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LNK501 Datasheet(PDF) 3 Page - Power Integrations, Inc.

Part No. LNK501
Description  Energy Efficient, CV/CC Switcher for Very Low Cost Chargers and Adapters
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Maker  POWERINT [Power Integrations, Inc.]
Homepage  http://www.powerint.com
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LNK501 Datasheet(HTML) 3 Page - Power Integrations, Inc.

 
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3
F
9/02
LNK501
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 outline
schematic which is used below to describe the LinkSwitch
operation.
Power Up
During power up, as V
IN is first applied (Figure 5), the
CONTROL 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 5.7 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 primary transformer winding ramp up, the feedback
CONTROL current I
C increases. As shown in Figure 4, the
internal current limit increases with I
C and reaches ILIM when IC
is equal to I
DCT. 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.
Constant Voltage (CV) Operation
When I
C exceeds IDCS, typically 2 mA (Figure 4), the maximum
duty cycle is reduced. At a value of I
C 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. R1
(Figure 5) is therefore initially selected to conduct a value of I
C
approximately equal to I
DCT 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
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 standby mode. In auto-restart,
LinkSwitch periodically restarts the power supply so that normal
power supply operation can be restored when the fault is
removed.
Figure 4. CONTROL Characteristics.
Figure 5. Power Supply Outline Schematic.
VOUT
VIN
D2
C4
D
S
C
C2
R1
R2
C1
D1
LinkSwitch
PI-2715-100801
PI-2799-112101
Internal Current Limit
CONTROL Current IC
IDCS
CONTROL Current IC
ICD1
CONTROL Current IC
Duty Cycle
Frequency
ILIM
IDCT
77%
30%
3.8%
f
OSC
f
OSC(low)
Auto-restart
Auto-restart
Auto-restart


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