AP1684 datasheet(9/13 Pages) DIODES

9 / 13 page

AP1684

Document number: DS36547 Rev. 3 - 2

9 of 13

www.diodes.com

February 2014

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 tOFF 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

RFB1

RFB2

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 VCC cap exceeds

Vcc_ovp or VFB_CV, the over voltage is triggered and the IC will discharge VCC. When the VCC is below the UVLO threshold voltage, the IC will start

a new work cycle and the VCC 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, VCC will drop down without auxiliary winding for power supply.

And the VCC will drop to UVLO threshold voltage, the IC will shut down and restart a new operating cycle, and the VCC is charged by startup

resistance. When the VCC is higher than Vcc_start voltage, the IC will output a bunch of pulse to control BJT on and off, which will consume the

energy stored in the VCC cap, because of no VCC supply from the auxiliary winding, the VCC will drop down to VCC 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

LED current

Output Current Ripple

Small current ripple is good for LED life

ton_initial

System initial on time, used to start up the system

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

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