LTC3205_15 Datasheet, PDF (10/16 Pages) Linear Technology – Multidisplay LED Controller

LTC3205
APPLICATIO S I FOR ATIO
necessary for LD to load the data once it has shifted into
the device. Command data is latched into the command
register on the falling edge of the LD signal. The LTC3205
will begin to act on new command data as soon as LD goes
low. Any general purpose microcontroller I/O line can be
configured to control the LD pin if the microcontroller
doesn’t provide this feature automatically.
V_IN, CPO Capacitor Selection
The style and value of capacitors used with the LTC3205
determine several important parameters such as regulator
control-loop stability, output ripple and charge pump
strength. To reduce noise and ripple, it is recommended
that low equivalent series resistance (ESR) multilayer
ceramic capacitors be used on both V_IN and CPO. Tanta-
lum and aluminum capacitors are not recommended be-
cause of their high ESR. The value of the capacitor on CPO
directly controls the amount of output ripple for a given
load current. Increasing the size of this capacitor will
reduce the output ripple. The peak-to-peak output ripple is
approximately given by the expression:
V_RIPPLEP-P
≅
I_OUT
3f_OSC • C_OUT
where f_OSC is the LTC3205’s oscillator frequency (typically
800kHz) and C_OUT is the output charge storage capacitor
on CPO. Both the style and value of the output capacitor
can significantly affect the stability of the LTC3205. The
LTC3205 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 LTC3205. The closed-loop output resis-
tance of the LTC3205 is designed to be 0.6Ω. For a 100mA
load current change, the error signal will change by about
60mV. If the output capacitor has 0.6Ω 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
10
capacitors typically have exceptional ESR performance.
MLCCs combined with a tight board layout will yield very
good stability. As the value of C_OUT 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
LTC3205 will be relatively constant while the charge pump
is on either the input charging phase or the output charg-
ing 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 tantalum will have higher input noise
due to the input current change times the ESR. Therefore,
ceramic capacitors are again recommended for their ex-
ceptional ESR performance. Input noise can be further
reduced by powering the LTC3205 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.
10nH
V_IN
V_IN
0.1µF
1µF LTC3205
GND
3205 F06
Figure 6. 10nH Inductor Used for Input Noise
Reduction (Approximately 1cm of Wire)
Flying Capacitor Selection
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 LTC3205.
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 0.7µF 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
3205f