LCS700-708
450
400
350
300
500
450
400
350
300
250
200
150
BT1
BT2
BT3
250
200
150
100
50
250
300
350
400
450
500
0
0
200
400
600
800
1000
Dead-Time (ns)
Frequency (kHz)
Figure 18. fSTART (Lower Burst Threshold Frequency) vs. Dead-Time Setting for
Different Burst Threshold Settings (BT1, BT2, BT3).
Figure 16. FEEDBACK Pin and DT/BF Pin Current vs. Frequency.
For example, if BT2 is selected, and fMAX is 800 kHz, then fSTART
= 300 kHz, and fSTOP = 350 kHz. If during normal operation the
load is reduced and the frequency rises to 350 kHz, the switching
will stop. This causes the output voltage to drop and the
feedback loop to decrease the FEEDBACK pin current. When
the current decreases to a value which corresponds to 300 kHz,
switching will commence, and the cycle will repeat. During
start-up mode, however, the outputs can switch at a frequency
between fSTOP and fMAX (250 kHz and 800 kHz in the above
example). Start-up mode is exited once the switching frequency
drops below fSTOP, and the HiperLCS will subsequently enter
burst mode if the feedback loop attempts to produce a
The fSTOP to fSTART ratio is fixed, and dependent on the Burst
Threshold setting (see Table 5).
Burst Threshold
fSTOP /fSTART
Setting
1
2
3
1.14
1.17
1.20
Table 5. Ratio of fSTOP /fSTART vs. Burst Threshold Selection.
As a first approximation, during burst mode, the frequency
ramps from fSTART to fSTOP; then switching stops, and then the cycle
repeats.
switching frequency >fSTOP
.
fMAX is the frequency at which the internal counters run when the
HiperLCS is in the off-state of the auto-restart cycle, or in the
power-up delay before switching.
FEEDBACK Pin
The FEEDBACK pin is the voltage regulation FEEDBACK pin. It
has a nominal Thevenin equivalent circuit of 0.65 V and 2.5 kW.
In normal operation, it sinks current. During the off-period of
auto-restart, and during the clocked delay before start-up, it pulls
up internally to VREF in order to discharge the soft-start capacitor.
The current entering the pin determines switching frequency.
Higher current yields higher frequency and thus reduces LLC
output voltage. In a typical application an optocoupler connected
to the VREF pin pulls up on the FEEDBACK pin, via a resistor
network. The optocoupler is configured to source increasing
FEEDBACK pin current, as the output rises. The resistor network
between the optocoupler, FEEDBACK pin, and VREF pin,
determine the minimum and maximum FEEDBACK pin current
(and thus the minimum and maximum operating frequency), that
the optocoupler can command as it goes from cutoff to saturation.
This network also contains the soft-start timing capacitor, CSTART
(Figure 19).
The minimum recommended dead-time is 275 ns, and thus the
maximum fMAX setting is 1 MHz.
To simplify the selection of RFMAX, see the selection curves in Figure 17.
13.0
BT1
12.0
BT2
BT3
11.0
10.0
9.0
8.0
7.0
6.0
5.0
The minimum frequency as set by this network must be lower
than the frequency required by the powertrain at minimum input
voltage. In Figure 19 this is determined by the sum of RFMIN and
RSTART. The FEEDBACK pin current is determined by these two
resistors when the optocoupler is cut off. CSTART can be ignored
during normal operation. Do not confuse RSTART, which determines
250
300
350
400
450
500
Dead-Time (ns)
Figure 17. RFMAX vs. Dead-Time, for the 3 Different Burst Threshold Settings.
14
Rev. B 062011
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