AP3987CT

Document number: DS41644 Rev. 2 - 2

9 of 15

www.diodes.com

June 2019

© Diodes Incorporated

AP3987CT

Operation Description (continued)

6.

Operating Frequency

For primary-side regulation, the primary current ip(t) is sensed by a current sense resistor RCS (R4 as shown in Typical Applications). The current

rises up linearly at a rate of:

M

IN

L

)

t

(

V

dt

)

t

(

dip



(6)

As illustrated in Figure 2, when the current ip(t) rises up to IPK, the switch Q1 turns off. The constant peak current is given by:

CS

CS

PK

R

V

I



(7)

The energy stored in the magnetizing inductance LM each cycle is therefore:

2

2

1

PK

M

G

I

L

E







(8)

So the power transferring from the input to the output is given by:

SW

2

PK

M

f

I

L

2

1

P









(9)

O

O

O

SW

2

PK

P

I

V

P

f

I

L

2

1

















(10)

Where, fSW is the switching frequency, η is the transferring efficiency.

In AP3987CT, the high load mode and light load mode adopt the Frequency Modulation (FM), and the middle load mode uses the Amplitude

Modulation (AM).

During FM, the peak current IPK is constant, the output power depends on the switching frequency fSW.

During AM, the frequency is fixed, VCS_M is varied. Below is the analysis of VCS_M。

The square root equation can be got from the following equation:

O

O

SW

2

CS

CS

P

SW

2

PK

P

I

V

f

R

V

L

2

1

f

I

L

2

1







































(11)

So,

SW

p

O

O

2

CS

CS

f

L

I

V

R

2

V















(12)

During AM, the frequency is fixed, assume VO and η are constants, then

O

CS

I

k

V





(

SW

p

O

2

CS

f

L

V

R

2

k













)

(13)

7.

NL Mode Operation (Typical Application with APR3415CT)

At no load and ultra light load, the AP3987CT works at No Load mode (NL mode) and the output voltage is detected by APR3415CT. In order to

achieve ultra low standby power at NL mode, the static current is reduced to ICC_NL.

• The conditions of exiting NL mode---VCBC> VCBC(EN) or tOFF< tOFF(EX)

• The conditions of entering NL mode-- VCBC< VCBC(EN) and tOFF≥ tOFF(EN)

At NL mode, the internal reference voltage VDD is pulled to ground. For normal NL working state, when the APR3415CT detects the output

voltage is lower than its trigger voltage, the APR3415CT VDET pin emits a periodical pulse current. This pulse current will generate a pulse

voltage on feedback winding through the transformer coupling. When the FB pin detects this pulse (>VTRIGGER is valid), the AP3987CT re-

establishes the VDD and turns on primary switch to provide one energy pulse to supply output terminal and primary VCC voltage.