LNK501
LinkSwitch Functional Description
Internal Current Limit
Auto-restart
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.
ILIM
Power Up
During power up, as VIN 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.
CONTROL Current I
C
IDCT
Duty Cycle
Auto-restart
77%
Constant Current (CC) Operation
30%
Astheoutputvoltage,andthereforethereflectedvoltageacross
the primary transformer winding ramp up, the feedback
CONTROL current IC increases. As shown in Figure 4, the
internal current limit increases with IC and reaches ILIM when IC
is equal to IDCT. The internal current limit vs. IC characteristic
isdesignedtoprovideanapproximatelyconstantpowersupply
output current as the power supply output voltage rises.
3.8%
ICD1
IDCS
CONTROL Current I
C
Frequency
Auto-restart
Constant Voltage (CV) Operation
fOSC
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
peakcurrentbelowtheinternalcurrentlimitvalue.Atthispoint
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 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
frequency is reduced, which reduces energy consumption
under light load conditions.
fOSC(low)
CONTROL Current I
C
PI-2799-112101
Figure 4. CONTROL Characteristics.
LinkSwitch
D
S
C
C1
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,
LinkSwitchperiodicallyrestartsthepowersupplysothatnormal
power supply operation can be restored when the fault is
removed.
C2
R1
V
C4
OUT
V
D1
R2
IN
D2
PI-2715-100801
Figure 5. Power Supply Outline Schematic.
F
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