AL1677 datasheet(7/13 Pages) DIODES

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AL1677

Document number: DS37526 Rev. 1 - 2

7 of 13

www.diodes.com

November 2015

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-

voltage MOSFET. The AL1677 adopts a novel method to detect the tOFF time and achieve an extremely low operation current, so the device does

not need the auxiliary winding for VCC supply and detecting the tOFF time. It also has good constant current control which can guarantee the

system current accuracy.

OUT

VCC

GND

U1 AL1677

Input

DB1

ROVP

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

exceeding the startup voltage (VTH (ST)).

Protections

Undervoltage Lockout (UVLO)

When the voltage on the VCC pin drops to below VOPR(Min), the IC stops switching. The IC can restart when the voltage on VCC is exceeds the

startup voltage (VTH(ST))

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

(tON_MIN).

Cycle-by-cycle Overcurrent Protection (OCP)

The AL1677 has a built-in peak current detector. It triggers when the voltage on CS pin reaches the peak level VCS_CLAMP. The R5 is connected to

the CS pin to sense the current of the inductor. The maximum peak current (IPEAK(MAX))of the inductor can be calculated as below:

_ REF

PEAK



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)

When LED is shorted, the device cannot detect the tOFF time, and the device controls the system operation at 5kHz low frequency.

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