LTC3200-5_15 Datasheet, PDF (8/12 Pages) Linear Technology – Low Noise, Regulated Charge Pump DC/DC Converters

LTC3200/LTC3200-5
U
OPERATIO
LTC3200/LTC3200-5 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 (~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 recom-
mended for their exceptional ESR performance.
Further input noise reduction can be achieved by powering
the LTC3200/LTC3200-5 through a very small series in-
ductor as shown in Figure 4. 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
0.22µF
1µF
LTC3200/
LTC3200-5
GND
32005 F02
Figure 4. 10nH Inductor Used for
Additional Input Noise Reduction
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the flying capacitor
since its voltage can reverse upon start-up of the LTC3200/
LTC3200-5. Low ESR ceramic capacitors should always
be used for the flying capacitor.
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 0.68µF of capacitance for the
flying capacitor.
For very light load applications the flying capacitor may be
reduced to save space or cost. The theoretical minimum
output resistance of a voltage doubling charge pump is
given by:
R_OL(MIN)
≡
2V_IN – V_OUT
I_OUT
≅
1
f_OSCC _FLY
Where f_OSC is the switching frequency (2MHz typ) and
C_FLY is the value of the flying capacitor. The charge pump
will typically be weaker than the theoretical limit due to
additional switch resistance, however for very light load
applications the above expression can be used as a guide-
line in determining a starting capacitor value.
Ceramic Capacitors
Ceramic capacitors of different materials lose their capaci-
tance with higher temperature and voltage at different
rates. For example, a capacitor made of X5R or 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 the rated voltage is applied. Therefore, when com-
paring different capacitors it is often more appropriate to
compare the amount of achievable capacitance for a given
case size rather than discussing the specified capacitance
value. For example, over rated voltage and temperature
conditions, a 1µF, 10V, Y5V ceramic capacitor in an 0603
case may not provide any more capacitance than a
0.22µF, 10V, X7R available in the same 0603 case. In fact
for most LTC3200/LTC3200-5 applications these capaci-
tors can be considered roughly equivalent . The capacitor
manufacturer’s data sheet should be consulted to deter-
mine what value of capacitor is needed to ensure the
desired capacitance at all temperatures and voltages.
Below is a list of ceramic capacitor manufacturers and
how to contact them:
AVX
Kemet
Murata
Taiyo Yuden
Vishay
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.vishay.com
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