10V, Y5V ceramic capaci-
tor in a 0603 case may not provide any more capacitance
than a 0.22µF, 10V, X7R available in the same 0603 case.
The capacitor manufacturer’s data sheet should be con-
sulted to determine what value of capacitor is needed to
ensure minimum capacitances at all temperatures and
voltages.
3206f
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