NCL30059
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10
The internal timing capacitor Ct is charged by current
which is proportional to the current flowing out from the
Rt pin. The discharging current I
DT
is applied when voltage
on this capacitor reaches 2.5 V. The output drivers are
disabled during discharge period so the dead time length is
given by the discharge current sink capability. Discharge
sink is disabled when voltage on the timing capacitor
reaches zero and charging cycle starts again. The charging
current and thus also whole oscillator is disabled during the
PFC delay period to keep the IC consumption below 400 mA.
This is valuable for applications that are supplied from
auxiliary winding and V
CC
capacitor is supposed to provide
energy during PFC delay period.
For resonant LED driver applications it is necessary to
adjust minimum operating frequency with high accuracy.
The designer also needs to limit maximum operating and
startup frequency. All these parameters can be adjusted
using few external components connected to the Rt pin as
depicted in Figure 22.
Figure 22. Typical Rt Pin Connection
R
fmax
R
t
V
CC
R
t
R
fstart
R
bias
R
fmax−CC
R
comp C
comp
C
SS
D1
TLV431 (to primary
current sensor)
(to secondary
voltage regulator)
NCL30059
Voltage Feedback Current Feedback
The minimum switching frequency is given by the Rt
resistor value. This frequency is reached if there is no
optocoupler or current feedback action and soft start period
has been already finished. The maximum switching
frequency excursion is limited by the Rf
max
selection. Note
that the F
max
value is influenced by the optocoupler
saturation voltage value. Resistor Rf
start
together with
capacitor C
SS
prepares the soft start period after PFC timer
elapses. The Rt pin is grounded via an internal switch during
the PFC delay period to assure that the soft start capacitor
will be fully discharged via Rfstart resistor.
Constant LED current is achieved using a feedback loop
monitoring the primary current. The sensing voltage must be
scaled by the turns ratio of the transformer. The Rt pin
reference voltage is Vref
Rt
= 3.5 V. The control regulator
operates on the difference between the Rt pin reference
voltage and the minimum voltage compliance of the
regulator. This voltage difference is applied across
R
fmax−CC
.
The TLV431 shunt regulator is used in Figure 22 as the
constant current control regulator. Diode D1 is used to
establish minimum regulator bias current via resistor R
bias
.
Total saturation voltage of this solution is 1.25 + 0.6 = 1.85
V for room temperature. Shottky diode will further decrease
saturation voltage. The Rf
max−CC
resistor limits the
maximum frequency delivered by this regulation loop. This
parameter is affected by D1 temperature drift.
Brown−Out Protection
The Brown−Out circuitry (BO) offers a way to protect the
application from low DC input voltages. Operation is
blocked below a set threshold. Hysteresis is provided by the
switched current source providing stable operation. The
internal circuitry, depicted by Figure 23, offers a way to
monitor the high−voltage (HV) rail.
