AL1677

Document number: DS37526 Rev. 1 - 2

7 of 13

www.diodes.com

November 2015

© Diodes Incorporated

AL1677

Application Information

Convertor Operation

The AL1677 is designed for single-inductance buck application and is an extremely low BOM cost solution widely used in non-isolated situations.

The AL1677 operates at boundary conduction mode (BCM) which can get good EMI performance. The device internally integrates a 500V high-

system current accuracy.

OUT

+

VCC

CS

GND

U1 AL1677

C3

R1

AC

Input

F1

DB1

C1

ROVP

NC

R2

CS

D

C2

D1

L1

R4

R5

R3

D

1

2

3

4

5

6

7

8

Figure 1 Typical Application Circuit

Start-up and Supply Voltage

Before start-up, the VCC capacitor C2 is charged by the startup resistors (R1, R2) from the high voltage mains. When the start-up voltage is

reached, the AL1677 starts switching. During normal operation, the VCC supply is provided by start-up resisters (R1, R2) and internal source

driver circuit.

The AL1677 has an internal VCC clamp voltage (typical 16.2V), which is limited by one internal active Zener diode.

When VCC voltage drops to below the undervoltage lockout (UVLO), switching is stop, the IC can restart when the voltage on VCC pin is

Protections

Undervoltage Lockout (UVLO)

Leading-Edge Blanking (LEB)

To prevent false detection of the peak current of the inductor, a blanking time following switch-on is designed. When the internal switch turns on, a

short current spike can occur because of the capacitive discharge of the voltage over the drain and source. It is disregarded during the LEB time

Cycle-by-cycle Overcurrent Protection (OCP)

_ REF

5

CS

PEAK

V

I

R



The detection circuit is activated after the LEB time. When the detection circuit sense the CS voltage is higher than 0.4V, the IC will turn off the

switching to limit the output current. It automatically provides protection for the maximum LED current during operation. A propagation delay exists

between overcurrent detection and actual source-switch off, so the actual peak current is a little higher than the OCP level set by the R5.

Output-Short Protection (OSP)