AL1677 datasheet(9/13 Pages) DIODES

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AL1677

Document number: DS37526 Rev. 1 - 2

9 of 13

www.diodes.com

November 2015

AL1677

Application Information (continued)

REF

peak 

Where,

VCS_REF is the reference of the current sense, and the typical value is 0.4V.

R5 is the current sense resistor.

In no PF buck topology, the system operates at boundary conduct mode. The output current is:

peak

mean





Therefore, the current sense resistor R5 is determined:

mean

REF





Inductance Selection (L1)

The AL1677 controls the system operating at boundary conduction mode, and the systems

’ operating frequency does not keep constant because

of the fluctuation of the bus voltage. Set the minimum switching frequency fmin at the maximum bus voltage, and the buck inductance value L1 can

be calculated by:

min

)

(

rms

ref

rms







Where,

Vo is the output voltage.

Vin_rms is the RMS value of the input voltage.

According to the Faraday

’s Law of Induction, the winding number of the inductance can be calculated by:

REF

peak









Where,

Ae is the core effective area.

Bm is the maximum magnetic flux density.

The AL1677 has designed the minimum tON time and maximum tON time. The tON_MIN is about 700ns and the tON_MAX time is about 35µs. In buck

topology we can get the equation Vin_rms-VO=L*Ipeak/tON. If the inductance is very small, leading the tON to become smaller, when the system’s tON

is smaller than tON_MIN, the device cannot detect the peak current of the system, leading to incorrect output current. If the inductance is very large,

leading tON to become larger when the system’s tON is larger than the tON_MAX, the system’s output current will decrease because of the limit of the

tON_MAX. Therefore, the suitable value of the inductance is very important.

The AL1677 tOFF_MIN time is about 6µS and tOFF_MAX time is about 200µs. In buck topology we use the equation VO=L*Ipeak/tOFF. If the inductance

is very small, leading the tOFF to become much smaller when the system’s tOFF is smaller than tOFF_MIN, the system will enter DCM mode, and the

output current will be incorrect. If the inductance is very large, leading tOFF to become much larger when the system’s tOFF is larger than the

tOFF_MAX, the system will enter CCM mode, and the output current will also be incorrect. Therefore, the suitable value of the inductance is very

important.

Consider these parameters, two examples of the typical application inductance is recommended as below:

System Spec.

Inductance Value

System Frequency

TON Min

TOFF

60V/150mA

(185~265VAC)

2.3mH

60K(230 VAC)

2.5µS(265 VAC)

11.5µS

42V/100mA (85~265 VAC)

2.5mH

62K(230 VAC)

1.7µs(265 VAC)

12.1µS

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