Data Sheet
PRIMARY SIDE CONTROL IC FOR OFF-LINE BATTERY CHARGERS AP3706
Operation Description (Continued)
Where the Vd is the diode forward drop voltage.
The relationship between the output constant-current
and secondary peak current Ipks is given by:
See equation 5
1
Tons
................(7)
Iout = ×Ipks×
2
Tons+Toffs
At the instant of D1 turn-on, the primary current
transfers to the secondary at an amplitude of:
0V
2/3 Tons
Tons
NP
................(8)
Ipks =
× Ipk
NS
Figure 12. Auxiliary Voltage Waveform
Thus the output constant-current is given by:
The output voltage is different from the secondary
voltage in a diode forward drop voltage. The diode
drop voltage depends on the current. If the secondary
voltage is always detected at a constant secondary
current, the difference between the output voltage and
the secondary voltage will be a fixed Vd. The voltage
detection point is at two-thirds of the D1 on-time. The
CV loop control function of AP3706 then generates a
D1 off-time to regulate the output voltage.
NP
NP
2
1
Tons
Iout = ×
2 NS
×Ipk×
= ×
×Ipk
Tons+Toffs 7 NS
................(9)
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.
Constant Current Operation
The AP3706 is designed to work in constant-current
(CC) mode. Figure 13 shows the secondary current
waveforms.
See equation 8
CCM Protection
The AP3706 is designed to operate in discontinuous
conduction mode (DCM) in both CV and CC modes.
To avoid operating in continuous conduction mode
(CCM), the AP3706 detects the falling edge of the FB
input voltage on each cycle. If a 0.1V falling edge of
FB is not detected, the AP3706 will stop switching.
Iout
Is
0A
Toffs
Tons
Figure 13. Secondary Current Waveform
OVP & OCkP
The AP3706 includes output over-voltage protection
(OVP) and open circuit protection (OCkP) circuitry as
shown in Figure 14. If the voltage at FB pin exceeds
8V, 100% above the normal detection voltage, or the
-0.7V falling edge of the FB input can not be
monitored, the AP3706 will immediately shut off and
enters hiccup mode. The AP3706 sends out a fault
detection pulse every 8ms in hiccup mode until the
fault has been removed.
In CC operation, the CC loop control function of
AP3706 will keep a fixed proportion between D1 on-
time Tons and D1 off-time Toffs by discharging or
charging the capacitance connected in COMP pin. The
fixed proportion is
Tons
Toffs
4
3
=
................(6)
Jul. 2008 Rev. 1.3
BCD Semiconductor Manufacturing Limited
9
BCDSEMI [ BCD SEMICONDUCTOR MANUFACTURING LIMITED ]
