ALTAIR05T-800 Datasheet, PDF (19/28 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

ALTAIR05T-800
Application information
Figure 17 presents the internal schematic.
Figure 17. CDC block: internal schematic
Rzc d
Dsec _{Vout_reg}
Vout
Rc able/2
N pr i
Nsec
Cout
R cable/ 2
GND
Rcdc ^CDC
Viref
CDC
LOGIC
ZCD/FB
F B Block
N aux
To C V Compar ator
Vr ef
Rf b
COMP
During CV regulation, as the CDC block is capable of sinking current, a resistor connect
between its output and ZCD/FB pin allows to increase the CV setpoint, by providing a
voltage lower than the internal reference voltage by an amount proportional to the average
load current.
If R_CABLE is the total cable resistance, the resistor value can be calculated by using the
following equation:
R_CDC
=
2_-----⋅_------N_{-----S---E----C-}
N_PRI
⋅
N_{-----S---E----C--}
N_AUX
⋅
-^R----^S---^E----^N----^S---^E-----^â‹------^R----^Z---^C----^D--
R_CABLE
(5)
In this equation R_CABLE is the total resistance of the output cable.
The CDC block acts as an outer control loop with a positive feedback that changes the CV
setpoint. As such, it can impact on the overall system’s stability. In order to avoid any issue
that could make unstable the loop, the CV setpoint response time must be much slower than
that of the inner voltage loop.
For this purpose the CDC block is designed with a time response of a few ten ms. For the
same reason, the minimum voltage on CDC pin is bottom limited at to 2.25 V.
If the function is not required, the pin can be connected to ground or left open.
Doc ID 17957 Rev 1
19/28