lnk520 Power Integrations, Inc., lnk520 Datasheet - Page 3

• Manufacturer Part Number: lnk520
• Description: Energy Ef?cient, Cv Or Cv/cc Switcher For Very Low Cost Adapters And Chargers
• Manufacturer: Power Integrations, Inc.

LinkSwitch Functional Description
The duty cycle, current limit and operating frequency
relationships with CONTROL pin current are shown in
Figure 4. Figure 5 shows a typical power supply schematic outline
which is used below to describe the LinkSwitch operation.
Power Up
During power up, as V
pin capacitor C1 is charged through a switched high voltage
current source connected internally between the DRAIN and
CONTROL pins (see Figure 2). When the CONTROL pin voltage
reaches approximately 5.6 V relative to the SOURCE pin, the
high voltage current source is turned off, the internal control
circuitry is activated and the high voltage internal MOSFET
starts to switch. At this point, the charge stored on C1 is used
to supply the internal consumption of the chip.
Constant Current (CC) Operation
As the output voltage, and therefore the reflected voltage across
the transformer bias winding ramp up, the feedback CONTROL
current I
the internal current limit increases with I
I
is designed to provide an approximately constant power supply
output current as the power supply output voltage rises.
Constant Voltage (CV) Operation
When I
duty cycle is reduced. At a value of I
supply input voltage, the duty cycle control limits LinkSwitch
peak current below the internal current limit value. At this point
the power supply transitions from CC to CV operation. With
minimum input voltage in a typical universal input design, this
transition occurs at approximately 30% duty cycle. Resistor R1
(Figure 5) is therefore initially selected to conduct a value of I
approximately equal to I
at the minimum power supply input voltage. The final choice
of R1 is made when the rest of the circuit design is complete.
When the duty cycle drops below approximately 4%, the
frequency is reduced, which reduces energy consumption under
light load conditions.
Auto-Restart Operation
When a fault condition, such as an output short circuit or open
loop, prevents flow of an external current into the CONTROL
pin, the capacitor C1 discharges towards 4.7 V. At 4.7 V, auto-
restart is activated, which turns the MOSFET off and puts the
control circuitry in a low current fault protection mode. In
auto-restart, LinkSwitch periodically restarts the power supply
so that normal power supply operation can be restored when
the fault is removed.
C
is equal to I
C
C
exceeds I
flowing through R1 increases. As shown in Figure 4,
DCT
. The internal current limit vs. I
DCS
, typically 2 mA (Figure 4), the maximum
IN
is first applied (Figure 5), the CONTROL
DCT
when V
OUT
C
that depends on power
C
is at the desired value
and reaches I
C
characteristic
LIM
when
C
Figure 4. CONTROL Characteristics.
Figure 5. Power Supply Schematic outline.
V
f
I
LIM
OSC(low)
IN
3.8%
Duty Cycle
Frequency
Internal Current Limit
77%
30%
f
OSC
Auto-restart
Auto-restart
Auto-restart
CONTROL Current I
CONTROL Current I
CONTROL Current I
I
CD1
LinkSwitch
R3
C
C
C
D
S
C
C3
D3
I
DCS
LNK520
I
C
C1
R1
PI-3579-031004
PI-3578-021405
D2
D1
2/05
C4
I
E
DCT
R2
C2
V
3
OUT