LTC3614_15 Datasheet, PDF (16/30 Pages) Linear Technology – 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3614
Applications Information
Output Capacitor (C_OUT ) Selection
The selection of C_OUT is typically driven by the required
ESR to minimize voltage ripple and load step transients
(low ESR ceramic capacitors are discussed in the next
section). Typically, once the ESR requirement is satisfied,
the capacitance is adequate for filtering. The output ripple
∆V_OUT is determined by:
∆V_OUT
≤
∆I_L
•

ESR +

8
•
1
f_SW •
C_OUT



where f_OSC = operating frequency, C_OUT = output capaci-
tance and ∆I_L = ripple current in the inductor. The output
ripple is highest at maximum input voltage since ∆I_L
increases with input voltage.
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 tantalum
capacitors are all available in surface mount packages.
Tantalum capacitors have the highest capacitance density,
but can have higher ESR and must be surge tested for
use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can often
be used in extremely cost-sensitive applications provided
that consideration is given to ripple current ratings and
long-term reliability.
Ceramic Input and Output Capacitors
Ceramic capacitors have the lowest ESR and can be cost
effective, but also have the lowest capacitance density,
high voltage and temperature coefficients, and exhibit
audible piezoelectric effects. In addition, the high Q of
ceramic capacitors along with trace inductance can lead
to significant ringing.
They are attractive for switching regulator use because
of their very low ESR, but great care must be taken when
using only ceramic input and output capacitors.
Ceramic capacitors are prone to temperature effects
which require the designer to check loop stability over
the operating temperature range. To minimize their large
temperature and voltage coefficients, only X5R or X7R
ceramic capacitors should be used.
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 until the feedback loop
raises the switch current enough to support the load. The
time required for the feedback loop to respond is dependent
on the compensation components and the output capaci-
tor size. Typically, 3 to 4 cycles are required to respond
to a load step, but only in the first cycle does the output
drop linearly. The output droop, V_DROOP , is usually about
2 to 4 times the linear drop of the first cycle; however,
this behavior can vary depending on the compensation
component values. Thus, a good place to start is with the
output capacitor size of approximately:
C_OUT
≈
3.5 • ∆I_OUT
f_SW • V_DROOP
This is only an approximation; more capacitance may
be needed depending on the duty cycle and load step
requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance
to the supply is very low.
3614fc
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