and reverse recovery of the rectifier diode D, capacitance of the printed circuit board traces CPCB and
output capacitance CDRAIN of the controller itself. These parasitic elements affect the efficiency of the
switching converter and could potentially cause false triggering of the current sense comparator if not
properly managed. Minimizing these parasitics is essential for efficient and reliable operation of the
IZ9921M/22M/23M. Coil capacitance of inductors is typically provided in the manufacturer’s data books
either directly or in terms of the self-resonant frequency (SRF).
1
SRF
(3)
2 L * CL
where L is the inductance value, and CL is the coil capacitance.
Charging and discharging this capacitance every switching cycle causes high-current spikes in
the LED string. Therefore, connecting a small capacitor CO (~10 nF) is recommended to bypass these
spikes. Using an ultra-fast rectifier diode for D is recommended to achieve high efficiency and reduce
the risk of false triggering of the current sense comparator. Using diodes with shorter reverse recovery
time tRR and lower junction capacitance CJ achieves better performance. The reverse voltage rating VR
of the diode must be greater than the maximum input voltage of the LED lamp. The total parasitic
capacitance present at the DRAIN pin of the IZ9921M/22M/23M can be calculated as:
CP CDRAIN CPCB CL CJ
(4)
where and CDRAIN is the DRAIN capacitance (CDRAIN < 5 pF), and CPSB is the printed-circuit board
capacitance.
When the switching MOSFET turns on, the capacitance CP is discharged into the DRAIN pin of
the IC. The discharge current is limited to about 150 mA typically. However, it may become lower at
increased junction temperature. The duration of the leading edge current spike can be estimated as:
V *CP
IN
TSPIKE tRR
(5)
ISAT
In order to avoid false triggering of the current sense comparator, CP must be minimized in
accordance with the following expression:
ISAT * (TBLANK_MIN tRR )
(6)
CP
VIN_MAX
where TBLANK_MIN is the minimum blanking time of 200 ns, and VIN
instantaneous input voltage.
is the maximum
_MAX
Estimating Power Loss
Discharging the parasitic capacitance CP into the DRAIN pin of the IZ9921M/22M/23M is
responsible for the bulk of the switching power loss. It can be estimated using the following equation:
Ver. 1.0
Dec. 2011
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