LNK520
LinkSwitch Functional Description
Internal Current Limit
Auto-restart
The duty cycle, current limit and operating frequency
relationships with CONTROL pin current are shown in
Figure4.Figure5showsatypicalpowersupplyschematicoutline
which is used below to describe the LinkSwitch operation.
ILIM
Power Up
Duringpowerup,asVIN isfirstapplied(Figure5),theCONTROL
pin capacitor C1 is charged through a switched high voltage
current source connected internally between the DRAIN and
CONTROLpins(seeFigure2).WhentheCONTROLpinvoltage
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.
CONTROL Current IC
IDCT
Duty Cycle
Auto-restart
77%
Constant Current (CC) Operation
As the output voltage, and therefore the reflectedvoltage across
thetransformerbiaswindingrampup,thefeedbackCONTROL
current IC flowing through R1 increases.As shown in Figure 4,
theinternalcurrentlimitincreaseswithIC andreachesILIM when
IC is equal to IDCT. The internal current limit vs. IC characteristic
is designed to provide an approximately constant power supply
output current as the power supply output voltage rises.
30%
3.8%
ICD1
IDCS
CONTROL Current IC
Frequency
Constant Voltage (CV) Operation
Auto-restart
When IC exceeds IDCS, typically 2 mA(Figure 4), the maximum
duty cycle is reduced. At a value of IC that depends on power
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 IC
approximately equal to IDCT when VOUT is at the desired value
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
frequencyisreduced,whichreducesenergyconsumptionunder
light load conditions.
fOSC
fOSC(low)
CONTROL Current IC
PI-3579-031004
Figure 4. CONTROL Characteristics.
D1
Auto-Restart Operation
VOUT
R3
C3
C4
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.
D3
D2
VIN
LinkSwitch
D
S
R2
R1
C1
IC
C
C2
PI-3578-021405
Figure 5. Power Supply Schematic outline.
E
2/05
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POWERINT [ Power Integrations ]
