LTC3446EDE Datasheet, PDF (13/20 Pages) Linear Technology – Monolithic Buck Regulator with Dual VLDO Regulators

LTC3446
applications information
Output Capacitor (C_OUTB) Selection
The selection of C_OUTB 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 (∆V_OUTB) is
determined by:
∆V_OUTB
≈
∆I_L

^{ESR +}
1
8f_O C_OUTB


where f = 2.25MHz, C_OUTB = output capacitance and
∆I_L = ripple current in the inductor. The output ripple is
highest at maximum input voltage since ∆I_L increases
with input voltage.
Once the ESR requirements for C_OUTB 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, offer very low ESR,
but have a lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density,
but have a larger ESR and 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 tantalums, avalable 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 specialty polymer
(SP) capacitors.
In most cases, 0.1µF to 1µF of ceramic capacitors should
also be placed close to the LTC3446 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.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
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