ALTAIR04-900TR Ver la hoja de datos (PDF) - STMicroelectronics
Número de pieza
componentes Descripción
Fabricante
ALTAIR04-900TR Datasheet PDF : 29 Pages
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ALTAIR04-900
5
Application information
Application information
The device is an all-primary-sensing switching regulator, based on quasi-resonant flyback
topology.
According to the load conditions of the converter, the device can work in different modes
(see Figure 9):
1. QR-mode at heavy load. Quasi-resonant operation synchronizes MOSFET turn-on and
the demagnetization of the transformer by detecting the resulting negative-going edge
of the voltage across any winding of the transformer. The system works close to the
boundary between discontinuous (DCM) and continuous conduction (CCM) of the
transformer. Therefore, the switching frequency is different according to different
line/load conditions (see the hyperbolic-like portion of the curves in Figure 9). Minimum
turn-on losses, low EMI emissions and safe behavior in short-circuit are the main
benefits of this operation.
2. Valley-skipping-mode at light-to-medium load. According to voltage on COMP pin, the
device defines the maximum operating frequency of the converter. As the load is
reduced, MOSFET turn-on doesn’t occur on the first valley but on the second one, the
third one and so on. In this manner, the switching frequency doesn’t rise (piecewise
linear portion in Figure 9).
3. Burst-mode with or without very light load. When the load is extremely light or
disconnected, the converter enters a controlled on/off operation with a constant peak
current. Decreasing the load results even few hundred hertz minimizes all frequency-
related losses and makes it easier to comply with energy saving regulations or
recommendations. Being the peak current very low, no issue of audible noise arises.
Figure 9. Multi-mode operation of the ALTAIR04-900
fosc
Input voltage
fsw
0
Val ley- skip ping
mode
Burst-mode
Quasi-resonant mode
Pi n
Pinmax
DocID18211 Rev 3
11/29
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