AS2845D-8 datasheet(19/20 Pages) ETC

19 / 20 page

ASTEC Semiconductor

Current Mode Controller

AS2842/3/4/5

graphically in Figure 22.

First, current-mode controllers detect and control

the peak

inductor current, where as the

converter’s output corresponds to the average

inductor current. Figure 22(a) clearly shows that

the average inductor current (I1 & I2) changes as

the duty ratio (D1 & D2) changes. Note that for a

fixed control voltage, the peak current is the

same for any duty ratio. The difference between

the peak and average currents represents an

error which causes the converter to deviate from

true current-mode control.

Second, Figure 22(b) depicts how a small pertur-

bation of the inductor current (

∆I) can result in an

unstable condition. For duty ratios less than 50

%, the disturbance will quickly converge to a

steady state condition. For duty ratios greater

than 50 %,

∆I progressively increases on each

cycle, causing an unstable condition.

Both of these problems are corrected simulta-

neously by injecting a compensating ramp into

either the control voltage (VE) as shown in Figure

22(c) & (d), or to the current sense waveform at

pin 3. Since VE is not directly accessible, and, a

positive ramp waveform is readily available from

the oscillator at pin 4, it is more practical to add

the slope compensation to the current waveform.

This can be implemented quite simply with the

addition of a single resistor, RSLOPE, between pin

4 and pin 3 as shown in Figure 23(a). RSLOPE, in

conjunction with the leading edge filter resistor,

R1 (Section 2.1), forms a divider network which

determines the amount of slope added to the

waveform. The amount of slope added to the

current waveform is inversely proportional to the

value of RSLOPE. It has been determined that the

amount of slope (m) required is equal to or

greater than 1/2 the downslope (m2) of the induc-

tor current. Mathematically stated:

≥

(7)

In some cases the required value of RSLOPE may

be low enough to affect the oscillator circuit and

thus cause the frequency to shift. An emitter

follower circuit can be used as a buffer for RSLOPE

as depicted in Figure 23(b).

Slope compensation can also be used to improve

noise immunity in current mode converters oper-

ating at less than 50% duty ratio. Power supplies

Figure 23. Slope Compensation

RSLOPE

ISENSE

VREG

RT/CT

AS2842

GND

RSLOPE

ISENSE

VREG

RT/CT

AS2842

GND

(a)(b)

OPTIONAL

BUFFER

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