Rev. F 05/18

4

LinkSwitch-4

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Functional Description

Power-Up/Power-Down Sequences

Refer to Figure 10 and Figure 7. When mains input voltage (V

IN) is

applied, current flows through the start-up resistors (R

HT) and BJT.

Some of this current flows into the LinkSwitch-4 internal circuits,

which are in Sleep mode; the remainder charges capacitor C

VCC.

As soon as the VOLTAGE SUPPLY pin voltage rises to V

VCC(RUN), the

LinkSwitch-4 changes to Initialise mode. Current consumption

increases to I

VCC(RUN) while internal circuits are enabled. The emitter

switch is held at low impedance to ground (GND) and a short drive

pulse is output on the BASE DRIVE pin, during which time the voltage

at feedback is held at GND potential by current sourced from the

FEEDBACK pin. This enables the LinkSwitch-4 control circuit to

compare the rectified mains input voltage with thresholds for allowing

or preventing the next stage of power-up. If the input voltage is too

low (I

FB < IFBHT(START)), the LinkSwitch-4 will not issue further drive

pulses, the VCC voltage will discharge to V

VCC(SLEEP), and the power-up

sequence will repeat. If the mains input voltage is high enough (I

FB >

I

FBHT(START)), the LinkSwitch-4 will enter Run mode and drive pulses will

be output on the BASE DRIVE pin. To achieve smooth power-up

(monotonic rise in V

OUT), CVCC must be large enough to power the

control circuitry during Initialize mode and the first few cycles of Run

mode, until sufficient power is provided by the transformer voltage

supply winding.

If the input voltage falls below V

MAINS(LO) (see Input Undervoltage

Protection), V

VCC will fall below VVCC(SLEEP) and the LinkSwitch-4 will go

VVCC(RUN)

VVCC(SLEEP)

Off

Sleep

Initialize

Sleep Off

Run

VVCC

PI-7457-010815

Figure 7.

VCC Waveforms.

Mode

Description

Sleep

From initial application of input power or from Run mode, if V

VCC falls below VVCC(SLEEP), the LinkSwitch-4 goes to Sleep mode.

Non-essential circuits are turned off. Base and Emitter drives are turned off so BASE DRIVE and EMITTER DRIVE pins

become high impedance, allowing the bootstrap resistor (R

HT) and BJT to start the circuit. Sleep mode is exited when VVCC

rises to V

VCC(RUN) and the control circuitry goes to Initialize mode.

Initialize

Internal circuits are enabled and the LinkSwitch-4 issues one switching cycle to sample the input voltage via the FEEDBACK

pin. If V

IN (hence VHT) is high enough, the LinkSwitch-4 changes to Run mode. If VIN is not high enough, no further base drive

pulses are issued and the LinkSwitch-4 returns to Sleep mode when V

VCC falls below VVCC(SLEEP).

Run

Power conversion: The control circuitry is powered from the VCC rail and the internal V

DD is regulated. If VVCC falls below

V

VCC(SLEEP), the IC ceases power conversion and goes to Sleep mode.

Table 2.

Summary of LinkSwitch-4 Operating Modes.

into Sleep mode, reducing its current consumption to I

VCC(SLEEP). The

control circuitry will re-initialize if the input voltage is restored and

V

VCC reaches VVCC(RUN).