AL8820

Document number: DS37369 Rev. 1 - 2

11 of 15

www.diodes.com

December 2014

© Diodes Incorporated

AL8820

Operation Frequency of SW2

33µH

756kHz

47µH

568kHz

68µH

397kHz

100µH

257kHz

Table 2

VIN OVP Protection

The AL8820 has two kinds of Over-voltage (OVP) protection both of which turn off the power switch SW1. When the voltage at the FB pin exceeds

threshold approximately 1.66V, the power switch of step-up stage is turned off. The power switch of boost section can be turned on again once the

voltage at the FB pin drops below 1.46V.

The AL8820 additionally has an internal over voltage protection to protect the AL8820 from excessive input voltage. When the voltage applied at

VCC Regulator

The VCC pin requires a capacitor for stable operation and to store the charge for the large GATE switching currents. Choose a 10V rated low ESR,

X7R or X5R, ceramic capacitor for best performance. A 4.7µF capacitor will be adequate for many applications. Place the capacitor close to the IC

to minimize the trace length between the VCC pin and the exposed pad.

An internal current limit on the VCC output protects against excessive on-chip power dissipation. The VCC pin has set the output to 5V (typ.) to

protect the internal FETs from excessive power dissipation caused by not being fully enhanced. If the VCC pin is used to drive extra circuits beside

the AL8820, the extra loads should be limited to less than 8mA.

Output Capacitor C1 and C2 of Boost

The capacitor C1 is used to hold the bus voltage when the electronic transformer has no output. For most applications, it is recommended to use

an aluminum electrolytic capacitor with greater than 220µF capacitance.

The output capacitor C2 is selected to handle the output ripple noise requirements. For the best performance, it is recommended to use X7R or

better grade ceramic capacitor greater than 1µF capacitance.

Compensation Capacitor C4

In applications powered by electronic transformer, the input voltage can change roughly in one cycle of AC power frequency. A 1µF ceramic

capacitor C4 connected from COMP pin to ground help to stabilize the control loop of the Boost regulator.

Output Capacitor C5 of Buck

Higher LED current ripple will shorten the LED life time and increase heat accumulation of LED. To reduce the LED current ripple, an output

capacitor in parallel with the LED should be added. Lower ripple can be achieved with higher capacitor values. For most applications, a value of

4.7µF is recommended.

Diode Selection

For maximum efficiency and performance, the rectifiers (D5, D6) should be fast low capacitance Schottky diodes with low reverse leakage at

maximum operating voltage and temperature. With its low power dissipation, the Schottky diode outperforms other silicon diodes and increases

overall efficiency.

Over Temperature Protection

A over temperature protection feature is to protect the AL8820 from excessive internal temperature. When the junction temperature exceeds

+160ºC, the internal FETs will be turned off. When junction temperature drops below +120ºC, IC will turn on both FETs and return to normal

operation.