LTC3549EDCB#TRPBF Linear Technology, LTC3549EDCB#TRPBF Datasheet - Page 10

• Manufacturer Part Number: LTC3549EDCB#TRPBF
• Description: IC REG BUCK 250MA 6-DFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3549EDCB#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.61 ~ 5.5 V
• Current - Output: 250mA
• Frequency - Switching: 2.25MHz
• Voltage - Input: 1.6 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 6-DFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC3549
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
(ESR). Typically, once the ESR requirement for C
been met, the RMS current rating generally far exceeds
the I
is determined by:
where f = operating frequency, C
and ΔI
voltage, the output ripple is highest at maximum input
voltage since ΔI
electrolytic and dry tantalum capacitors are both available
in surface mount confi gurations. 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 specifi c 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
10
OUT
∆
V
RIPPLE(P-P)
is driven by the required effective series resistance
OUT
L
= ripple current in the inductor. For a fi xed output
Figure 1. Setting the LTC3549 Output Voltage
= ∆
I ESR
L
L
⎛
⎜
⎝
requirement. The output ripple ΔV
increases with input voltage. Aluminum
U
LTC3549
+
8 • •
U
GND
V
FB
f C
1
OUT
OUT
V
OUT
W
⎞
⎟
⎠
= output capacitance
R1
R2
3549 F01
U
OUT
has
OUT
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
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
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:
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
0.85V
V
1.2V
1.5V
1.8V
OUT
=
0 611
.
V
⎛
⎝ ⎜
1
+
R
R
2
1
53.6k
133k
200k
267k
R1
⎞
⎠ ⎟
IN
. At best, this ringing can
IN
, large enough
137k
137k
137k
137k
R2
3549f
(2)