ADP1823 Datasheet, PDF (16/32 Pages) Analog Devices – Dual, Interleaved, Step-Down DC-to-DC Controller with Tracking

ADP1823
APPLICATIONS INFORMATION
SELECTING THE INPUT CAPACITOR
The input current to a buck converter is a pulse waveform. It is
zero when the high-side switch is off and approximately equal
to the load current when it is on. The input capacitor carries the
input ripple current, allowing the input power source to supply
only the dc current. The input capacitor needs sufficient ripple
current rating to handle the input ripple and also ESR that is
low enough to mitigate input voltage ripple. For the usual current
ranges for these converters, good practice is to use two parallel
capacitors placed close to the drains of the high-side switch
MOSFETs, one bulk capacitor of sufficiently high current rating
as calculated in Equation 1, along with 10 μF of ceramic capacitor.
Select an input bulk capacitor based on its ripple current rating.
If both Channel 1 and Channel 2 maximum output load
currents are about the same, the input ripple current is less than
half of the higher of the output load currents. In this case, use
an input capacitor with a ripple current rating greater than half
of the highest load current.
I _RIPPLE
>
I_L
2
(1)
If the Output 1 and Output 2 load currents are significantly
different (if the smaller is less than 50% of the larger), then the
procedure in Equation 1 yields a larger input capacitor than
required. In this case, the input capacitor can be chosen as in
the case of a single phase converter with only the higher load
current, so first determine the duty cycle of the output with the
larger load current:
D = ^VOUT
(2)
V_IN
In this case, the input capacitor ripple current is approximately
I_RIPPLE ≈ I_L D(1 − D)
(3)
where IL is the maximum inductor or load current for the
channel and D is the duty cycle. Use this method to determine
the input capacitor ripple current rating for duty cycles between
20% and 80%.
For duty cycles less than 20% or greater than 80%, use an input
capacitor with ripple current rating IRIPPLE > 0.4 IL.
Selecting the Output LC Filter
The output LC filter attenuates the switching voltage, making
the output an almost dc voltage. The output LC filter
characteristics determine the residual output ripple voltage.
Choose an inductor value such that the inductor ripple current
is approximately 1/3 of the maximum dc output load current.
Using a larger value inductor results in a physical size larger
than is required, and using a smaller value results in increased
losses in the inductor and MOSFETs.
Choose the inductor value by the equation
L = ^{VIN − VOUT}
ΔI _L f_SW
⎜⎜⎝⎛
V_OUT
V_IN
⎟⎟⎠⎞
(4)
where:
L is the inductor value.
fSW is the switching frequency.
VOUT is the output voltage.
VIN is the input voltage.
ΔIL is the inductor ripple current, typically 1/3 of the maximum
dc load current.
Choose the output bulk capacitor to set the desired output voltage
ripple. The impedance of the output capacitor at the switching
frequency multiplied by the ripple current gives the output
voltage ripple. The impedance is made up of the capacitive
impedance plus the nonideal parasitic characteristics, the
equivalent series resistance (ESR) and the equivalent series
inductance (ESL). The output voltage ripple can be
approximated with
ΔV_OUT
=
ΔI _L
⎜⎛
⎜⎝
ESR
+
1
8 f _SW C_OUT
+ 4 f _SW
ESL
⎟⎞
⎟⎠
(5)
where:
ΔVOUT is the output ripple voltage.
ΔIL is the inductor ripple current.
ESR is the equivalent series resistance of the output capacitor
(or the parallel combination of ESR of all output capacitors).
ESL is the equivalent series inductance of the output capacitor
(or the parallel combination of ESL of all capacitors).
Note that the factors of 8 and 4 in Equation 5 would normally
be 2π for sinusoidal waveforms, but the ripple current
waveform in this application is triangular. Parallel combinations
of different types of capacitors, for example, a large aluminum
electrolytic in parallel with MLCCs, may give different results.
Usually, the impedance is dominated by ESR at the switching
frequency, as stated in the maximum ESR rating on the
capacitor data sheet, so this equation reduces to
ΔV_OUT ≅ ΔI _L ESR
(6)
Electrolytic capacitors have significant ESL also, on the order of
5 nH to 20 nH, depending on type, size, and geometry, and PCB
traces contribute some ESR and ESL as well. However, using the
maximum ESR rating from the capacitor data sheet usually
provides some margin such that measuring the ESL is not
usually required.
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