LTC3549 Datasheet, PDF (10/16 Pages) Linear Technology – 250mA Low VIN Buck Regulator in 2mm × 3mm DFN

LTC3549
APPLICATIO S I FOR ATIO
tions do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based
on 2000 hours of life. This makes it advisable to further
derate the capacitor, or choose a capacitor rated at a
higher temperature than required. Always consult the
manufacturer if there is any question. The selection of
C_OUT is driven by the required effective series resistance
(ESR). Typically, once the ESR requirement for C_OUT has
been met, the RMS current rating generally far exceeds
the I_RIPPLE(P-P) requirement. The output ripple ΔV_OUT
is determined by:
∆V_OUT
=
∆I_L ^{⎛⎝⎜}ESR
+
8
•
f
1
• C_OUT
⎞
⎠⎟
where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ΔI_L increases with input voltage. Aluminum
electrolytic and dry 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
The LTC3549 typically will require an output capacitor
in the 4.7µF to 10µF range for optimum stability. Higher
V_OUT
R1
LTC3549
V_FB
R2
GND
3549 F01
value, lower cost ceramic capacitors are now available in
smaller case sizes. Their high ripple current, high voltage
rating and low ESR make them ideal for switching regulator
applications. Because the LTC3549’s control loop does not
depend on the output capacitor’s ESR for stable operation,
ceramic capacitors can be used to achieve very low output
ripple and small circuit size.
However, care must be taken when these capacitors are
used at the input and the output. When a ceramic capacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can
induce ringing at the input, V_IN. At best, this ringing can
couple to the output and be mistaken as loop instability. At
worst, a sudden inrush of current through the long wires
can potentially cause a voltage spike at V_IN, large enough
to damage the part.
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.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
V_OUT
=
0.611V
⎛
⎝⎜
1+
R1⎞
R2 ⎠⎟
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 1
Table 2 gives 1% resistor values for selected output
voltages.
Table 2. Resistor Values for Selected Output Voltages
V_OUT
R1
R2
0.85V
53.6k
137k
1.2V
133k
137k
1.5V
200k
137k
1.8V
267k
137k
Figure 1. Setting the LTC3549 Output Voltage
3549f
10