L6566BH View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566BH' PDF : 51 Pages View PDF
L6566BH
Application information
If FF operation is selected:
1. FF mode from heavy to light load. The system operates exactly like a standard current
mode control, at a frequency fsw determined by the externally programmable oscillator:
both DCM and CCM transformer operations are possible, depending on whether the
power that it processes is greater or less than:
Equation 1
where Vin is the input voltage to the converter, VR the reflected voltage (i.e. the
regulated output voltage times the primary-to-secondary turn ratio) and Lp the
inductance of the primary winding. PinT is the power level that marks the transition from
continuous to discontinuous operation mode of the transformer.
2. Burst-mode with no or very light load. This kind of operation is activated in the same
way and results in the same behavior as previously described for QR operation.
The L6566BH is specifically designed for applications with no PFC front-end; pin 6 (FMOD)
features an auxiliary oscillator that can modulate the switching frequency (when FF
operation is selected) in order to mitigate EMI emissions by a spread-spectrum action.
5.1
High voltage startup generator
Figure 5 shows the internal schematic of the high voltage startup generator (HV generator).
It is made up of a high voltage N-channel FET, whose gate is biased by a 15 MΩ resistor,
with a temperature-compensated current generator connected to its source.
Figure 5. High voltage startup generator: internal schematic
HV
L6566BH
15 MW
1
V c c _OK
HV_EN
IHV
CONTROL
3
GND
5 Vcc
Icharge
AM11481v1
With reference to the timing diagram of Figure 6, when power is first applied to the converter
the voltage on the bulk capacitor (Vin) builds up and, at about 80 V, the HV generator is
enabled to operate (HV_EN is pulled high) so that it draws about 1 mA. This current, minus
the device consumption, charges the bypass capacitor connected from the Vcc pin (5) to
ground and makes its voltage rise almost linearly.
Doc ID 16610 Rev 2
17/51
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