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AP3703

LOW-POWER OFF-LINE PRIMARY SIDE REGULATION CONTROLLER

BCD Semiconductor 

Preliminary Datasheet

LOW-POWER OFF-LINE PRIMARY SIDE REGULATION CONTROLLER AP3703

Operation Description (Continued)

where the Vd is the diode forward drop voltage.

See equation 5

The relationship between the output constant-current

and secondary peak current Ipks is given by:

Iout = 1 × Ipks× Tons

2

Tons+ Toffs

......(7)

0V

2/3 Tons

Tons

Figure 9. Auxiliary Voltage Waveform

The output voltage is different from the secondary

voltage in a diode forward drop voltage that depends

on the current. If the secondary voltage is always

detected at a fixed secondary current, the difference

between the output voltage and the secondary voltage

will be a fixed Vd. For AP3703, the voltage detection

point is at two-thirds of the D1 on-time, which means

the the secondary voltage is detected at a fixed

secondary current of one-third of Ipks. The CV loop

control function of AP3703 then generates a D1 off-

time to regulate the output voltage.

Constant Current Operation

Figure 10 shows the secondary current waveforms.

See equation 8

Is

Iout

0A

Tons

Toffs

Figure 10. Secondary Current Waveform

In CC operation, the CC loop control function of

AP3703 will keep a fixed proportion between D1 on-

time Tons and D1 off-time Toffs by discharging or

charging a capacitor embedded in the IC. The fixed

proportion is:

Tons = 4

Toffs 3

......(6)

At the instant of D1 turn-on, the primary current

transfers to the secondary at an amplitude of:

Ipks = NP × Ipk

NS

......(8)

Thus the output constant-current is given by:

Iout = 1 × NP × Ipk× Tons = 2 × NP × Ipk ......(9)

2 NS

Tons+ Toffs 7 NS

Leading Edge Blanking

When the power switch is turned on, a turn-on spike

will occur on the sense-resistor. To avoid false-

termination of the switching pulse, a 430ns leading-

edge blanking is built in. During this blanking period,

the current sense comparator is disabled and the gate

driver can not be switched off.

CCM Protection

The AP3703 is designed to operate in discontinuous

conduction mode (DCM) in both CV and CC modes.

To avoid operating in continuous conduction mode

(CCM), the AP3703 detects the falling edge of the FB

input voltage on each cycle. If a 0.1V falling edge of

FB is not detected, the AP3703 will stop switching.

OVP & OCkP

The AP3703 includes output over-voltage protection

(OVP) and open circuit protection (OCkP) circuitry as

shown in Figure 11. If the voltage at FB pin exceeds

8V, 100% above the normal detection voltage, or the

-1.5V falling edge of the FB input can not be

monitored, the AP3703 will immediately shut off and

enters hiccup mode. The AP3703 sends out a fault

detection pulse every 24ms in hiccup mode until the

fault has been removed.

Feb. 2009 Rev. 1.0

BCD Semiconductor Manufacturing Limited

8

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