IR3863MPBF
COMPONENT SELECTION
The output voltage drop, V DROP , initially depends
on the characteristic of the output capacitor.
V DROP is the sum of the equivalent series
inductance (ESL) of the output capacitor times
the rate of change of the output current and the
ESR times the change of the output current.
Boot Capacitor Selection
The boot capacitor starts the cycle fully charged
to a voltage of VB(0). Cg equals 0.6nF in
IR3863. Choose a sufficiently small ΔV such that
VB(0)- ΔV exceeds the maximum gate threshold
voltage to turn on the upper MOSFET.
C BOOT ? C g ? ? B
? 1 ? (9)
VESR is usually much greater than VESL. The
IR3863 requires a total ESR such that the ripple
? V (0)
? ΔV
?
?
voltage at the FB pin is greater than 7mV. The
second purpose of the output capacitor is to
minimize the overshoot of the output voltage
when the load decreases as shown in Figure
26. By using the law of energy before and after
the load removal, equation 8 shows the output
capacitance requirement for a load step down.
Choose a boot capacitor value larger than the
calculated C BOOT in equation 9. Equation 9 is
based on charge balance at CCM operation.
Usually the boot capacitor will be discharged to a
much lower voltage when the circuit is operating
in DCM mode at light load, due to much longer
lower MOSFET off time and the bias current
C OUT ?
V OUT
V L
V DROP
V ESR
I OUT
L ? I STEP 2
V OS 2 ? V OUT 2
I STEP
(8)
V OS
drawn by the IC. Boot capacitance needs to be
increased if insufficient turn-on of the upper
MOSFET is observed at light load, typically
larger than 0.1μF is needed. The voltage rating
of this part needs to be larger than VB(0) plus
the desired derating voltage. Its ESR and ESL
needs to be low in order to allow it to deliver the
large current and di/dt’s which drive MOSFETs
most efficiently. In support of these requirements
a ceramic capacitor should be chosen.
Figure 26. Typical Output Voltage Response
Waveform
8/8/2012 Rev3.2
15
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