ALTAIR04-900 Datasheet, PDF (18/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

Application information
ALTAIR04-900
Figure 16. Feedforward compensation: internal schematic
R zcd ZCD/FB
F eedf orward .
Logic
Rfb
A ux
CC
I FF Block
CC
DRAIN
P WM
LOGI C
Rff
SOURCE
Rs ens e
The R_ZCD resistor can be calculated as follows:
5.7
18/29
R_ZCD
=
N_AUX
N_PRI
⋅
L_P ⋅ R_FF
T_d⋅ R_SENSE
(4)
In this case the peak drain current does not depend on input voltage anymore.
One more consideration concerns the R_ZCD value: during MOSFET’s ON-time, the current
sourced by the ZCD/FB pin, I_ZCD, is compared with an internal reference current I_ZCDON (-50
µA typical).
If I_ZCD < I_ZCDON, the brownout function is activated and the IC is shut-down.
This feature is especially important when the auxiliary winding is accidentally disconnected
and considerably increases the end-product’s safety and reliability.
Burst-mode operation at no load or very light load
When the voltage at the COMP pin falls 65 mV below a threshold fixed internally at a value,
V_COMPBM, the IC is disabled with the MOSFET kept in OFF state and its consumption
reduced at a lower value to minimize Vcc capacitor discharge.
In this condition the converter operates in burst-mode (one pulse train every T_START=500
µs), with minimum energy transfer.
As a result of the energy delivery stop, 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 V_COMPL threshold, the device restarts switching, otherwise it stays
OFF for another 500 µs period.
In this way the converter works in burst-mode with a nearly constant peak current defined by
the internal disable level. Then a load decrease causes a frequency reduction, which can go
down even to few hundred hertz, thus minimizing all frequency-related losses and making it
easier to comply with energy saving regulations. This kind of operation, shown in the timing
Doc ID 18211 Rev 2