LTC3419 Datasheet, PDF (10/16 Pages) Linear Technology – Dual Monolithic 600mA Synchronous Step-Down Regulator

LTC3419
APPLICATIONS INFORMATION
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 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
⎛
⎝⎜ ^{E SR}
+
1
8f_OC_OUT
⎞
⎠⎟
where f_O = 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.
If tantalum capacitors are used, it is critical that the capaci-
tors 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
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:
V_OUT
=
0.6V
⎛
⎝⎜
1+
R2 ⎞
R1⎠⎟
(2)
Keeping the current small (< 10μA) in these resistors
maximizes efficiency, 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 fixed output voltages available for the
LTC3419.
Table 2. Fixed Output Voltage Versions
PART NUMBER
V_OUT1
LTC3419
Adjustable
LTC3419-1
1.575V
V_OUT2
Adjustable
1.8V
3419f
10