AL1677

Document number: DS37526 Rev. 1 - 2

8 of 13

www.diodes.com

November 2015

© Diodes Incorporated

AL1677

Output Open Protection (OVP)

AL1677 has output open-voltage protection controlled when the LED is open, which can prevent the output voltage from increasing to a very high

value. This feature can help the system designer select a smaller volume capacitor. The output voltage is set by the external resistor R3, shown in

Figure 1.

5

R

1

_







OVP

REF

CS

off

V

V

L

t

Where

The output voltage is set by R3, and R3 is:

5

12

1

12

3

_

R

_

_

R

V

C

L

V

V

C

t

V

R

OVP

REF

REF

OVP

REF

CS

REF

off

REF

ROVP



















Where

Thermal Foldback Protection (TFP)

AL1677 has a thermal foldback function and adopts a self-adaptive control method, which can prevent the system from breaking down due to high

temperatures. The overheating temperature is set at +140°C typical. When the junction temperature of the IC is higher than +140°C typical, the

device will linearly decrease the internal reference voltage to decrease the output current. As a result of this feature, the device can control the

system

’s output power at high ambient temperatures, to control the quantity of heat of the system. This enhances the safety of the system at high

temperatures.

The thermal foldback waveform is shown below:

100%

50%

Junction temperature/°C

Output current

OTP

Figure 2. Thermal foldback waveform

Over-Temperature Protection (OTP)

The AL1677 has OTP protection function. When the junction temperature reach to +170°C typical, the IC will trigger an over-temperature

protection condition which causes the device to shut down and latched. Once OTP has triggered, the system will resume after the system

’s AC

source supply has been reset and power up.

Design Parameters

Setting the Current Sense Resistor R5

The AL1677 uses peak current control method to obtain constant current control and senses the peak current of the inductance cycle by cycle.

The sample value is compared with the internal 0.4V reference. When the sampling value is up to 0.4V, the IC control of the internal MOSFET will

turn off.

The peak current of the inductance is: