FAN501AMPX datasheet(11/16 Pages) FAIRCHILD

No de pièce	FAN501AMPX

Description	Offline DCM / CCM Flyback PWM Controller for Charger Applications

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Fabricant	FAIRCHILD [Fairchild Semiconductor]

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FAN501A

• Rev. 1.0.0

11

operation range. It is typical to design the voltage divider

for the VS pin such that frequency change occurs at

170 VAC (VDC-170 VAC = 240 V); calculated as:

1

/

240

AP

VS

VS H

NN

R

I -



(8)

With the value of RVS1 determined from Equation (8), the

switching frequency drops to 85 kHz as line voltage

increases above 170 VAC, while switching frequency

increases to 140 kHz, as line voltage drops <155 VAC.

1/140kHz

Low line

High line

IDS

VDS

1/140kHz

1/85kHz

IDS

VDS

Low line

High line

(a) Single frequency operation

(b) Dual frequency operation

Figure 24.

Peak Switch Current, Single- and

Dual-Frequency Operation

Brownout Protection

Line voltage information is also used for brownout

protection. When the IVS current out of the VS pin during

the MOSFET conduction time is less than 160

μA for

longer than 30 ms, the brownout protection is triggered.

When setting RVS1 as calculated in Equation (8), the

brownout level is set at 30 VAC.

Aux.

VS

IVS

Line Voltage

Detector

5V

RVS1

RVS2

IVS

VAux

NA

NP

Pri.

VBLK

GATE

Line signal

VS_Offset

Figure 25.

Line Voltage Detection Circuit

tON

VAux

VS

GATE

tS

-

A

BLK

S

N

V

N

0.5V

Figure 26.

Waveforms for Line Voltage Detection

Maximum Power Limit by Precision CC Control

Primary-side current-sensing voltage is used to estimate

the output current for CC regulation. However, the

actual output current regulation is also affected by the

turn-off delay of the MOSFET, as illustrated in Figure

27. While FAN501A samples the CS pin voltage at the

half on-time of gate drive signal, the actual turn-off is

delayed by the MOSFET gate charge and driving

current resulting in peak current detection error as:

.

PK

DL

DS

OFF DLY

m

V

It

L



(9)

where Lm is the primary side magnetic inductance.

As can be seen, the error is proportional to the line

voltage. FAN501A has an internal correction function to

improve CC regulation, as shown in Figure 28. Line

information is obtained through the line voltage detector

as shown in Figure 25 and Figure 26 and this

information is used for the CC regulation correction. The

correction gain can be programmed using external

resistor RCOMP on the COMP pin. This correction

current, ILVF, flows through internal resistor, RLVF, and

external resistor, RCSF, to introduce offset voltage on

current sensing voltage. Thus, the primary current

detection error affected by line voltage and turn-off

delay is corrected for better CC regulation. The RCOMP

resistor can be calculated as:

.

1

CS

OFF DLY

P

COMP

VS

COMP

A

LVF

CSF

m

Rt

N

R

K

N

R

L





+

(10)

where RLVF is the internal resistor on the IC, which is

2.0

kΩ, and KCOMP is the design factor of the IC, which

is 3.745

MΩ.

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