L6564TD STMicroelectronics, L6564TD Datasheet - Page 19

• Manufacturer Part Number: L6564TD
• Description: Power Factor Correction ICs 10-Pin Trans Mode PFC Bi 6mA 0.75W
• Manufacturer: STMicroelectronics

Specifications of L6564TD

• Rohs: yes
• Maximum Power Dissipation: 0.75 W
• Maximum Operating Temperature: + 150 C
• Mounting Style: SMD/SMT
• Package / Case: SSOP-10
• Minimum Operating Temperature: - 40 C

L6564T
6.2
6.3
Feedback failure protection (FFP)
The OVP function described above handles “normal” overvoltage conditions, i.e. those
resulting from an abrupt load/line change or occurring at startup. In case the overvoltage is
generated by a feedback disconnection, for instance when the upper resistor of the output
divider (R1) fails to open, the comparator detects the voltage at pin INV. If the voltage is
lower than 1.66 V and the OVP is active, the FFP is triggered, the gate drive activity is
immediately stopped, the device is shut down, its quiescent consumption is reduced below
180 µA and the condition is latched as long as the supply voltage of the IC is above the
UVLO threshold. To restart the system it is necessary to recycle the input power, so that the
Vcc voltage of the L6564T goes below 6 V.
The pin PFC_OK doubles its function as a not-latched IC ‘Disable’: a voltage below 0.23 V
shuts down the IC, reducing its consumption below 2 mA. To restart the IC simply let the
voltage at the pin go above 0.27 V.
Note that these functions offer complete protection against not only feedback loop failures or
erroneous settings, but also against a failure of the protection itself. Either resistor of the
PFC_OK divider failing short or open or a PFC_OK pin floating results in shutting down the
IC and stopping the pre-regulator.
Voltage feedforward
The power stage gain of PFC pre-regulators varies with the square of the RMS input
voltage. So does the crossover frequency FC of the overall open-loop gain because the gain
has a single pole characteristic. This leads to a large trade-off in the design.
For example, setting the gain of the error amplifier to get FC = 20 Hz @ 264 Vac means
having FC 4 Hz @ 88 Vac, resulting in a sluggish control dynamics. Additionally, the slow
control loop causes large transient current flow during rapid line or load changes that are
limited by the dynamics of the multiplier output. This limit is considered when selecting the
sense resistor to let the full load power pass under minimum line voltage conditions, with
some margin. But a fixed current limit allows excessive power input at high line, whereas a
fixed power limit requires the current limit to vary inversely with the line voltage.
Voltage feedforward can compensate for the gain variation with the line voltage and allow
the minimizing of all the above-mentioned issues. It consists in deriving a voltage
proportional to the input RMS voltage, feeding this voltage into a squarer/divider circuit (1/V
corrector) and providing the resulting signal to the multiplier that generates the current
reference for the inner current control loop (see
Doc ID 022671 Rev 1
Figure 28
).
Application information
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