AS2844 datasheet(13/20 Pages) ETC

13 / 20 page

ASTEC Semiconductor

Current Mode Controller

AS2842/3/4/5

Table 1. RT vs Maximum Duty

Ratio

RT (

Ω)

Dmax

470

22%

560

37%

683

50%

750

54%

820

58%

910

63%

1,000

66%

1,200

72%

1,500

77%

1,800

81%

2,200

85%

2,700

88%

3,300

90%

3,900

91%

4,700

93%

5,600

94%

6,800

95%

8,200

96%

10,000

97%

18,000

98%

that compensates for all of the tolerances within

the device (such as the tolerances of VREG,

propagation delays, the oscillator trip points,

etc.) which have an effect on the frequency and

maximum duty ratio. For example, if the com-

bined tolerances of a particular device are 0.5%

above nominal, then ID is trimmed to 0.5% above

nominal. This method of trimming virtually

eliminates the need to trim external oscillator

components during power supply manufactur-

REG

−

≈

()

REG

0.736

REG

−

≈

()

0.4

where fosc is the oscillator frequency, D is the

maximum duty ratio, VH is the oscillator’s upper

trip point, VL is the lower trip point, VR is the

Reference voltage, ID is the discharge current.

Table 1 lists some common values of RT and the

corresponding maximum duty ratio. To select the

timing components; first, use Table 1 or equation

(2) to determine the value of RT that will yield the

desired maximum duty ratio. Then, use equation

(1) to calculate the value of CT. For example, for

a switching frequency of 250 kHz and a maxi-

mum duty ratio of 50%, the value of RT, from

Table 1, is 683

Ω. Applying this value to equation

(1) and solving for CT gives a value of 4700 pF. In

practice, some fine tuning of the initial values

may be necessary during design. However, due

to the advanced design of the AS2842 oscillator,

once the final values are determined, they will

yield repeatable results, thus eliminating the need

for additional trimming of the timing components

during manufacturing.

1.3.2 Oscillator enhancements

The AS2842 oscillator is trimmed to provide

guaranteed duty ratio clamping. This means that

the discharge current (ID ) is trimmed to a value

REG

=⋅ ()

( )

()

( )

()

=⋅ () − ()

−

−−

582

0 736

0 432

0 736

0 432

–

)

( .

)

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