FIP4F8S datasheet(20/33 Pages) FUJI

20 / 33 page

AN-087E Rev.1.0

April-2011

FA5526/5527/5528/5536/5537/5538

Fuji Electric Co., Ltd.

http://www.fujielectric.co.jp/products/semiconductor/

When the output voltage momentarily drops due to

abrupt load change and FB pin voltage restores to the

voltage at steady state before CS pin voltage reaches 8.5V,

the 4V clamp circuit restarts, producing no latch mode.

The latch mode can be reset through cutting off input

voltage or through forcibly decreasing CS pin voltage to

7.4V or less.

Cutting off the input voltage decreases VH pin voltage,

supplying no current to the VCC pin. Thereafter, the latch

mode is reset when Vcc drops below the OFF threshold

voltage, 8.0Vmin.

In addition, when CS pin voltage is forcibly decreased,

the latch mode comparator is re-inverted and the IC

re-starts switching operation.

In the case of typical IC, delay time td (OLP), the time from

overload detection to the latch mode, is given by the

following equation.

td1 (OLP) [ s ] = 0.93 * Cs [ uF ]

( typical value )

Here, Cs is a capacitance connected to CS pin [ uF ].

Delay time td(OLP) is inversely proportional to CS

charging current and proportional to the difference

between CS pin clamp voltage “4V” at steady condition and

latch-mode threshold voltage “8.5V”. Pay attention to

variations in delay time resulting from variations in

numerical values.

In addition, be aware that when the VH pin is connected

after rectification, it takes rather long time, approximately

several minutes, before the latch mode is reset.

(See “9-(1) Start-up circuit.”)

(9) Overload protection of FA5536/37/38

FA5536, FA5537 and FA5538 have built-in auto-recovery

type overload protection. Fig.14 shows VCC pin voltage

and the drain voltage of power MOSFET “Q1” in the circuit

on page 33 at “6A” overload when 90Vac input is applied

and Fig.15 shows same parameters mentioned above at

“7A” overload when 264Vac input is applied.

Switching period after the overload occurs and stop

period are calculated as follows.

Steady State to Overload : td2 (OLP) [ s ] = td1 (OLP),

After overload starts : td2 (OLP) [ s ] = 1.65* Cs [ uF ],

stop period t (stop) [ s ] = Cvcc [ uF ] *

[ Vcc(sw/OL) – Vccoff ] / ( ICCL )

Here,

Vcc(sw/OL) : Vcc in switching period at overload [ V ]

Vccoff : OFF threshold voltage of U.V.L.O ( 9V (typ.))

ICCL : Consumption current in latch mode ( 290uA (typ.))

OUT pin

Voltage

3.6V

8.5V

8.8V

Short time

Overload

Overload Latch Off

Secondary

Side DC

Output

Voltage

CS pin

Voltage

FB pin

Voltage

Fig.13

Overload protection timing chart

Fig.14

Overload protection waveform

of FA5538 as Auto-Recovery ( 90Vac )

Fig.15

Overload protection waveform

of FA5538 as Auto-Recovery

( 264Vac )

Vcc

(20V

/div)

Vds

(100V

/div)

Vcc

(20V

/div)

Vds

(100V

/div)

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