LT3651-8.2/LT3651-8.4
19
36518284fa
For more information www.linear.com/LT3651-8.2
APPLICATIONS INFORMATION
Consideration should be given for power dissipation and
overall efficiency in a LT3651-8.2/LT3651-8.4 charger. A
detailed analysis is beyond the scope of the data sheet,
however following are general guidelines.
The major components of power loss are: conduction and
transition losses of the LT3651-8.2/LT3651-8.4 switches;
losses in the inductor and sense resistors; and AC losses
in the decoupling capacitors. Switch conduction loss is
fixed. Transition losses are adjustable by changing switcher
frequency. Higher input voltages cause an increase in
transition losses, decreasing overall efficiency. However
transition losses are inversely proportional to switcher
oscillator frequency so lowering operating frequency
reduces these losses. But lower operating frequency
usually requires higher inductance to maintain inductor
ripple current (inversely proportional). Inductors with
larger values typically have more turns, increasing ESR
unless you increase wire diameter making them physically
larger. So there is an efficiency and board size trade-off.
Secondarily, inductor AC losses increase with frequency
and lower ripple reduces AC capacitor losses.
The following simple rules of thumb assume a charge
current of 4A and battery voltage of 7.5V, with 1MHz os
-
cillator, 24mΩ sense
resistor and 3.3µH/20mΩ inductor.
A 1% increase in efficiency represents a 0.35W reduction
in
power loss at 85% overall efficiency. One way to do
this is to decrease resistance in the high current path. A
reduction of 0.2W at 4A requires a 22mΩ reduction in
resistance. This can be done by reducing inductor ESR.
It is also possible to lower the sense resistance (with a
reduction in R
RNG/SS
as well), with a trade-off of slightly
less accurate current accuracy. All high current board
traces should have the lowest resistance possible. Addition
of input current limit sense resistance reduces efficiency.
Charger efficiency drops approximately linearly with in
-
creasing frequency
all other things constant. At 15V V
IN
there is a 1% improvement in efficiency for every 200kHz
reduction in frequency (100kHz to 1MHz); At 28V V
IN
, 1%
for every 100kHz.
Of course all of these must be experimentally confirmed
in the actual charger.
