HV9921 datasheet(7/19 Pages) MICROCHIP

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 2014 Microchip Technology Inc.

DS20005311A-page 7

HV9921/HV9922/HV9923

Coil capacitance of inductors is typically provided in the

manufacturer’s data books either directly or in terms of

the self-resonant frequency (SRF).

where L is the inductance value, and CL is the coil

capacitance.) Charging and discharging this capaci-

tance every switching cycle causes high-current spikes

in the LED string. Therefore, connecting a small capac-

itor CO (~10nF) is recommended to bypass these

spikes.

Using an ultra-fast rectifier diode for D1 is recom-

mended 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 HV9921/22/23 can be calculated as:

When the switching MOSFET turns on, the capaci-

tance CP is discharged into the DRAIN pin of the IC.

The discharge current is limited to about 150mA typi-

cally. However, it may become lower at increased junc-

tion temperature. The duration of the leading edge

current spike can be estimated as:

In order to avoid false triggering of the current sense

comparator, CP must be minimized in accordance with

the following expression:

where TBLANK(MIN) is the minimum blanking time of

200ns, and VIN(MAX) is the maximum instantaneous

input voltage.

4.2

Estimating Power Loss

Discharging the parasitic capacitance CP into the

DRAIN pin of the HV9921/22/23 is responsible for the

bulk of the switching power loss. It can be estimated

using the following equation:

where FS is the switching frequency, ISAT is the satu-

rated DRAIN current of the HV9921/22/23. The switch-

ing loss is the greatest at the maximum input voltage.

The switching frequency is given by the following equa-

tion.

When the HV9921/22/23 LED driver is powered from

the full-wave rectified AC input, the switching power

loss can be estimated as:

VAC is the input AC line voltage.

The switching power loss associated with turn-off tran-

sitions of the DRAIN pin can be disregarded. Due to the

large amount of parasitic capacitance connected to this

switching node, the turn-off transition occurs essen-

tially at zero-voltage.

Conduction power loss in the HV9921/22/23 can be

calculated as:

where D = VO/VIN is the duty ratio, RON is the on-resis-

tance, IDD is the internal linear regulator current.

When the LED driver is powered from the full-wave rec-

tified AC line input, the exact equation for calculating

the conduction loss is more cumbersome. However, it

can be estimated using the following equation:

where VAC is the input AC line voltage. The coefficients

KC and Kd can be determined from the minimum duty

ratio of the HV9921/22/23.

SRF



LCL











CP CDRAIN CPCB CL CJ

TSPIKE

VIN CP



ISAT

-------------------- trr

ISAT TBLANK MIN

 trr

–





VIN MAX



---------------------------------------------------------



PSWITCH

VIN

------------------ VIN I



SAT

trr





 F



VIN VO

–

VIN TOFF



-------------------------

PSWITCH

2TOFF



-------------------- VAC CP 2ISAT trr



 V

–





PCOND DIO

RON IDD VIN 1D

–





PCOND KC IO

RON Kd IDD VAC



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