LTC3550 Datasheet, PDF (17/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550
APPLICATIO S I FOR ATIO
where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. For a fixed output
voltage, the output ripple voltage is highest at maximum
input voltage since ΔI_L increases with input voltage.
Aluminum electrolytic and solid tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
Using Ceramic Input and Output Capacitors
Higher capacitance values, lower cost ceramic capacitors
are now becoming available in smaller case sizes. Their
high ripple current, high voltage rating and low ESR make
them ideal for switching regulator applications. Because
the LTC3550’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
V_FB
LTC3550
GND
0.6V ≤ V_OUT ≤ 5.5V
R2
R1
3550 F03
Figure 3. Setting the LTC3550 Output Voltage
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of
the losses in LTC3550 circuits: V_CC quiescent current
and I²R losses. The V_CC quiescent current loss dominates
the efficiency loss at very low load currents whereas the
I²R loss dominates the efficiency loss at medium to high
load currents. In a typical efficiency plot, the efficiency
curve at very low load currents can be misleading since
the actual power lost is of no consequence as illustrated
in Figure 4.
1. The V_CC quiescent current is due to two components:
the DC bias current as given in the Electrical Charac-
teristics and the internal main switch and synchronous
Output Voltage Programming
1
The output voltage is set by a resistive divider according
to the following formula:
0.1
V_OUT
=
0.6V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
0.01
(4)
0.001
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
0.0001
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
0.00001
0.1
1
10
100
LOAD CURRENT (mA)
1000
3550 F04
Figure 4. Power Lost vs Load Current
3550fa
17