LTC3208 Datasheet, PDF (17/24 Pages) Linear Technology – High Current Software Confi gurable Multidisplay LED Controller

LTC3208
APPLICATIO S I FOR ATIO
V_BAT, CPO Capacitor Selection
The value and type of capacitors used with the LTC3208
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 LTC3208 oscillator frequency or typically
900kHz 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 value and type of output capacitor can significantly
affect the stability of the LTC3208. As shown in the block
diagram, the LTC3208 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 2.2µF of capacitance over all conditions.
In addition, excessive output capacitor ESR will tend to
degrade the loop stability. The closed loop output resis-
tance is about 80m . For a 100mA load current change,
the error signal will change by about 8mV. If the output
capacitor has 80m or more of ESR, the closed loop fre-
quency response will cease to roll off in a simple one-pole
fashion and poor load transient response or instability may
occur. Multilayer ceramic chip capacitors typically have
exceptional 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 CV_BAT controls the amount of ripple present at
the input pin (V_BAT). The LTC3208 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
LTC3208 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.
V_BAT
LTC3208
GND
3208 F06
Figure 6. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Board Trace)
Flying Capacitor Selection
Warning: Polarized capacitors such as tantalum or
aluminum should never be used for the flying capaci-
tors since their voltage can reverse upon start-up of the
LTC3208. Ceramic capacitors should always be used for
the flying capacitors.
The flying capacitors control the strength of the charge
pump. In order to achieve the rated output current it is
necessary to have 2.2µF 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 coefficient
causing them to lose 60% or more of their capacitance when
3208fa
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