LTC3407A Datasheet, PDF (9/16 Pages) Linear Technology – Dual Synchronous 600mA, 1.5MHz Step-Down DC/DC Regulator

LTC3407A
APPLICATIONS INFORMATION
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case is commonly used to
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple cur-
rent 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 ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance
is adequate for filtering. The output ripple (ΔV_OUT) is
determined by:

V_OUT


I_L
 _ESR
+
8f_O
1
C_OUT


where f_O = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI_L increases
with input voltage. With ΔI_L = 0.3 • I_LIM the output ripple
will be less than 100mV at maximum V_IN and f_O = 1.5MHz
with:
ESR_COUT < 150mΩ
Once the ESR requirements for C_OUT have been met, the
RMS current rating generally far exceeds the I_RIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or
RMS current handling requirement of the application.
Aluminum electrolytic, special polymer, ceramic and dry
tantulum capacitors are all available in surface mount
packages. The OS-CON semiconductor dielectric capacitor
available from Sanyo has the lowest ESR(size) product
of any aluminum electrolytic at a somewhat higher price.
Special polymer capacitors, such as Sanyo POSCAP, of-
fer very low ESR, but have a lower capacitance density
than other types. Tantalum capacitors have the highest
capacitance density. However, they also have a larger
ESR and it is critical that they are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalums, available in case
heights ranging from 2mm to 4mm. Aluminum electrolytic
capacitors have a significantly larger ESR, and are often
used in extremely cost-sensitive applications provided that
consideration is given to ripple current ratings and long
term reliability. Ceramic capacitors have the lowest ESR
and cost, but also have the lowest capacitance density,
a high voltage and temperature coefficient, and exhibit
audible piezoelectric effects. In addition, the high Q of
ceramic capacitors along with trace inductance can lead
to significant ringing. Other capacitor types include the
Panasonic Special Polymer (SP) capacitors.
In most cases, 0.1μF to 1μF of ceramic capacitors should
also be placed close to the LTC3407A in parallel with the
main capacitors for high frequency decoupling.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now be-
coming available in smaller case sizes. These are tempting
for switching regulator use because of their very low ESR.
Unfortunately, the ESR is so low that it can cause loop
stability problems. Solid tantalum capacitor ESR gener-
ates a loop “zero” at 5kHz to 50kHz that is instrumental in
giving acceptable loop phase margin. Ceramic capacitors
remain capacitive to beyond 300kHz and usually resonate
with their ESL before ESR becomes effective. Also, ceramic
caps are prone to temperature effects which requires the
designer to check loop stability over the operating tem-
perature range. To minimize their large temperature and
voltage coefficients, only X5R or X7R ceramic capacitors
should be used. A good selection of ceramic capacitors
is available from Taiyo Yuden, TDK, and Murata.
Great care must be taken when using only ceramic input
and output capacitors. When a ceramic capacitor is used
at the input and the power is being supplied through long
wires, such as from a wall adapter, a load step at the output
can induce ringing at the V_IN pin. At best, this ringing can
couple to the output and be mistaken as loop instability.
At worst, the ringing at the input can be large enough to
damage the part.
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