LCS700-708
VCCH Pin UVLO
HiperLCS Basic Operation
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.
The HiperLCS is designed for half-bridge LLC converters, which
are high-efficiency resonant, variable frequency converters. The
HiperLCS is an LLC controller chip with built-in drivers and
half-bridge MOSFETs.
LLC converters require a fixed 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.
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.
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
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 fixed fractions of fMAX. The internal oscillator
runs the internal counters at fMAX whenever the FEEDBACK pin
internal pull-up is on.
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
first 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 determines 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.
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 first 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
first time, after VCC is powered up.
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.
Remote-Off
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 flowing from the
resistor divider to the DT/BF pin determines fMAX. The ratio of the
resistors selects from 3 discrete, burst threshold frequency ratios,
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 sufficient 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.
which are fixed 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 fixed; 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.
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 rectifier 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
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 VUVLO(+). HiperLCS will turn off when the VCC drops to
the VCC Shutdown Threshold VUVLO(-)
.
5
www.powerint.com
Rev. B 062011
POWERINT [ Power Integrations ]
