ACT30AYT datasheet(5/10 Pages) ACTIVE-SEMI

5 / 10 page

ACT30

Rev 5, 05-Jun-09

Active-Semi

Innovative Power

- 5 -

www.active-semi.com

ActiveSwitcher

TM is a trademark of Active-Semi.

Normal Operation

In normal operation, the feedback signal from the

secondary

side

transmitted

through

the

optocoupler as a current signal into VVDD pin, which

has dynamic impedance of 9kΩ. The resulting VVDD

voltage affects the switching of the IC. As seen in

the Functional Block Diagram, the Current Limit VC

Generator uses the VVDD voltage difference with

4.75V to generate a proportional offset at the

negative input of the Error Comparator.

The drivers turn on at the beginning of each

switching cycle. The current sense resistor current,

which is a fraction of the transformer primary

current, increases with time as the primary current

increases. When the voltage across this current

sense resistor plus the oscillator ramp signal equals

Error Comparator's negative input voltage, the

drivers turn off. Thus, the peak DRV1 current has a

negative voltage coefficient of -0.29A/V and can be

calculated from the following:

for VVDD < 4.75V and duty cycle < 50%.

When the output voltage is lower than regulation,

the current into VVDD pin is zero and VVDD voltage

decreases. At VVDD = VUV = 3.35V, the peak DRV1

current has maximum value of 400mA.

Current Limit Adjustment

The IC's proprietary driver arrangement allows the

current limit to be easily adjusted between 400mA

and 1.2A. To understand this, the drivers have to be

utilized as linear resistive devices with typically

3.6Ω (rather than as digital output switches). The

current limit can then be calculated through linear

combination as shown in Figure 3. For TO-92

package, the ACT30A are preprogrammed to

400mA

current

limit

and

the

ACT30B

are

preprogrammed to 800mA current limit, for SOT23-

B package, the ACT30A are preprogrammed to

400mA current limit.

Figure 3:

Driver Output Configurations

()

VDD

PEAK

DRV

−

⎟

⎠

⎞

⎜

⎝

⎛ +

Ω

400

LIM

Figure 2:

Startup Waveforms

pulse-skipped

VAC

VDRV1

VVDD

IPRIMARY

VOUT

VDRVST

400

LIM =

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

LIM

Ω

400

800

LIM =

DRV1

DRV2

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