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IR3863MPBF
COMPONENT SELECTION
Input Capacitor Selection
The main function of the input capacitor bank is
to provide the input ripple current and fast slew
rate current during the load current step up. The
Output Capacitor Selection
Selection of the output capacitor requires
meeting voltage overshoot requirements during
load removal, and meeting steady state output
input capacitor bank must have adequate ripple
ripple
voltage
requirements.
The
output
current carrying capability to handle the total
RMS current. Figure 24 shows a typical input
current. Equation 6 shows the RMS input
current. The RMS input current contains the DC
load current and the inductor ripple current. As
shown in equation 5, the inductor ripple current
is unrelated to the load current. The maximum
RMS input current occurs at the maximum
output current. The maximum power dissipation
in the input capacitor equals the square of the
maximum RMS input current times the input
capacitor’s total ESR.
capacitor is the most expensive converter
component and increases the overall system
cost. The output capacitor decoupling in the
converter typically includes the low frequency
capacitor, such as Specialty Polymer Aluminum,
and mid frequency ceramic capacitors.
The first purpose of output capacitors is to
provide current when the load demand exceeds
the inductor current, as shown in Figure 25.
Equation 7 shows the charge requirement for a
certain load step. The advantage provided by
the IR3863 at a load step is the reduced delay
? ? f 2 ? t ? ? dt
1
I IN_RMS ?
Ts
Ts 0
compared to a fixed frequency control method.
If the load increases right after the PWM signal
goes low, the longest delay will be equal to the
minimum lower gate on-time as shown in the
1 ? ΔI ?
2
? I OUT ? T ON ? Fs ? 1 ? ? ? ? (6)
3 ? I OUT ?
The voltage rating of the input capacitor needs
to be greater than the maximum input voltage
because of high frequency ringing at the phase
node. The typical percentage is 25%.
Electrical Specification table. The IR3863 also
reduces the inductor current slew time, the time
it takes for the inductor current to reach equality
with the output current, by increasing the
switching frequency up to 1/( T ON + Min Off
Time ). This results in reduced recovery time.
Load
Current
Output
I STEP
Charge
Δ t
Inductor
Slew
Rate
t
Figure 25. Charge Requirement during Load Step
Q ? C ? V ? 0.5 ? I STEP ? ? t
(7a)
? ?
V DROP ? 2 ? V IN ? V OUT ? ?
8/8/2012 Rev3.2
C OUT ?
1 ? 1 L ? I STEP 2 ?
? (7b)
14
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