Datasheet

LTC3419
10
3419fa
deviations do not offer much relief. Note that capacitor
manufacturer’s ripple current ratings are often based on
only 2000 hours lifetime. This makes it advisable to further
derate the capacitor, or choose a capacitor rated at a higher
temperature than required. Several capacitors may also be
paralleled to meet the size or height requirements of the
design. An additional 0.1μF to 1μF ceramic capacitor is
also recommended on V
IN
for high frequency decoupling
when not using an all-ceramic capacitor solution.
Output Capacitor (C
OUT
) Selection
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:
ΔΔVIESR
fC
OUT L
O OUT
≈+
⎛
⎝
⎜
⎞
⎠
⎟
1
8
where f
O
= operating frequency, C
OUT
= output capacitance
and ΔI
L
= ripple current in the inductor. For a fi xed output
voltage, the output ripple is highest at maximum input
voltage since ΔI
L
increases with input voltage.
If tantalum capacitors are used, 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
Higher 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 LTC3419 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. 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. For more information, see Application Note 88.
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.
Setting the Output Voltage
The LTC3419 regulates the V
FB1
and V
FB2
pins to 0.6V
during regulation. Thus, the output voltage is set by a
resistive divider according to the following formula:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1
2
1
2.()
Keeping the current small (<10μA) in these resistors
maximizes effi ciency, but making it too small may allow
stray capacitance to cause noise problems or reduce the
phase margin of the error amp loop.
To improve the frequency response of the main control
loop, a feedback capacitor (C
F
) may also be used. Great
care should be taken to route the V
FB
line away from noise
sources, such as the inductor or the SW line.
Fixed output versions of the LTC3419 (e.g. LTC3419-1)
include an internal resistive divider, eliminating the need
for external resistors. The resistor divider is chosen
such that the V
FB
input current is approximately 3μA. For
these versions the V
FB
pin should be connected directly
to V
OUT
. Table 2 lists the fi xed output voltages available
for the LTC3419.
APPLICATIONS INFORMATION
Table 2. Fixed Output Voltage Versions
PART NUMBER V
OUT1
V
OUT2
LTC3419 Adjustable Adjustable
LTC3419-1 1.575V 1.8V