AS2845D-8 datasheet(18/20 Pages) ETC

18 / 20 page

AS2842/3/4/5

Current Mode Controller

ASTEC Semiconductor

latch is still fully operative, and the normal termi-

nation of this cycle by the current sense com-

parator will latch the output low until the over-

temperature condition is rectified. Cycling the

power will reset the over-temperature disable

mechanism, or the chip will re-start after cooling

through a nominal hysteresis band.

Section 2 – Design Considerations

2.1 Leading edge filter

The current sensed by RS contains a leading

edge spike as shown in Figure 20. This spike is

caused by parasitic elements within the circuit

including the interwinding capacitance of the

power transformer and the recovery characteris-

tics of the rectifier diode(s). The spike, if not

properly filtered, can cause stability problems by

prematurely terminating the output pulse.

A simple RC filter is used to suppress the spike.

The time constant should be chosen such that

it approximately equals the duration of the spike.

A good choice for R1 is 1 k

Ω, as this value

is optimum for the filter and at the same

time, it simplifies the determination of RSLOPE

(Section 2.2). If the duration of the spike is, for

example, 100 ns, then C is determined by:

Time Constant

(6)

100

Ω

2.2 Slope compensation

Current-mode controlled converters can experi-

ence instabilities or subharmonic oscillations

when operated at duty ratios greater than 50%.

Two different phenomena can occur as shown

Figure 22. Slope Compensation

IPK

IL2

IL1

IAVG 2

IAVG 1

(a)

∆I

(b)

∆I'

IL2

IL1

IAVG 1 = IAVG 2

(c)

m = m

2/2

∆I

(d)

∆I'

m = m

2/2

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