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IR3841MPbF
Output Capacitor Selection
The voltage ripple and transient requirements
determine the output capacitors type and values.
The criteria is normally based on the value of the
Effective Series Resistance (ESR). However the
actual capacitance value and the Equivalent Series
The output LC filter introduces a double pole,
–40dB/decade gain slope above its corner
resonant frequency, and a total phase lag of 180 o
(see figure 13). The resonant frequency of the LC
filter is expressed as follows:
Inductance (ESL) are other contributing
components. These components can be described
as
F LC =
1
2 ? π L o ? C o
.......... .......... .......... .. (16)
Δ V o = Δ V o ( ESR ) + Δ V o ( ESL ) + Δ V o ( C )
Δ V o ( ESR ) = Δ I L * ESR
Figure 13 shows gain and phase of the LC filter.
Since we already have 180 o phase shift from the
output filter alone, the system runs the risk of
being unstable.
Δ V o ( ESL ) = ?? in
?? * ESL
? V ? V o ?
? L ?
Gain
Phase
Δ V o ( C ) =
Δ I L
8 * C o * F s
.......... .......... ..... (15)
0 dB
-40dB/decade
0 0
Δ V o = Output
voltage
ripple
Δ I L = Inductor ripple current
Since the output capacitor has a major role in the
F LC Frequency
-180 0
F LC Frequency
overall performance of the converter and
determines the result of transient response,
selection of the capacitor is critical. The IR3841
can perform well with all types of capacitors.
As a rule, the capacitor must have low enough
ESR to meet output ripple and load transient
requirements.
The goal for this design is to meet the voltage
ripple requirement in the smallest possible
capacitor size. Therefore it is advisable to select
ceramic capacitors due to their low ESR and ESL
and small size. Six of the Panasonic ECJ-
2FB0J226ML (22uF, 6.3V, 3mOhm) capacitors is
a good choice.
Feedback Compensation
Fig. 13. Gain and Phase of LC filter
The IR3841 uses a voltage-type error amplifier
with high-gain (110dB) and wide-bandwidth. The
output of the amplifier is available for DC gain
control and AC phase compensation.
The error amplifier can be compensated either in
type II or type III compensation.
Local feedback with Type II compensation is
shown in Fig. 14.
This method requires that the output capacitor
should have enough ESR to generate a zero
typically at 5kHz to 50kHz, which is essential for
an acceptable phase margin and to satisfy
stability requirements.
The IR3841 is a voltage mode controller. The
control loop is a single voltage feedback path
including error amplifier and error comparator. To
achieve fast transient response and accurate
F ESR =
1
2 ? π *ESR*C o
.......... .......... ....... (17)
output regulation, a compensation circuit is
necessary. The goal of the compensation
network is to provide a closed-loop transfer
function with the highest 0 dB crossing frequency
and adequate phase margin (greater than 45 o ).
06/18/09
19
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