AN4154

Electrical specifications

Doc ID 023578 Rev 2

5/21

Figure 2.

ALTAIR04-900 pin connection (top view)

Figure 3.

Package (SO-16)

ALTAIR04-900 operation modes

The device is an all-primary sensing switching regulator, based on quasi-resonant flyback

topology. Depending on the converter load condition, the device is able to work in different

modes (see

Figure 4):

1.

QR mode at heavy load. Quasi-resonant operation lies in synchronizing MOSFET turn-

on to the transformer's demagnetization by detecting the resulting negative-going edge

of the voltage across any winding of the transformer. The system then works close to

the boundary between discontinuous (DCM) and continuous conduction (CCM) of the

transformer. As a result, the switching frequency is different for different line/load

conditions (see the hyperbolic-like portion of the curves in

Figure 4). Minimum turn-on

losses, low EMI emission and safe behavior in short-circuit are the main benefits of this

kind of operation.

2.

Valley-skipping mode at medium/ light load. Depending on voltage on the COMP pin,

the device defines the maximum operating frequency of the converter. As the load is

reduced MOSFET turn-on no longer occurs on the first valley but on the second one,

the third one and so on. In this way the switching frequency is no longer increased

(piecewise linear portion in

Figure 4).

3.

Burst-mode with no or very light load. When the load is extremely light or disconnected,

the converter enters a controlled on/off operation with constant peak current.

Decreasing the load results in frequency reduction, which can go down even to few a

hundred hertz, therefore minimizing all frequency-related losses and making it easier to

comply with energy saving regulations or recommendations. The peak current being

very low, no issue of audible noise arises.