Diodes AP1684 - Manual (Page 9)

AP1684
Document number: DS36547 Rev. 3 - 2
9 of 13
www.diodes.com
February 2014
© Diodes Incorporated
AP1684
A Product Line of
Diodes Incorporated
Application Information
(Cont.)
According to Figure 2, when the falling edge of 0.1V is sensed by the FB pin, the AP1684 will see the t
OFF
time is over and delay 1µs to start a
new operating cycle. In this way we can realize valley on function.
Fault Protection
Over Voltage Protection and Output Open Protection
VCC
FB
R
FB1
R
FB2
AP1684
Figure 3. OVP Circuit
The output voltage is sensed by the auxiliary winding voltage of the buck transformer, the VCC pin and FB pin provide over voltage protection
function. When the output is open or large transient happens, the output voltage will exceed the rated value. When the voltage of V
CC cap exceeds
V
cc_ovp
or V
FB_CV
, the over voltage is triggered and the IC will discharge V
CC
. When the V
CC
is below the UVLO threshold voltage, the IC will start
a new work cycle and the V
CC
cap is charged again by start resistance. If the over voltage condition still exists, the system will work in hiccup
mode.
Output Short Protection
When the output is shorted, the output voltage will be clamped at 0. At this condition, V
CC
will drop down without auxiliary winding for power supply.
And the V
CC
will drop to UVLO threshold voltage, the IC will shut down and restart a new operating cycle, and the V
CC
is charged by startup
resistance. When the V CC is higher than V cc_start voltage, the IC will output a bunch of pulse to control BJT on and off, which will consume the
energy stored in the V CC cap, because of no V
CC
supply from the auxiliary winding, the V CC will drop down to V CC UVLO threshold voltage again. If
output short condition still exists, the system will operate in hiccup mode.
Over Temperature Protection
The AP1684 has two kinds of over temperature protection processes. First, the system is operating normally, the ambient temperature is changed
to +170°C suddenly, the IC will trigger over temperature protection which leads to a latch work mode. Second, if the system starts, the over
temperature protection will be triggered when the ambient temperature is higher than +150°C. So the AP1684 can startup successfully when the
ambient temperature is less than +150°C.
Recommended Applications
The AP1684 is design
ed to drive BJT as the power switch, because of the BJT’s current limit, the maximum output current is limited. In buck
structure, the output voltage has some limitation because of the dead zone. The device is designed for single voltage application, so the
recommended application is given in the table below.
AC Power Input Output Voltage Range Max Output Current
Low Mains Input 20V to 70V 200mA (13005)
High Mains Input 20V to 120V 200mA (13005)
Components Selection Guide
If the system’s output spec is changed, please refer to the design sheet of the AP1684 and select the compatible system parameter. When the
system needs to be adjusted slightly, please refer to the table below and adjust the value of the related component.
Item Description Related Components
I
O
LED current R8
Output Current Ripple Small current ripple is good for LED life C4
t
on_initial
System initial on time, used to start up the system R1
Output Open Voltage Setting the output voltage when the LED is open R5, R6
Line Compensation To get a good line regulation R7, R9
Startup Time System startup time R3, C3, T1
EMI Pass EN 55022 class B with 6DB margin L1, C1, C2