LTC3607
applicaTions inForMaTion
be less than 100mV at maximum V and f = 2.25MHz
Input Capacitor (C ) Selection
IN
O
IN
with: ESR
< 150mΩ.
COUT
In continuous mode, the input current of the converter is a
Once the ESR requirements for C
have been met, the
square wave with a duty cycle of approximately V /V .
OUT
OUT IN
RMS current rating generally far exceeds the I
Topreventlargevoltagetransients,alowequivalentseries
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
RIPPLE(P-P)
requirement, except for an all ceramic solution.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement.Duringaloadstep,theoutputcapacitormust
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
looptorespondisdependentonthecompensationandthe
output capacitor size. Typically, 3 to 4 cycles are required
to respond to a load step, but only in the first cycle does
V
(V – V
)
OUT IN
OUT
I
≈I
RMS OUT(MAX)
V
IN
where the maximum average output current I
equals
MAX
the peak current minus half the peak-to-peak ripple cur-
rent, I = I – ΔI /2.
MAX
LIM
L
This formula has a maximum at V = 2V , where I
RMS
IN
OUT
the output drop linearly. The output droop, V
, is
DROOP
= I /2. This simple worst-case is commonly used to
OUT
usually about five times the linear drop of the first cycle.
Thus, a good place to start is with the output capacitor
size of approximately:
design because even significant deviations do not offer
muchrelief. Notethatcapacitormanufacturer’sripplecur-
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.Severalcapacitorsmayalsobeparalleledtomeet
thesizeorheightrequirementsofthedesign.Anadditional
0.1μF to 1μF ceramic capacitor is also recommended on
∆IOUT
COUT ≈5
fO •VDROOP
Thoughthisequationprovidesagoodapproximation,more
capacitance may be required depending on the duty cycle
and load step requirements.
V for high frequency decoupling, when not using an all
IN
ceramic capacitor solution.
Ceramic Input and Output Capacitors
Output Capacitor (C ) Selection
OUT
High value, low cost ceramic capacitors are available in
small case sizes. Their high ripple current, high voltage
rating,andlowESRmakethemidealforswitchingregulator
applications.However,duetotheself-resonantandhigh-Q
characteristics of some types of ceramic capacitors, care
mustbetakenwhenthesecapacitorsareusedattheinput.
The selection of C
is driven by the required ESR to
OUT
minimizevoltagerippleandloadsteptransients.Typically,
once the ESR requirement is satisfied, the capacitance
is adequate for filtering. The output ripple (ΔV ) is
OUT
determined by:
1
∆VOUT ≈∆I ESR+
L
Whenaceramiccapacitorisusedattheinputandthepower
is being supplied through long wires, such as from a wall
adapter, a load step at the output can induce ringing at
8f C
O OUT
wheref =operatingfrequency,C
=outputcapacitance
the V pin. At best, this ringing can couple to the output
O
L
OUT
IN
and ΔI = ripple current in the inductor. The output ripple
and be mistaken as loop instability. At worst, the ringing
at the input can be large enough to damage the part. For
a more detailed discussion, refer to Application Note 88.
is highest at maximum input voltage since ΔI increases
L
with input voltage. With ΔI = 240mA the output ripple will
L
3607fb
10
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Linear Systems [ Linear Systems ]
