ICE3AS02G datasheet(13/22 Pages) INFINEON

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ICE3AS02 / ICE3AS02G / ICE3BS02 / ICE3BS02G

Functional Description

Version 1.2

02 Sep 2005

integrated pull up resistor RSoftS. The comparator C3 releases

the gates G5 and G6 once VSofts has exceeded 5.4V.

Therefore there is no entering of Auto Restart Mode possible

during this charging time of the external capacitor CSoftS. The

same procedure happens to the external Soft Start capacitor

if a low load condition is detected by comparator C5 when

VFB is falling below 1.32V. Only after VSoftS has exceeded

5.4V and VFB is still below 1.32V Active Burst Mode is

entered.

3.6.2

Active Burst Mode

The controller provides Active Burst Mode for low load

conditions at VOUT. Active Burst Mode increases

significantly the efficiency at light load conditions while

supporting a low ripple on VOUT and fast response on load

jumps. During Active Burst Mode which is controlled only

by the FB signal the IC is always active and can therefore

immediately response on fast changes at the FB signal. The

Startup Cell is kept switched off to avoid increased power

losses for the self supply.

Figure 16

Active Burst Mode

The Active Burst Mode is located in the Control Unit. Figure

16 shows the related components.

3.6.2.1

Entering Active Burst Mode

The FB signal is always observed by the comparator C5 if

the voltage level falls below 1.32V. In that case the switch S1

is released which allows the capacitor CSoftS to be charged

starting from the clamped voltage level at 4.4V in normal

operating mode. If VSoftS exceeds 5.4V the comparator C3

releases the gate G6 to enter the Active Burst Mode. The

time window that is generated by combining the FB and

SoftS signals with gate G6 avoids a sudden entering of the

Active Burst Mode due to large load jumps. This time

window can be adjusted by the external capacitor CSoftS.

After entering Active Burst Mode a burst flag is set and the

internal bias is switched off in order to reduce the current

consumption of the IC down to approx. 1.05mA. In this Off

State Phase the IC is no longer self supplied so that therefore

CVCC has to provide the VCC current (see Figure 17).

Furthermore gate G11 is then released to start the next burst

cycle once VFB has 3.4V exceeded.

It has to be ensured by the application that the VCC remains

above the Undervoltage Lockout Level of 8.5V to avoid that

the Startup Cell is accidentally switched on. Otherwise

power losses are significantly increased. The minimum VCC

level during Active Burst Mode is depending on the load

conditions and the application. The lowest VCC level is

reached at no load conditions at VOUT.

3.6.2.2

Working in Active Burst Mode

After entering the Active Burst Mode the FB voltage rises as

VOUT starts to decrease due to the inactive PWM section.

Comparator C6a observes the FB signal if the voltage level

4V is exceeded. In that case the internal circuit is again

activated by the internal Bias to start with switching. As now

in Active Burst Mode the gate G10 is released the current

limit is only 0.257V to reduce the conduction losses and to

avoid audible noise. If the load at VOUT is still below the

starting level for the Active Burst Mode the FB signal

decreases down to 3.4V. At this level C6b deactivates again

the internal circuit by switching off the internal Bias. The

gate G11 is released as after entering Active Burst Mode the

burst flag is set. If working in Active Burst Mode the FB

voltage is changing like a saw tooth between 3.4V and 4V

(see Figure 17).

3.6.2.3

Leaving Active Burst Mode

The FB voltage immediately increases if there is a high load

jump. This is observed by comparator C4. As the current

limit is ca. 26% during Active Burst Mode a certain load

jump is needed that FB can exceed 4.8V. At this time C4

resets the Active Burst Mode which also blocks C12 by the

5.4V

4.8V

C6a

4.0V

1.32V

Control Unit

Active

Burst

Mode

4.4V

Internal Bias

SoftS

6.5V

SoftS

G10

Current

Limiting

C6b

3.4V

G11

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