GA1706 Goodark Electronics, GA1706 Datasheet - Page 9

• Manufacturer Part Number: GA1706
• Description: Preliminary Datasheet Primary Side Control Ic For Off-line Battery Chargers Ap3706 General Description
• Manufacturer: Goodark Electronics

Operation Description (Continued)
Jun. 2009 Rev. 1. 0
Where the Vd is the diode forward drop voltage.
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 GA1706 then generates a
D1 off-time to regulate the output voltage.
Constant Current Operation
The GA1706 is designed to work in constant-current
(CC) mode. Figure 13 shows the secondary current
waveforms.
In CC operation, the CC loop control function of
GA1706 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
0V
Toffs
Tons
Is
0A
Figure 13. Secondary Current Waveform
Figure 12. Auxiliary Voltage Waveform
=
4
3
Tons
2/3 Tons
Tons
Toffs
See equation 8
See equation 5
................
(6)
Iout
9
The relationship between the output constant-current
and secondary peak current Ipks is given by:
At the instant of D1 turn-on, the primary current
transfers to the secondary at an amplitude of:
Thus the output constant-current is given by:
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 GA1706 is designed to operate in discontinuous
conduction mode (DCM) in both CV and CC modes.
To avoid operating in continuous conduction mode
(CCM), the GA1706 detects the falling edge of the FB
input voltage on each cycle. If a 0.1V falling edge of
FB is not detected, the GA1706 will stop switching.
OVP & OCkP
The GA1706 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 GA1706 will immediately shut off and
enters hiccup mode. The GA1706 sends out a fault
detection pulse every 8ms in hiccup mode until the
fault has been removed.
Iout
Iout
Ipks
=
=
=
1
2
1
2
N
N
⋅
⋅
N
P
S
N
Ipks
⋅
P
S
Ipk
⋅
Ipk
⋅
Tons
⋅
Tons
Tons
Preliminary Datasheet
+
Tons
Toffs
+
Toffs
................
................
................
=
7
2
⋅
N
N
P
S
(7)
(8)
(9)
⋅
Ipk