LTC3214 Datasheet, PDF (8/12 Pages) Linear Technology – 500mA Camera LED Charge Pump

LTC3214
APPLICATIO S I FOR ATIO
V_IN, CPO Capacitor Selection
The value and type of capacitors used with the LTC3214
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 be
used for both C_VIN and C_CPO. Tantalum and aluminum
capacitors are not recommended because of their 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 at the expense of higher
start-up current. The peak-to-peak output ripple for 1.5x
mode is approximately given by the expression:
V_RIPPLE(P-P) = I_OUT/(3f_OSC • C_CPO)
Where f_OSC is the LTC3214’s oscillator frequency (typi-
cally 900kHz) and C_CPO is the output storage capacitor.
Both the style and value of the output capacitor can
significantly affect the stability of the LTC3214. As shown
in the Block Diagram, the LTC3214 uses a 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 as the dominant
pole for the control loop. To prevent ringing or instability,
it is important for the output capacitor to maintain at least
3µF of actual capacitance over all conditions.
Likewise, excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC3214. To prevent
poor load transient response and instability, the ESR of the
output capacitor should be kept below 50mΩ. Multilayer
ceramic chip capacitors typically have exceptional ESR
performance. MLCCs 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_VIN controls the
amount of ripple present at the input pin (V_IN). The input
current to the LTC3214 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 nonoverlap time is
small (~15ns), 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 LTC3214
through a very small series inductor as shown in Figure 3.
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.
0.1µF
10nH
2.2µF
V_IN
LTC3214
GND
3214 F03
Figure 3. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Wire)
Flying Capacitor Selection
Warning: Polarized capacitors such as tantalum or alu-
minum should never be used for the flying capacitors
since their voltage can reverse upon start-up of the
LTC3214. 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 1.6µF of actual capacitance for
each of the flying capacitors. Capacitors of different mate-
rials 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 capacitance
when the rated voltage is applied. Therefore, when compar-
ing different capacitors, it is often more appropriate to
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