ALTAIR04-900TR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ALTAIR04-900TR' PDF : 29 Pages View PDF
Application information
ALTAIR04-900
Figure 16. Feed-forward compensation: internal schematic
DRAIN
R zcd ZCD/FB
Rfb
Aux
F eedf orward .
Logic
CC
I FF Block
P WM
CC
LOGI C
Rff
SOURCE
Rsens e
RZCD resistor can be calculated as follows:
Equation 4
5.7
18/29
RZCD
=
NAUX
NP RI
⋅
LP⋅ RFF
Td⋅ RSENSE
The peak drain current does not depend on the input voltage.
Concerning RZCD value: during the MOSFET on-time, the current, sourced from ZCD/FB
pin, IZCD, is compared with an internal reference current IZCDON ( - 50 µA typical).
If IZCD < IZCDON, the brownout function is active and IC shuts down.
This feature is important when the auxiliary winding is accidentally disconnected and
considerably increases the end-product safety and reliability.
Burst-mode operation (no load or very light load)
When the voltage on COMP pin falls 65 mV below a fixed threshold, VCOMPBM, the IC is
disabled, the MOSFET is in off-state and its consumption reduced to a lower value to
minimize Vcc capacitor discharge.
Due to this condition, the converter operates in burst-mode (one pulse train every TSTART =
500 µs), with a minimum energy transfer.
Therefore, the output voltage decreases: after 500 µs the controller switches on the
MOSFET again and the sampled voltage on the ZCD pin is compared with the internal
reference. If the voltage on the EA output, as a result of the comparison, exceeds the
VCOMPL threshold, the device restarts switching, otherwise it is off for another period of 500
µs.
The converter works in burst-mode with a nearly constant peak current. A load decrease
causes a frequency reduction, which can go down even to few hundreds hertz, thus
minimizing all frequency-related losses and meeting energy saving regulations. This kind of
operation, shown in the timing diagrams (see Figure 17) along with the others previously
described, is noise-free since the peak current is low.
DocID18211 Rev 3
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