LTC3202 Datasheet, PDF (9/12 Pages) Linear Technology – Low Noise, High Efficiency Charge Pump for White LEDs

U
OPERATIO
10nH
V_IN
0.1µF
4
V_IN
1µF LTC3202
5, 11
GND
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Figure 5. 10nH Inductor Used for Input Noise Reduction
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 LTC3202.
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 lose considerable capaci-
tance 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 comparing different capaci-
tors 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 0603 case. The capacitor manufacturer’s data sheet
should be consulted to determine what value of capacitor
is needed to ensure minimum capacitances at all tempera-
tures and voltages.
Table 2 shows a list of ceramic capacitor manufacturers
and how to contact them:
LTC3202
Table 2 Recommended Capacitor Vendors
AVX
www.avxcorp.com
Kemet
www.kemet.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay
www.vishay.com
For very light load applications the flying capacitor may be
reduced to save space or cost. The theoretical minimum
output resistance of a 2:3 fractional charge pump is given
by:
R_OL(MIN)
≡
1.5V_IN – V_OUT
I_OUT
=
1
2f_0SCC_FLY
Where f_OSC is the switching frequency (1.5MHz typ) and
C_FLY is the value of the flying capacitors. Note that 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 guideline in determining a starting capacitor value.
Power Efficiency
The power efficiency (η) of the LTC3202 is similar to that
of a linear regulator with an effective input voltage of 1.5
times the actual input voltage. This occurs because the
input current for a 2:3 fractional charge pump is approxi-
mately 1.5 times the load current. In an ideal regulating 2:3
charge pump the power efficiency would be given by:
η_{IDEAL ≡}
P_OUT
P_IN
=
V_OUT • I_OUT
V_IN
•
3
2
I_OUT
=
V_OUT
1.5V_IN
At moderate to high output power the switching losses
and quiescent current of the LTC3202 are negligible and
the expression above is valid. For example with V_IN = 3.2V,
I_OUT = 80mA and V_OUT regulating to 4.2V the measured
efficiency is 82% which is just under the theoretical 87.5%
calculation.
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