L4990AD datasheet(15/24 Pages) STMICROELECTRONICS

15 / 24 page

ing conditions range, in terms of input voltage and

output current, that the converter is able to cover

but, on the other hand, the higher is the peak cur-

rent on the secondary side.

As to this point, the L4990 turns out to be particu-

larly useful since it allows to set any maximum

duty cycle greater (and lower) than 50% with very

good precision. In the present case, a maximum

duty cycle of 60% for steady state operation has

been selected and an extra 5% is allowed to take

transients into account.

Since it is not requested a very tight tolerance on

the output, the feedback employs a primary side

voltage sensing technique to reduce cost and

complexity of the circuit. The same technique has

been used to protect against output overvoltages.

The electric schematic is shown in fig. 30. The

PCB layout is shown in figg. 31 and 32. Table 1

and 2 summarize typical system performance,

while table 3 lists the relevant bill of material,

where details are given only for critical compo-

nents and/or where useful.

Warning: the NTC for inrush current limitation is

not assembled, thus use caution when connecting

the demo board to the mains directly. The use of

a variac or an isolation transformer is recom-

mended.

Table 1. System efficiency.

Iout =1A

Iout =2A

Vin (Vac)

Vout (V)

Effic. %

Vout (V)

Effic. %

14.93

83.7

14.53

84.3

110

14.95

82.5

14.55

84.9

220

14.95

81.4

14.57

85.2

270

14.96

14.59

81.6

Table 2. System performance.

Line regulation

Vin = 85 to 270 Vac

Iout = 0.5A

30mV

Load regulation

Iout = 0.5A to 2A

Vin = 85 Vac

Vin = 270 Vac

0.95V

0.90V

Maximum effic.

Vin = 190 Vac

Iout =2A

86.2%

Output ripple

Vin = 85 to 270 Vac

Iout =2A

< 200mV

Minimum load

Vin = 270 Vac

Vout < 20V

150mA

Transition Volt.

From C.C.M to D.C.M

Iout =2A

160V

85 to

270

Vac

R12

R19

NTC

R24

R11

R15

R10

R23

R16

C12

R17

GND

D97IN499B

L4990

R13

C10

C13

R14

DCC

VREF

RCT

R22

C15

DC-LIM

SGND

C14

R20

R21

C11

PGND

ISEN

OUT

VCC

DIS

COMP

VFB

15V/2A

Figure 30. AC-DC adaptor electric schematic

L4990 - L4990A

15/24

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