LTC3215 Datasheet, PDF (8/12 Pages) Linear Technology – 700mA Low Noise High Current LED Charge Pump

LTC3215
APPLICATIO S I FOR ATIO
V_IN, CPO Capacitor Selection
The value and type of capacitors used with the LTC3215
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)
(3)
Where f_OSC is the LTC3215’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 LTC3215. As shown
in the block diagram, the LTC3215 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
2.2µF of actual capacitance over all conditions.
Likewise, excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC3215. The closed
loop output resistance of the LTC3215 is designed to be
76mΩ. For a 100mA load current change, the error signal
will change by about 7.6mV. If the output capacitor has
76mΩ or more of ESR, the closed-loop frequency re-
sponse will cease to roll off in a simple one-pole fashion
and poor load transient response or instability could
result. 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 LTC3215 will be relatively
constant while the charge pump is on either the input
8
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 capaci-
tors are again recommended for their exceptional ESR
performance. Input noise can be further reduced by pow-
ering the LTC3215 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.
Flying Capacitor Selection
10nH
0.1µF
2.2µF
V_IN
LTC3215
GND
3215 F03
Figure 3. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Wire)
Warning: Polarized capacitors such as tantalum or alumi-
num should never be used for the flying capacitors since
their voltage can reverse upon start-up of the LTC3215.
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 2.2µF 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^oC to 85^oC 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
3215f