HVLED007 datasheet(21/33 Pages) STMICROELECTRONICS

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DS12866 Rev 1

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HVLED007

Application information

and the programmed peak current is actually:

Equation 13

which can be derived from equations #3 and #4 in Table 7. As previously mentioned, if Ct is

selected with the criterion proposed in "Section 4.4: Shaping capacitor (Ct) selection (pin

CT)", this peak is no more than 5% lower than the value provided by equation #1 in Table 8.

The control loop easily compensates for that generating a Vcsref (θ) only slightly larger than

that predicted by equation # 3 in Table 7 by positioning the control voltage VCOMP at a level

slightly higher than that predicted by equation #1 in Table 7. However there is one more

point to be considered and that can be clarified with the aid of Figure 9:

 Case a) is that of normal operation just mentioned, i.e. with Vcsref (θ) more than ΔVcsref

(θ) / 2 away from Vcsmax so that the current sense signal is entirely in its linear operation

range.

 Case b) occurs when Vcsref (θ) is less than ΔVcsref (θ) / 2 away from Vcsmax; the peaks

of the ripple (Vcsref (θ) + ΔVcsref (θ) / 2) is clamped at Vcsmax. This impacts indirectly on

the envelope of the valleys Vcsref (θ) - ΔVcsref (θ) / 2 and, then, on Ippk (θ). In fact, as the

peaks of the ripple start being clamped, the smoothing resistor Rf has a loading effect on

the ICS circuit: the current flowing through Rf to the clamp is stolen from that charging Ct

so that the slope of the ascending ramps gets lower, whereas the descending ramps get

steeper. This alters the operation of the ICS circuit and results in a Vcsref (θ) and an

envelope of the valleys that are slightly flattened around the top of the sinusoid. In turn,

this flattens the peak of the input current sinusoid, increasing the THD.

 Case c) occurs when the peaks of the ripple (Vcsref (θ) + ΔVcsref (θ) / 2) not only exceed

Vcsmax with all the consequences discussed in the previous case but also the level

VCT_OL, which trips the overload protection (see "Section 4.9: Overload and short-circuit

protection (OCP function)"). This should be the case of a real overload but note that this

may occur - normally at low input voltage - even before Vcsref (θ) is clamped at Vcsmax

when ΔVcsref (θ) is too large (too small Ct and/or too large Rs). This is not the case if Ct

is selected with the criterion proposed in "Section 4.4: Shaping capacitor (Ct) selection

(pin CT)": ΔVcsref (θ) / 2 does not exceed 0.05·1.4 = 70 mV, while the difference VCT_OL

- Vcsmax is always greater than 100 mV.

Therefore, as a conclusion, for a given Ct (again, selected as per proposed criterion), if the

additional distortion caused at low line by the peaks of the ripple being clamped is

acceptable one can consider using the full dynamics of the current sense or nearly so, so

that the value of Rs is found from:

Equation 14

If the additional distortion is to be prevented, the full dynamics needs to be reduced

accordingly; then:

Ip pk 

 = 4

V R

------- sin



1+Kv sin

--------------------------- 1

2Rt Ct

----------------

4Lp Pin

Kv VR

-------------------- sin



–







Vcsmax_min

Ip pk_max

-------------------------- =

1.2

Ip pk_max

----------------------



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