LTC3216 Datasheet, PDF (9/12 Pages) Linear Technology – 1A Low Noise High Current LED Charge Pump with Independent Torch/Flash Current Control

LTC3216
APPLICATIO S I FOR ATIO
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
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
appropriate to compare the amount of achievable capaci-
tance for a given case size rather than comparing the
specified capacitance value. For example, over rated volt-
age 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
case. The capacitor manufacturer’s data sheet should be
consulted to determine what value of capacitor is needed
to ensure minimum capacitances at all temperatures and
voltages.
Table 2 shows a list of ceramic capacitor manufacturers
and how to contact them.
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
TDK
www.tdk.com
Layout Considerations and Noise
Due to its high switching frequency and the transient
currents produced by the LTC3216, careful board layout is
necessary. A true ground plane and short connections to
all capacitors will improve performance and ensure proper
regulation under all conditions.
The flying capacitor pins C1⁺, C2⁺, C1^– and C2^– will have
very high edge rate waveforms. The large dv/dt on these
pins can couple energy capacitively to adjacent PCB runs.
Magnetic fields can also be generated if the flying capaci-
tors are not close to the LTC3216 (i.e., the loop area is
large). To decouple capacitive energy transfer, a Faraday
shield may be used. This is a grounded PCB trace between
the sensitive node and the LTC3216 pins. For a high quality
AC ground, it should be returned to a solid ground plane
that extends all the way to the LTC3216.
Power Efficiency
To calculate the power efficiency (η) of a white LED driver
chip, the LED power should be compared to the input
power. The difference between these two numbers repre-
sents lost power whether it is in the charge pump or the
current sources. Stated mathematically, the power effi-
ciency is given by:
η ≡ ^PLED
P_IN
(4)
The efficiency of the LTC3216 depends upon the mode in
which it is operating. Recall that the LTC3216 operates as
a pass switch, connecting V_IN to CPO, until dropout is
detected at the I_LED pin. This feature provides the optimum
efficiency available for a given input voltage and LED
forward voltage. When it is operating as a switch, the
efficiency is approximated by:
η ≡ ^PLED = ^{VLED •ILED} ≈ ^VLED
P_IN
V_IN •I_IN
V_IN
(5)
since the input current will be very close to the LED
current.
At moderate to high output power, the quiescent current
of the LTC3216 is negligible and the expression above is
valid.
Once dropout is detected at the I_LED pin, the LTC3216
enables the charge pump in 1.5x mode.
In 1.5x boost mode, the efficiency is similar to that of a
linear regulator with an effective input voltage of 1.5 times
the actual input voltage. This is because the input current
for a 1.5x charge pump is approximately 1.5 times the load
current. In an ideal 1.5x charge pump, the power efficiency
would be given by:
η_IDEAL
≡
P_LED
P_IN
=
V_LED •I_LED
V_IN •1.5I_LED
≈
V_LED
1.5V_IN
(6)
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