Rev. F 05/18

5

LinkSwitch-4

www.power.com

Figure 8. Typical Waveforms at the Feedback and Primary Current Sense Inputs.

PI-7458-010815

tSAMP

tCSB

VFBREG

VCSTHR

VVCC(RUN)

tFON

0 A

0 V

0 V

ON

OFF

FB

CS

BD

ED

Transformer

Flux

Switching Waveforms

Typical waveforms at the feedback and primary current sense inputs

are shown in Figure 8.

Constant Voltage (CV) Regulation

Constant output voltage regulation is achieved by sensing the voltage

at the feedback input, which is connected to the voltage supply

winding as shown in Figure 10 or to a dedicated feedback winding.

An internal current source prevents the feedback voltage from going

negative. A typical feedback voltage waveform is shown in Figure 8.

The feedback waveform is continuously analyzed and sampled at time

t

SAMP to measure the reflected output voltage. tSAMP is identified by the

slope of the feedback waveform and is coincident with zero flux in the

transformer. The sampled voltage is regulated at V

FB(REG) by the

voltage control loop. The (typical) CV mode output voltage is set by

the ratio of resistors R

FB1 and RFB2 (see Figure 10) and by the

transformer turns ratio, according to the following formula (where

output diode voltage is neglected):

VV

R

R

N

N

1

OUTCVFBREG

FB

FB

F

S

2

1

=+

a

a

^^

k k

hh

Where N

F is the number of turns on the feedback (or voltage supply

if used for feedback) winding and N

S is the number of turns on the

secondary winding. The tolerances of R

FB1 and RFB2 affect output

voltage regulation and mains estimation so should typically be chosen

to be 1% or better.

The current required to clamp the feedback voltage to ground

potential during the on-time of the primary switch depends on the

primary winding voltage (approximately equal to the rectified mains

input voltage), the primary to feedback turns ratio, and resistor R

FB1.

The controller measures feedback source current and so enables R

FB1

to set the input voltage start threshold and the input undervoltage

protection threshold, as described below.

Input Voltage Start Threshold

In Initialise mode, the LinkSwitch-4 issues a single short-duration

drive pulse in order to measure the primary voltage and so the

approximate mains input voltage. If the input voltage is below

V

MAINS(START) then the LinkSwitch-4 will not start. Instead it will pause

while V

VCC discharges below VVCC(SLEEP) then it will begin a new power-up

cycle. If the input voltage exceeds V

MAINS(START), the converter will

power-up. V

MAINS(START) is set by RFB1 using this equation:

VI

R

N

N

2

1

MAINSSTART

FBHT STARTFB

F

P

1

##

#

= -

^^

hh

Input Undervoltage Protection

In Run mode, if the mains voltage falls to V

MAINS(LO), the LinkSwitch-4

will stop issuing drive pulses, V

VCC will reduce to VVCC(SLEEP) and the

LinkSwitch-4 will enter Sleep mode. V

MAINS(LO) is set by RFB1 using this

equation:

VI

R

N

N

2

1

MAINSLOFBHTLOFB

F

P

1

##

#

= -

^^

hh

Constant Current (CC Mode) Regulation

Constant current output (I

OUT(CC)) is achieved by regulating the CS

input to the primary side estimate of the output current scaled by R

CS,

V

CS(CC). The regulated output current, IOUT(CC) is set by the value of the

current sense resistor, R

CS, and the transformer primary to secondary

turns ratio (N

P/NS). The value of RCS is determined using the formula:

R

N

N

ITyp

VTyp

CS

S

P

OUTCC

CS CC

.

a d ^^^

^

k

h

hn

h

h

The tolerance of R

CS affects the accuracy of output the current

regulation so is typically chosen to be 1%. The LinkSwitch-4 can

maintain CC regulation down to much lower levels of V

SHUTDN(MAX)

normally specified for mobile phones chargers (see Figure 11).