HV9922 Supertex, Inc., HV9922 Datasheet - Page 5

• Manufacturer Part Number: HV9922
• Description: 3-pin Switch-mode Led Lamp Driver Ic
• Manufacturer: Supertex, Inc.

P
When the switching MOSFET turns on, the capacitance C
is discharged into the DRAIN pin of the IC. The discharge
current is limited to about 150mA typically. However, it
may become lower at increased junction temperature. The
duration of the leading edge current spike can be estimated
as:
In order to avoid false triggering of the current sense
comparator, C
following expression:
where T
V
Estimating Power Loss
Discharging the parasitic capacitance C
the HV9922 is responsible for the bulk of the switching power
loss. It can be estimated using the following equation:
where F
DRAIN current of the HV9922. The switching loss is the
greatest at the maximum input voltage.
The switching frequency is given by the following:
F
where η is the effi ciency of the power converter.
When the HV9922 LED driver is powered from the full-wave
rectifi ed AC input, the switching power loss can be estimated
as:
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.
C
P
T
SWITCH
AC
IN(MAX)
S
SWITCH
SPIKE
P
= (V
is the input AC line voltage.
<
= ((V
I
IN
is the maximum instantaneous input voltage.
SAT
=
BLANK(MIN)
≈
- η
S
⎛
⎜
⎝
2 T
⋅
V C
is the switching frequency, I
IN
-1
(
IN
⋅
T
• V
• C
V
1
P
2
BLANK MIN
2
OFF
IN MAX
must be minimized in accordance with the
is the minimum blanking time of 200ns, and
O
P
) / V
) / (I
(
P
(
+
(
V
V I
SAT
IN
AC
)
IN SAT
• T
)) +t
)
⋅
−
C
OFF
t
P
r
rr
+ ⋅
⋅
)
t
2 I
rr
⎞
⎟
⎠
⋅
SAT
F
S
P
⋅
into the DRAIN pin of
t
SAT
rr
)
is the saturated
(
V
AC
(4)
(5)
(6)
(7)
(8)
−
η
−
1
⋅
V
O
P
)
5
Conduction power loss in the HV9922 can be calculated as:
P
where D = V
I
When the LED driver is powered from the full-wave
rectifi ed AC line input, the exact equation for calculating the
conduction loss is more cumbersome. However, it can be
estimated using the following equation:
P
where V
and K
the HV9922.
Fig. 1. Conduction Loss Coeffi cients K
EMI Filter
As with all off-line converters, selecting an input fi lter 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 fi lter is
shown in Figure 2 for the following design example.
Design Example
Let us design an HV9922 LED lamp driver meeting the
following specifi cations:
Input:
Output Current: 50mA
Load:
DD
COND
COND
is the internal linear regulator current.
Kd Dm
Kc Dm
= (D • I
= (K
d
(
(
can be determined from the minimum duty ratio of
AC
)
)
C
0.7
0.6
0.5
0.4
0.3
0.2
0.1
is the input AC line voltage. The coeffi cients K
• I
O
O
0
/ηV
O
2
2
• R
• R
Universal AC, 85 - 135VAC
String of 12 LED (Power TOPLED OSRAM
V
IN
F
0.1
ON
= 2.5V max. each)
ON
is the duty ratio, R
) + (I
) + (K
0.2
DD
D
• V
• I
0.3
DD
IN
Dm
• (1 - D))
• V
0.4
AC
ON
)
C
is the on-resistance,
and K
0.5
HV9922
d
0.6
(9)
(10)
0.7
C
®