Rev. G 05/15
5
LCS700-708
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HiperLCS Basic Operation
The HiperLCS is designed for half-bridge LLC converters, which are
high-efciency resonant, variable frequency converters. The HiperLCS
is an LLC controller chip with built-in drivers and half-bridge MOSFETs.
LLC converters require a xed dead-time between switching
half-cycles. The dead-time, maximum frequency at start-up, and
burst threshold frequencies, are programmed with a resistor divider on
the DT/BF pin from the VREF to the GROUND pins.
The FEEDBACK (FB) pin is the frequency control input for the
feedback loop. Frequency is proportional to FEEDBACK pin current.
The FEEDBACK pin V-I characteristic resembles a diode to ground.
Burst Mode
If the frequency commanded by the FEEDBACK pin current exceeds
the upper burst threshold frequency (fSTOP, ISTOP) programmed by the
resistor divider on the DT/BF pin, the output MOSFETs will turn off,
and will resume switching when the current drops below the value
which corresponds to the frequency equal to the lower burst
threshold frequency (fSTART, ISTART). As a rst approximation, burst
mode control resembles a hysteretic controller where the frequency
ramps from fSTART to fSTOP, stops and repeats. An external component
network connected from the VREF pin to the FEEDBACK pin deter-
mines the minimum and start-up FEEDBACK pin currents, and thus
the minimum and start-up switching frequencies. A soft-start
capacitor in this network determines soft-start timing.
The VREF pin provides a nominal 3.4 V as a reference for this
FEEDBACK pin external network and other functions. Maximum
current from this pin must be ≤4 mA.
The Dead-Time/Burst Frequency (DT/BF) pin also has a diode-to-
ground V-I characteristic. A resistor divider from VREF to GROUND
programs dead-time, maximum start-up switching frequency (fMAX),
and the burst threshold frequencies. The current owing from the
resistor divider to the DT/BF pin determines fMAX. The ratio of the
resistors selects from 3 discrete, burst threshold frequency ratios,
which are xed fractions of fMAX.
The OV/UV pin senses the high-voltage B+ input through a
resistor divider. It implements brown-in, brown-out, and OV with
hysteresis. The ratios of these voltages are xed; the user must
select the resistor divider ratio such that the brown-in voltage is
below the minimum nominal bulk (input) voltage regulation set-point
to ensure start-up, and the OV (lower) restart voltage is above the
maximum nominal bulk voltage set-point, to ensure that the LCS will
restart after a voltage swell event that triggers the OV upper
threshold. If different brown-in to brown-out to OV ratios are
required, external circuitry needs to be added to the resistor divider.
VCC Pin UVLO
The VCC pin has an internal UVLO function with hysteresis. The
HiperLCS will not start until the voltage exceeds the VCC start
threshold VU VL O (+). HiperLCS will turn off when the VCC drops to the
VCC Shutdown Threshold VU VLO (-).
VCCH Pin UVLO
The VCCH pin is the supply pin for the high-side driver. It also
has a UVLO function similar to the VCC pin, with a threshold
lower than the VCC pin. This is to allow for a VCCH voltage that is
slightly lower than VCC because the VCCH pin is fed by a bootstrap
diode and series current-limiting resistor from the
VCC supply.
Start-Up and Auto-Restart
Before start-up the FEEDBACK pin is internally pulled up to the VREF
pin to discharge the soft-start capacitor and to keep the output
MOSFETs off. When start-up commences the internal pull-up
transistor turns off, the soft-start capacitor charges, the outputs
begin switching at fMAX, the FEEDBACK pin current diminishes, the
switching frequency drops, and the PSU output rises. When the
output reaches the voltage set-point, the optocoupler will conduct,
closing the loop and regulating the output.
Whenever the VCC pin is powered up, the DT/BF pin goes into high
impedance mode for 500 ms in order to sense the voltage divider ratio
and select the Burst Threshold. This setting is stored until the next
VCC recycle. The DT/BF pin then goes into normal mode, resembling
a diode to ground, and the sensed current continuously sets the fMAX
frequency. The burst threshold frequencies are xed fractions of fMAX.
The internal oscillator runs the internal counters at fMAX whenever the
FEEDBACK pin internal pull-up is on.
When a fault is detected on the IS, OV/UV, or VCC pin (UVLO), the
internal FEEDBACK pin pull-up transistor turns on for 131,072 clock
cycles, to discharge the soft-start capacitor completely, then a restart
is attempted. The rst power-up after a VCC recycle only waits 1024
cycles, including the condition where the OV/UV pin rises above the
brown-in voltage for the rst time, after VCC is powered up.
Remote-Off
Remote-off can be invoked by pulling down the OV/UV pin to ground,
or by pulling up the IS pin to >0.9 V. Both will invoke a 131,072 cycle
restart cycle. VCC can also be pulled down to shut the device off, but
when it is pulled up, the FEEDBACK pin is pulled up to the VREF pin to
discharge the soft-start capacitor for only 1024 fMAX clock cycles. If
this scheme is used, the designer must ensure that the time the VCC
is pulled down, plus 1024 cycles, is sufcient to discharge the
soft-start capacitor, or if not, that the resulting lower starting
frequency is high enough so as not to cause excessive primary
currents that may cause the over-current protection to trip.
Current Sense
The IS pin senses the primary current. It resembles a reverse diode
to the GROUND pin. It is tolerant of negative voltages provided the
negative current is limited to <5 mA. Therefore it must be connected
to the current sense resistor (or primary capacitive voltage divider +
sense resistor) via a series current limiting resistor of >220 W. Thus
it can accept an AC waveform and does not need a rectier or peak
detector circuit. If the IS pin senses a nominal positive peak voltage
of 0.5 V for 7 consecutive cycles, an auto-restart will be invoked. The
IS pin also has a second, higher threshold at nominally 0.9 V, which
will invoke an auto-restart with a single pulse. The minimum pulse
width requirement for detection of both voltage thresholds is
nominally 30 ns. i.e. the thresholds have to be exceeded for >30 ns
for proper detection.
Over-Temperature Shutdown
The HiperLCS has latching OTP. VCCH must be cycled to resume
operation once the unit drops down below the OTP threshold.