Ap1694a – Diodes AP1694A User Manual
Page 11
AP1694A
Document number: DS37094 Rev.
1 - 2
11 of 15
May 2014
© Diodes Incorporated
AP1694A
A Product Line of
Diodes Incorporated
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Application Information Based on Buck Structure
(Cont.)
Fault Protection
Over Voltage Protection and Output Open Protection
VCC
FB
R
FB1
R
FB2
AP1694A
Figure 4. 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, 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.
Attention: If the external fast startup circuit is adding in the application and the over voltage protection and output open protection happen, the IC
will trigger latch.
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 V
CC
is higher than V
cc_start
voltage, IC will output a bunch of pulse to control power switch on and off. When still no FB signal
detected the device will not output more pulse. So the V
CC
will drop to V
CC
UVLO threshold again. If output short condition still exists, the system
will operate in hiccup mode.
Attention: If the external fast startup circuit is adding in the application, the device will not work at UVLO mode, and the device will work at
minimum toff mode.
Over Temperature Protection
AP1694A 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 when the ambient
temperature is higher than +150°C, over temperature protection will be triggered. So the AP1694A can startup successfully when the ambient
temperature is less than +150°C.
Components Selection Guide
If the system’s spec is changed, please refer to the design sheet of the AP1694A 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
Output current
R8
Output Current Ripple
Small current ripple is good for LED life
C4
t
on_initial
System initial on time, used to startup the system
R1
Output Open Voltage
Setting the output voltage when the LED is open
R5, R6
Dimming Performance
Improve the dimming performance
R1, RF, R9, C2, C4
EMI
Pass EN 55022 class B with 6DB margin
L1, C1
Line Compensation
To get a good line regulation
R7