NCL30088
www.onsemi.com
19
As illustrated in Figure 59, the V
S
pin provides the
sinusoidal reference necessary for shaping the input current.
The obtained current reference is further modulated so that
when averaged over a half−line period, it is equal to the
output current reference (V
REFX
). This averaging process is
made by an internal Operational Trans−conductance
Amplifier (OTA) and the capacitor connected to the COMP
pin (C1 of Figure 59). Typical COMP capacitance is 1 mF
and should not be less than 470 nF to ensure stability. The
COMP ripple does not affect the power factor performance
as the circuit digitally eliminates it when generating the
current setpoint.
If the V
S
pin properly conveys the sinusoidal shape, power
factor will be close to unity and the Total Harmonic
Distortion (THD) will be low. In any case, the output current
will be well regulated following the equation below:
I
out
+
V
REFX
2N
PS
R
sense
(eq. 1)
Where:
• N
PS
is the secondary to primary transformer turns
N
PS
= N
S
/N
P
. N
PS
is 1 in the case of non−isolated
buck−boost or SEPIC converter.
• R
sense
is the current sense resistor (see Figure 1).
• V
REFX
is the output current internal reference.
V
REFX
= V
REF
(250 mV in A and B versions and
200 mV in C and D versions, typically) at full load.
The output current reference (V
REFX
) is V
REF
unless the
temperature is high enough to activate the thermal fold−back
(see “protections” section).
If a major fault is detected, the circuit enters the
latched−off or auto−recovery mode and the COMP pin is
grounded (except in an UVLO condition). This ensures a
clean start−up when the circuit resumes operation.
Start−up Sequence
Generally an LED lamp is expected to emit light in < 1 sec
and typically within 300 ms. The start−up phase consists of
the time to charge the V
CC
capacitor, begin switching and
the time to charge the output capacitor until sufficient
current flows into the LED string. To speed−up this phase,
the following defines the start−up sequence:
• The COMP pin is grounded when the circuit is off. The
average COMP voltage needs to exceed the V
S
pin
peak value to have the LED current properly regulated
(whatever the current target is). To speed−up the COMP
capacitance charge and shorten the start−up phase, an
internal 80−mA current source adds to the OTA sourced
current (60 mA max typically) to charge up the COMP
capacitance. The 80−mA current source remains on until
the OTA starts to sink current as a result of the COMP
pin voltage sufficient rise. At that moment, the COMP
pin being near its steady−state value, it is only driven
by the OTA.
• If V
CC
drops below the V
CC(off)
threshold because the
circuit fails to start−up properly on the first attempt, a
new try takes place as soon as V
CC
is recharged to
V
CC(on)
. The COMP voltage is not reset at that
moment. Instead, the new attempt starts with the
COMP level obtained at the end of the previous
operating phase.
• If the load is shorted, the circuit will operate in hiccup
mode with V
CC
oscillating between V
CC(off)
and
V
CC(on)
until the AUX_SCP protection trips
(AUX_SCP is triggered if the ZCD pin voltage does
not exceed 1 V within a 90−ms operation period of time
thus indicating a short to ground of the ZCD pin or an
excessive load preventing the output voltage from
rising). The NCL30088A and NCL30088C latch off in
this case. With the B and D versions, the AUX_SCP
protection forces the 4−s auto−recovery delay to reduce
the operation duty−ratio. Figure 60 illustrates a start−up
sequence with the output shorted to ground, in this
second case.
