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Product data sheet

Rev. 01 — 17 March 2004

4 of 16

Philips Semiconductors

TEA1622P

7.2 Pin description

8.

Functional description

The TEA1622P is the heart of a compact flyback converter, with the IC placed at the

primary side. The auxiliary winding of the transformer can be used for indirect feedback to

control the isolated output. This additional winding also powers the IC. A more accurate

control of the output voltage and/or current can be implemented with an additional

secondary sensing circuit and optocoupler feedback.

The TEA1622P uses voltage mode control. The frequency is determined by the maximum

transformer demagnetizing time and the time of the oscillator. In the first case, the

converter operates in the Self Oscillating Power Supply (SOPS) mode. In the latter case, it

stroke is started only in a valley of the secondary ringing. This valley switching principle

minimizes capacitive switch-on losses.

8.1 Start-up and undervoltage lock-out

Initially, the IC is self supplying from the rectified mains voltage. The IC starts switching as

the line is stopped for high efficiency operation.

and restarts from the rectified mains voltage.

8.2 Oscillator

The frequency of the oscillator is set by the external resistor and capacitor on pin RC. The

relative sensitivity of the duty factor to the regulation voltage at low duty factor is almost

equal to the sensitivity at high duty factors. This results in a more constant gain over the

duty factor range compared to PWM systems with a linear sawtooth oscillator. Stable

operation at low duty factors is easily realized. For high efficiency, the frequency is

reduced as soon as the duty factor drops below a certain value. This is accomplished by

increasing the oscillator charge time.

Table 3:

Pin description

Symbol

Pin

Description

1

supply voltage

GND

2

ground

RC

3

frequency setting

REG

4

regulation input

AUX

5

input for voltage from auxiliary winding for timing (demagnetization)

SOURCE

6

source of internal MOS switch

n.c.

7

not connected

DRAIN

8

drain of internal MOS switch; input for start-up current and valley sensing