LTC3545 LINER [Linear Technology], LTC3545 Datasheet - Page 13

• Manufacturer Part Number: LTC3545
• Description: Triple 800mA Synchronous Step-Down Regulator-2.25MHz
• Manufacturer: LINER [Linear Technology]

The selection of C
series resistance (ESR). Typically, once the ESR require-
ment for C
generally far exceeds the I
output ripple ΔV
where f = operating frequency, C
and ΔI
voltage, the output ripple is highest at maximum input
voltage since ΔI
Using Ceramic Input and Output Capacitors
Higher value, lower cost, ceramic capacitors are now
widely available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them
ideal for switching regulator applications. Because the
LTC3545/LTC3545-1’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.
However, care must be taken when ceramic 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.
APPLICATIONS INFORMATION
Δ
V
OUT
L
= ripple current in the inductor. For a fi xed output
≅
OUT
Δ
I ESR
L
has been met, the RMS current rating
L
OUT
⎛
⎝ ⎜
increases with input voltage.
OUT
is determined by:
is driven by the required effective
+
8 • •
RIPPLE(P-P)
ƒ
IN
1
. At best, this ringing can
C
OUT
OUT
= output capacitance
⎞
⎠ ⎟
requirement. The
IN
, large enough
Output Voltage Programming
The output voltage is set by tying V
according to the following formula:
The external resistive divider is connected to the output
allowing remote voltage sensing as shown in Figure 2.
Effi ciency Considerations
The effi ciency 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
limiting the effi ciency and which change would produce
the most improvement. Effi ciency can be expressed as:
Effi ciency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc.
are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC3545/LTC3545-1 circuits: V
rent and I
the effi ciency loss at low load currents, whereas the I
loss dominates the effi ciency loss at medium to high load
currents. In a typical effi ciency plot, the effi ciency curve at
very low load currents can be misleading since the actual
power lost is of little consequence as illustrated on the
front page of the data sheet.
V
OUT
=
Figure 2. Setting the LTC3545 Output Voltage
2
R losses. V
0 6
.
V
LTC3545
⎛
⎝ ⎜
LTC3545/LTC3545-1
1
+
R
R
GND
IN
V
2
1
FB
quiescent current loss dominates
⎞
⎠ ⎟
0.6V ≤ V
FB
OUT
to a resistive divider
R2
R1
3545 F02
≤ 5.5V
IN
quiescent cur-
13
35451fb
2
R