LNK501GN Datasheet, PDF (3/20 Pages) Power Integrations, Inc. – Energy Efficient, CV/CC Switcher for Very Low Cost Chargers and Adapters

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 5.6 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 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 I_LIM when I_C
is equal to I_DCT. The internal current limit vs. I_C 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 I_DCS, 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. Resistor R1
(Figure 5) is therefore initially selected to conduct a value of I_C
approximately equal to I_DCT when V_OUT 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 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.
Internal Current Limit
Auto-restart
I_LIM
CONTROL Current I_C
I_DCT
Duty Cycle
77%
Auto-restart
30%
3.8%
Frequency
I_CD1
I_DCS
CONTROL Current I_C
Auto-restart
f_OSC
f_OSC(low)
CONTROL Current I_C
Figure 4. CONTROL Characteristics.
LinkSwitch
D
S
C _C1
C2
R1
V_IN
D1
R2
Figure 5. Power Supply Outline Schematic.
PI-2799-112102
C4
V_OUT
D2
PI-2715-112102
I
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