LTC3201_15 Datasheet, PDF (6/8 Pages) Linear Technology – 100mA Ultralow Noise Charge Pump LED Supply with Output Current Adjust

LTC3201
APPLICATIO S I FOR ATIO
Power Efficiency
The power efficiency (η) of the LTC3201 is similar to that
of a linear regulator with an effective input voltage of twice
the actual input voltage. This occurs because the input
current for a voltage doubling charge pump is approxi-
mately twice the output current. In an ideal regulator the
power efficiency would be given by:
η = ^POUT = ^{VOUT •IOUT} = ^VOUT
P_IN V_IN • 2I_OUT 2V_IN
At moderate to high output power the switching losses
and quiescent current of LTC3201 are relatively low. Due
to the high clocking frequency, however, the current used
for charging and discharging the switches starts to reduce
efficiency. Furthermore, due to the low V_F of the LEDs,
power delivered will remain low.
Short-Circuit/Thermal Protection
The LTC3201 has short-circuit current limiting as well as
overtemperature protection. During short-circuit condi-
tions, the output current is limited to typically 150mA.
On-chip thermal shutdown circuitry disables the charge
pump once the junction temperature exceeds approxi-
mately 160°C and re-enables the charge pump once the
junction temperature drops back to approximately 150°C.
The LTC3201 will cycle in and out of thermal shutdown
indefinitely without latchup or damage until the short-
circuit on V_OUT is removed.
V_OUT Capacitor Selection
The style and value of capacitors used with the LTC3201
determine several important parameters such as output
ripple, charge pump strength and minimum start-up time.
To reduce noise and ripple, it is recommended that low
ESR (<0.1Ω) capacitors be used for C_FILTER, C_IN, C_OUT.
These capacitors should be ceramic.
The value of C_OUT controls the amount of output ripple.
Increasing the size of C_OUT to 10µF or greater will reduce
the output ripple at the expense of higher turn-on times
and start-up current. See the section Output Ripple. A 1µF
C_OUT is recommended.
6
V_IN, V_FILTER Capacitor Selection
The value and resonant frequency of C_FILTER and C_IN
greatly determine the current noise profile at V_IN. C_FILTER
should be a high frequency 0.22µF capacitor with a reso-
nant frequency over 30MHz. Input capacitor C_IN should be
a 1µF ceramic capacitor with a resonant frequency over
1MHz. The X5R capacitor is a good choice for both. The
values of C_FILTER (0.22µF) and C_IN (1µF) provide optimum
high and low frequency input current filtering. A higher
filter cap value will result in lower low frequency input
current ripple, but with increased high frequency ripple.
The key at the FILTER node is that the capacitor has to be
very high frequency. If capacitor technology improves the
bandwidth, then higher values should be used. Similarly,
increasing the input capacitor value but decreasing its
resonant frequency will not really help. Decreasing it will
help the high frequency performance while increasing the
low frequency current ripple.
Direct Connection to Battery
Due to the ultra low input current ripple, it is possible to
connect the LTC3201 directly to the battery without using
regulators or high frequency chokes.
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or alumi-
num should never be used for the flying capacitor since its
voltage can reverse upon start-up. 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 0.22µF of capacitance for the
flying capacitor. 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 and Y5V style capacitor will
lose considerable capacitance over that range. Z5U and
Y5V capacitors may also have a very strong voltage
coefficient causing them to lose 60% or more of their
capacitance when the rated voltage is applied. Therefore,
when comparing different capacitors it is often more
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