ALTAIR04-900TR データシートの表示（PDF） - STMicroelectronics

部品番号

コンポーネント説明

メーカー

ALTAIR04-900TR

Off-line all-primary-sensing switching regulator

STMicroelectronics 

ALTAIR04-900

Application information

5.4

Constant voltage operation

The IC is specifically designed to work in the primary regulation and the output voltage is

sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier

diode.

Figure 13 shows the internal schematic of the constant voltage-mode and the external

connections.

Figure 13. Voltage control principle: internal schematic

R zcd ZCD/FB

Rf b

Aux

S/ H

EA

2. 5V

D EMAG

LOGI C

To PWM Logic

CV

F rom Rsense

COMP

R

C

Due to the parasitic wire resistance, the auxiliary voltage is representative of the output just

when the secondary current becomes zero. For this purpose, the signal on ZCD/FB pin is

sampled-and-held at the end of the transformer demagnetization to get an accurate image

of the output voltage and it is compared with the error amplifier internal reference.

The COMP pin is used for the frequency compensation: usually, an RC network, which

stabilizes the overall voltage control loop, is connected between this pin and ground.

The output voltage can be defined according to the following formula:

Equation 1

RFB = -NN---------AS--------UE------CX-------⋅---V-V---O-R---U-E---TF----–-----V----R----E---F-- ⋅ RZCD

where NSEC and NAUX are the numbers of secondary and auxiliary turns respectively.

RZCD value depends on the application parameters (see “Section 5.6: Voltage feed-forward

block”).

DocID18211 Rev 3

15/29

29

Share Link: