LTC3230_15 Datasheet, PDF (12/16 Pages) Linear Technology – 5-LED Main/Sub Display Driver with Dual LDO

LTC3230
APPLICATIONS INFORMATION
V_IN and CPO Capacitor Selection
The style and value of the capacitors used with the LTC3230
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 on both V_IN and 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 the output ripple but will increase start-up
time. The peak-to-peak output ripple of the 1.5x mode is
approximately given by the expression:
V_RIPPLE(P-P)
=
3
•
I_OUT
f_OSC • C_CPO
where f_OSC is the oscillator frequency, 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 style and value of the output capacitor can signifi-
cantly affect the stability of the LTC3230. As shown in the
Block Diagram, the LTC3230 uses a control loop to adjust
the strength of the charge pump to match the required
output current. The error signal for 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 0.6μF of capacitance over
all conditions.
In addition, excessive output capacitor ESR >100mΩ will
tend to degrade the loop stability. Multilayer ceramic chip
capacitors typically have exceptional ESR performance and
when 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 LTC3230’s 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 overlapping
12
times. Since the nonoverlapping time is small (~10ns),
these missing “notches” will result in only a small pertur-
bation on the input power supply line. Note that a higher
ESR capacitor such as tantalum will cause a higher input
noise due to the higher ESR. Input noise can be further
reduced by powering the LTC3230 through a very small
series inductor as shown in Figure 5. A 10nH inductor will
reject the fast current notches, thereby presenting a nearly
constant current load to the input power supply.
V_BAT
LTC3230
GND
3230 F05
Figure 5. 10nH Inductor Used for Input Noise Reduction
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
LTC3230. 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 at least 0.6μF 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. Capacitors
may also have a very poor voltage coefficient causing them
to lose 60% or more of their capacitance when the rated
voltage is applied. Therefore, when comparing different
capacitors, it is often more appropriate 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 capacitor in a 0603 case may not
provide any more capacitance than a 0.22μF, 10V, X7R
available in the same case. The capacitor manufacturer’s
data sheet should be consulted to determine what value
3230fa