HR1001A – ENHANCED LLC CONTROLLER
APPLICATION INFORMATION
Oscillator
The RC network connected externally to FSET
determines the normal switching frequency as
well as the soft start switching frequency.
Figure 2 shows the oscillator block diagram. A
modulated current charges and discharges the
CT capacitor repeatedly between its peak valley
thresholds, which determines the oscillator
frequency.
•
Rfmin from FSET to GND contributes the
maximum resistance of the external RC
network when the phototransistor does not
conduct. This sets the FSET minimum
source current, which defines the minimum
switching frequency.
•
Under normal operation, the phototransistor
adjusts the current flow through Rfmax to
modulate the frequency for output voltage
regulation. When the phototransistor is
saturated, the current through Rfmax is at its
maximum as setting the frequency at its
maximum).
•
An RC in series connected between FSET
and GND shifts the frequency at start-up.
(Please see the “Soft-Start Operation”
section for details.)
Figure 2: Oscillator Block Diagram
FSET sets the CT charging current, Iset (IS-1).
When CT passes its peak threshold (3.8V), the
filp-flop is set and a discharge current source of
twice the charge current is enabled. The
difference between these two currents forces
the charge and discharge of CT to be equal.
When the voltage on the CT capacitor falls
below its valley threshold (0.9 V), the flip-flop is
reset and turns off IS-2. This starts a new
switching cycle. Figure 3 shows the detailed
waveform of the oscillator.
Equation (1) and Equation (2) are used to set
the minimum and maximum frequency:
1
fmin
=
(1)
(2)
3⋅CT ⋅Rfmin
1
fmax
=
3⋅CT ⋅(Rfmin || Rfmax
)
Typically, the CT capacitance is between 0.1nF
and 1nF. Equation (3) and Equation (4)
calculate the values of Rfmin and Rfmax
:
1
Rfmin
=
(3)
(4)
3⋅CT ⋅ fmin
Rfmin
=
Rfmax
fmax
−1
fmin
It is recommended to use a CT capacitor
(<=330pF) for best overall temperature
performance.
Figure 3: CT Waveform and Gate Signal
HR1001A Rev.1.1
3/14/2016
www.MonolithicPower.com
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