AOZ7111

Rev. 1.0 January 2013

www.aosmd.com

Page 9 of 15

Figure 3. Soft-Start Sequence

On Time Control (Ramp Control Oscillator)

The switching pattern consists of constant on times and

variable off times for a given RMS input voltage and

output load. The AOZ7111 controls the on-time with the

capacitor connected to the Ct pin. A current source

charges the Ct capacitor to a voltage derived from the

below:

The ramp oscillator consists of three phases:

Charge Phase:

The oscillator capacitor voltage grows

linearly from its bottom value (ground) until it exceeds

gets low and the oscillator discharge sequence is set.

Discharge Phase:

The oscillator capacitor is discharged

down to its valley value of 0V.

Standby Phase:

At the end of the discharge sequence,

the oscillator voltage is maintained in a low state until the

PWM latch is set again.

load, the on time is constant during the AC line cycle if

the values of the compensation components are

maximum. The Ct capacitor is sized to ensure that the

required on time is reached at maximum output power

and the minimum input voltage condition.

The minimum Ct value is calculated as below:

Current Detection Block

The current detection circuit is composed of zero current

detection and over-current detection.

The inductor current is converted into a voltage by

the input diode bridge and the input filtering capacitor.

Therefore, a negative voltage proportional to the inductor

current is built:

where,

L is the inductor current;

Zero Current Detection

The zero current detection function guarantees that the

MOSFET can not turn on as long as the inductor current

has not reached zero.

pin 5. The pin 5 voltage is compared to the -15mV

threshold, the current sense comparator resets the PWM

latch to force the gate drive signal low state.

Consequently, it is not possible to turn on the power

MOSFET until the inductor current is measured smaller

V

Ct off



V

COMP

C

t

offset



–

2 P

OUT

LI

ch

e

arg







EFF

V

AC

2

C

t





-----------------------------------------------------------------

==

Ct

min



2 P

OUT

LI

ch

e

arg







EFF

V

AC

2

V

Ct max







--------------------------------------------------------------

=

V

CS

R



=

t

t

Rated output

95% rated