NCL30083BDMR2G ON Semiconductor, NCL30083BDMR2G Datasheet - Page 17

• Manufacturer Part Number: NCL30083BDMR2G
• Description: LED Lighting Drivers Step-Dimm Quasi-Res I-Mode Cntlr
• Manufacturer: ON Semiconductor

Specifications of NCL30083BDMR2G

• Rohs: yes
• Input Voltage: 18 V
• Operating Frequency: 65 kHz
• Maximum Supply Current: 1.2 mA
• Maximum Operating Temperature: + 125 C
• Mounting Style: SMD/SMT
• Package / Case: Micro-8
• Minimum Operating Temperature: - 40 C

Constant Current Control
a flyback converter in discontinuous conduction mode
(DCM). Figure 53 shows the basic circuit of a flyback
converter.
V
and L
first charges C
across L
The output diode current increase is limited by the leakage
inductor. As a consequence, the secondary peak current is
reduced:
The diode current reaches its peak when the leakage inductor
is reset. Thus, in order to accurately regulate the output
current, we need to take into account the leakage inductor
current. This is accomplished by sensing the clamping
network current. Practically, a node of the clamp capacitor
is connected to R
Then, by reading the voltage on the CS pin, we have an
image of the primary current (red curve in Figure 54).
bulk
Figure 54 portrays the primary and secondary current of
During the on−time of the MOSFET, the bulk voltage
When the MOSFET is turned−off, the inductor current
leak
is applied to the magnetizing and leakage inductors L
p
and the current ramps up.
reverses and reaches:
lump
sense
. The output diode is off until the voltage
V
bulk
N
I
Clamping
network
D,pk
sp
instead of the bulk voltage V
V
C
t
clp
out
I
N
L,pk
) V
sp
DRV
f
R
Figure 53. Basic Flyback Converter Schematic
clp
http://onsemi.com
(eq. 1)
(eq. 2)
bulk
L
leak
L
p
p
.
17
R
sense
Transformer
linearly from I
turned off, the drain voltage begins to oscillate because of
the resonating network formed by the inductors (L
and the lump capacitor. This voltage is reflected on the
auxiliary winding wired in flyback mode. Thus, by looking
at the auxiliary winding voltage, we can detect the end of the
conduction time of secondary diode. The constant current
control block picks up the leakage inductor current, the end
of conduction of the output rectifier and controls the drain
current to maintain the output current constant.
We have:
The output current value is set by choosing the sense
resistor:
current is independent of the inductor value. Moreover, the
leakage inductance does not influence the output current
value as the reset time is taken into account by the controller.
When the diode conducts, the secondary current decreases
From Equation 3, the first key point is that the output
.
N
C
sp
.
lump
D,pk
I
R
to zero. When the diode current has
out
sense
+
2N
+
V
sp
2N
REF
R
V
sp
sense
ref
I
out
V
out
p
+L
(eq. 3)
(eq. 4)
leak
)