TEA1751T/N1,518【PDF 17/29 页】IC CTRLR GREENCHIP SMPS 16-SOIC

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Sheet目录46

13页 14页 15页 16页 [17页]18页 19页 20页 21页

TEA1751T_LT_2

Product data sheet

Rev. 02 23 December 2009

17 of 29

NXP Semiconductors

TEA1751T; TEA1751LT

GreenChip III SMPS control IC

7.3.6 Soft start-up (pin FBSENSE)

To prevent audible transformer noise during start-up, the transformer peak current, I

slowly increased by the soft start function. This can be achieved by inserting a resistor and

a capacitor between pin 10, FBSENSE, and the current sense resistor.

An internal current source charges the capacitor to V = I

start(soft)fb

SS2

, with a maximum

of approximately 0.5 V.

The start level and the time constant of the increasing primary current level can be

adjusted externally by changing the values of R

SS2

and C

SS2

The soft start current I

start(soft)fb

is switched on as soon as V

reaches V

startup

. When the

voltage on pin FBSENSE has reached 0.5 V, the flyback converter starts switching.

The charging current I

start(soft)(PFC)

flows as long as the voltage on pin FBSENSE is below

approximately 0.5 V. If the voltage on pin FBSENSE exceeds 0.5 V, the soft start current

source starts limiting the current. After the flyback converter has started, the soft start

current source is switched off.

7.3.7 Maximum on-time

The flyback controller limits the on-time of the external MOSFET to 40 約 (typical). When

the on-time is longer than 40 約, the IC stops switching and enters the safe restart mode.

7.3.8 Overvoltage protection (FBAUX pin)

An output overvoltage protection is implemented in the GreenChip III series. This works

for the TEA1751(L)T by sensing the auxiliary voltage via the current flowing into

pin FBAUX during the secondary stroke. The auxiliary winding voltage is a well-defined

replica of the output voltage. Voltage spikes are averaged by an internal filter.

If the output voltage exceeds the OVP trip level, an internal counter starts counting

subsequent OVP events. The counter has been added to prevent incorrect OVP detection

which might occur during ESD or lightning events. If the output voltage exceeds the OVP

trip level a few times and not again in a subsequent cycle, the internal counter counts

down at twice the speed it uses when counting up. However, when typically eight cycles of

Fig 14. Soft start-up of flyback.

?SPAN class="pst TEA1751T-N1-518_2409373_10">softstart 3 R

SS2

?/DIV>

014aaa02

SOFT START

CONTROL

OCP

FBSENSE

0.5 V

start(soft)fb d 60 糀

SS2

SENSE2

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