Current Sense Pin; Voltage Supply Pin - Power integrations LinkSwitch-4 LNK4*15D Series Application Note

AN-69
The voltage control loop aims to maintain the sampled value at
V , about 1.98 V, see data sheet.
FB(REG)
R =
FB 2
V
OUT(CV)
Small changes can be made to R
If no signal is present on the FEEDBACK pin i.e. open circuit R
short-circuit R or open circuit tracking, switching will cease and the
FB2
circuit will continuously run the start-up cycle (hiccup) due to the lack
of signal.
Quasi-Resonant Switching
The primary switch is only turned on when the voltage across it rings
down to a minimum (voltage-valley, quasi-resonant switching). This
reduces EMI and switch turn-on loss. Again, a slope detector is used
to sense the idle ring valleys in the FEEDBACK pin signal. If the idle
ring has decayed and is undetectable, the next switching cycle will
start without waiting for a valley detection.
Cable Compensation
Cable compensation is fixed per variant. The controller alters the
voltage control signal produced during the comparison of V
V , thus making the necessary adjustment to compensate for the
FB(REG)
output voltage drop across the cable. The set value of cable
compensation is accurate at the top right of the VI output curve i.e.
the CC current setting, not necessarily the label current rating, which
is usually lower.

VOLTAGE SUPPLY Pin

The VCC pin supplies power to the internal circuitry. There is an ultra
low power series regulator supplying logic and analogue circuitry.
Constant current sources that drive the BJT are sourced directly from
the VCC pin. Being an ultra low power regulator results in it being
not very fast, so it is important that the VCC pin is very closely
decoupled to the GROUND pin via C
The VCC pin has several thresholds that prompt specific actions form
the controller.
1. V ; If V is below V
VCC(SLEEP) CC VCC(SLEEP)
and enter sleep mode and the current drawn through the
VOLTAGE SUPPLY pin falls to a few mA (I
2. V ; If in sleep mode and V
VCC(RUN)
initializes and the VOLTAGE SUPPLY pin current increases to
I and a probe pulse is issued (see Start-up sequence
VCC(RUN)
section and Step 15, First time start- up).
3. V ; If the controller is in Run mode and the V
VCC(LOW)
to V , an extra switching pulse is issued immediately as long
VCC(LOW)
as it does not overlap a pulse already issued. This function is
useful when there is a high load to no-load transient. The output
voltage will overshoot a little and will cause a frequency lower
than the design no-load frequency to be set. The period of this
frequency may be so long that the V
would during a stable no-load situation. To prevent the V
voltage falling to V and causing a shutdown and power
VCC(SLEEP)
cycle sequence, an extra no-load type pulse is issued which adds
charge to C and prevents a shutdown. The side effect is that it
VCC
will take a little longer to recover from the output voltage over-
shoot, but at least the switching operation is continuous and a
power cycle glitch is avoided.
4. ∆V ; During no-load operation, this is the difference
VCC(PFM)
between the peak and minimum V
and discharging of C during a switching cycle. During no-load
VCC
operation, at least a 50 mV rise must be detected in the C
voltage to proceed to the next stage of the switching cycle,
www.power.com
R # V
( )
FB 1 FB REG
N
B - V
#
N ( )
FB REG
S
to fine tune the output voltage.
FB1
FB1
and
FB
.
VCC
the controller will stop switching
).
VCC(SLEEP)
rises to V , the controller
CC VCC(RUN)
voltage falls
CC
voltage falls lower than it
CC
CC
voltage due to the charging
CC
VCC
otherwise switching will cease, V
sequence will occur. If V
is issued as if V
CC
V .
VCC(LOW)

CURRENT SENSE Pin

The CURRENT SENSE pin serves two functions;
1. During Initialize mode, during start-up, the value of R
or by sourcing a constant current from the CURRENT SENSE pin and
measuring the resulting voltage with a 2 bit ADC. This results in
one of four possible values being detected, which sets the chosen
level of V in an internal register. See data sheet for available
CS(MIN)
values of resistor and corresponding levels of V
LNK40X2S parts only have one fixed level of V
perform this routine. However, R
value of 1 kW, as an ESD protective measure.
2. The CURRENT SENSE pin measures the –V
resistor R , which is due to the volt drop across R
CS
the primary current.
This voltage is scaled internally and compared to the control
voltage and the over-current protection threshold (V
determine when the switching BJT should be turned off. Note
that if the over-current sense threshold is reached, there is a
small delay (t
CS(OFF)
propagation delay. This results in the observed OCP level being
higher than the expected value and it is dependant upon the HT
voltage at that time. Conversely, when the voltage across R
governed by the control voltage, the observed R
exactly as expected because the delays are compensated by the
control feedback loop. In normal regulation the maximum value
of voltage across R
power limit during transients or running at low HT voltage. Note
that the transient power limit will usually be higher than the V
× I steady state circuit power limit.
CC
There is a CURRENT SENSE pin leading edge blanking time of
t , so charging of stray capacitance does not cause a false
CS(B)
triggering of BJT turn off.
If R resistor is open circuit then the primary current will flow
CS
through the internal ESD protection diode between the GROUND pin
and the CURRENT SENSE pin, the voltage on the CURRENT SENSE
pin will very quickly reach the OCP or control voltage governed levels
and the switching BJT will be turned off. This will result in very little
energy in the primary, so at no-load, the output voltage may be
correct, but when a load is applied, the output voltage will fall.
If the R resistor is short-circuited or the connection to the CURRENT
CS
SENSE pin is open circuit at start-up, the controller will not start. It
will issue the probe pulse and immediately go to SLEEP mode. It will
then attempt to re-start. It will hiccup. If there is no-load on the
output, the output capacitor will charge to a level dependent on the
value of bleed (dummy load) resistor and the hiccup rate. Because
the controller doesn't wake up properly, the output overvoltage
protection will not be active.
The controller is able to detect if R
circuit during normal running. This is done by detecting switching
BJT desaturation on the FEEDBACK pin signal. If the condition
persists for a predetermined period, then the controller goes to
SLEEP mode. If the output voltage reaches the output overvoltage
threshold before the predetermined period expires, then the
controller goes to SLEEP mode.
It is difficult to predict the output voltage with R
be measured at no-load and maximum input voltage.
Application Note
will fall and a power cycle
CC
falls by 1.6 V, an extra switching pulse
CC
had fallen to V , even if V
VCC(LOW) CC
CS(MIN)
CS(MIN)
should still be present with a
CS2
voltage across the CS
E
) before the BJT is turner off due to internal
CS
is limited to V . This acts as a maximum
CS CS(MAX)
is shorted or connection open
CS
shorted so it must
CS
is higher than
is measured
CS2
.
and does not
caused by
CS
) to
CS(OCP)
is
CS
voltage is
OUT
29
Rev. B 10/17