LTC3545-1_15 Datasheet, PDF (13/20 Pages) Linear Technology – Triple 800mA Synchronous Step-Down Regulator–2.25MHz

LTC3545/LTC3545-1
APPLICATIONS INFORMATION
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment 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:
ΔV_OUT
≅
ΔI_L
⎛
⎝⎜ ^ESR
+
8
•
ƒ
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.
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_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 tying V_FB to a resistive divider
according to the following formula:
V_OUT
=
0.6V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
The external resistive divider is connected to the output
allowing remote voltage sensing as shown in Figure 2.
0.6V ≤ V_OUT ≤ 5.5V
V_FB
LTC3545
GND
R2
R1
3545 F02
Figure 2. Setting the LTC3545 Output Voltage
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
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 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_IN quiescent cur-
rent and I²R losses. V_IN quiescent current loss dominates
the efficiency loss at 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 little consequence as illustrated on the
front page of the data sheet.
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