6

NR092005

HV9921/HV9922/HV9923

as:

(4)

In order to avoid false triggering of the current sense

comparator, C

following expression:

(5)

where T

V

Estimating Power Loss

Discharging the parasitic capacitance C

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

switching power loss. It can be estimated using the following

equation:

(6)

where Fs is the switching frequency, I

DRAIN current of the HV9921/22/23. The switching loss is

the greatest at the maximum input voltage.

The switching frequency is given by the following:

(7)

where

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

full-wave rectified AC input, the switching power loss can be

estimated as:

(8)

V

The switching power loss associated with turn-off transitions

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 essentially at zero-voltage.

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

calculated as:

(9)

where D = V

I

When the LED driver is powered from the full-wave

rectified AC line input, the exact equation for calculating the

conduction loss is more cumbersome. However, it can be

estimated using the following equation:

(10)

where V

and K

the HV9921/22/23

Fig. 1. Conduction Loss Coefficients K

EMI Filter

As with all off-line converters, selecting an input filter is critical

to obtaining good EMI. A switching side capacitor, albeit of

small value, is necessary in order to ensure low impedance

to the high frequency switching currents of the converter. As

a rule of thumb, this capacitor should be approximately 0.1-

0.2

µF/W of LED output power. A recommended input filter is

shown in Figure 2 for the following design example.

Design Example 1

Let us design an HV9921 LED lamp driver meeting the

following specifications:

Input:

Universal AC, 85-264VAC

Output Current: 20mA

Load:

String of 10 LED (LW541C by OSRAM

V

Step 1. Calculating L1.

The output voltage V

(1) assuming a 30% peak-to-peak ripple.

()

()

()

SAT

BLANK MIN

rr

P

IN MAX

IT

t

C

V

⋅−

<

IN

P

SPIKE

rr

SAT

VC

Tt

I

⋅

=+

2

IN

P

SWITCH

IN SAT

rr

S

VC

PV I

t

F

2

⎛⎞

=+

⋅

⋅

⎜⎟

⎝⎠

1

IN

O

S

IN

OFF

VV

F

VT

−⋅

=

⋅

()

()

1

SWITCH

AC

P

SAT

rr

AC

O

OFF

1

PV

C

2 I

t

V

V

2T

≈⋅

+ ⋅

⋅

−

⋅

⋅

()

2

COND

O

ON

DD

IN

PD I

R

I

V

1 D

=⋅

⋅

+

⋅

⋅ −

2

COND

C

O

ON

d

DD

AC

PK

I

R

K

I

V

=⋅

⋅

+

⋅

⋅

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Kd Dm

(

)

Kc Dm

()

Dm

.

.

41V 10 5 s

L1

72mH

03 20mA

µ

⋅

==

⋅