HVLED007 datasheet(17/33 Pages) STMICROELECTRONICS

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DS12866 Rev 1

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HVLED007

Application information

With the HVLED007, since Rt is fixed inside the control IC (Rt = 8.3 k) the design objective

is to properly select the value of Ct. This is done looking basically at two interrelated factors:

the amplitude of the ripple voltage

VCt (t, θ) across Ct at the switching frequency and the

Total Harmonic Distortion (THD) of the input current. The amplitude of

VCt (t, θ) affects also

the maximum available signal on the current sense input and, then, the selection of the

sense resistor Rs. Please refer to "Section 4.7: Current sensing input (pin CS). Sense

resistor (Rs) selection" for more information concerning this point.

The ripple voltage, generated by the charge and discharge of Ct through IMULT (θ) and Rt,

can be considered essentially triangular and symmetrically superimposed on the slowly

varying (at 2 fline) DC value VCt (θ). Its peak-to-peak amplitude is given in equation #4

contained in Table 7. As discussed in [Reference 1], this ripple causes distortion of the

current reference and a reduction of the programmed peak current. Both can be significant if

the ripple amplitude is not adequately low.

As a rule of thumb, the maximum peak-to-peak value of

VCt (t, θ) should not exceed 10%

of the maximum amplitude of VCt (θ). Both amplitudes are at a maximum at the maximum

input power (Pin = Pin_max) and minimum input voltage (Kv = Kv_min).

Combining equations #3 and #4 of Table 7, this condition can be expressed as:

Equation 10

With this choice the programmed peak current Vcsref (θ) - Vcsref (θ) /2 is less than 5%

lower than the value calculated with equations #1 and #2 in Table 8, which neglect the

contribution of the ripple (see "Section 4.7: Current sensing input (pin CS). Sense resistor

(Rs) selection" for more details). The distortion caused by the ripple does not exceed 2.5%

and the resulting time constant Rt Ct is typically in the range of 4 to 5 times the maximum

switching period T(θ)max. Experience shows that this value is close to the value that

minimizes the THD of the input current for line frequencies in the range of 47 to 63 Hz,

which is the range typically specified for applications running from most of the power lines

worldwide.

4.5

Control input for isolated feedback and optocoupler driving

(pin COMP)

Pin COMP is to be driven by the phototransistor (emitter-grounded) of an optocoupler to

modulate its voltage by modulating the current sourced by the pin. Figure 8 shows the

output characteristic of the pin.

Vcs

ref

Vcsref

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

Rt Ct

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

Lp Pin_max

Kv_min VR

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

 0.1



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