AN4461 datasheet(20/70 Pages) STMICROELECTRONICS

20 / 70 page

ZVS LC resonant stage description

AN4461

20/70

DocID026130 Rev 2

Step 3: Calculate the inductance value of the Lres, which is given by Equation 8 and

Equation 13:

Equation 15

Step 4: determine the capacitance value of the resonant capacitor Cres so that the peak

amplitude of the AC ripple voltage is much lower (e.g. always 10%) than the DC offset.

Indicatively:

Equation 16

Step 5: select the output capacitors Co so that they are rated for the maximum output

voltage VLED, they meet the output voltage ripple specification (if any) and have an

adequate AC current ripple rating.

The output current ripple specification must be met. To do so, at first we need to find the

value of the overall RLED for the LED string by considering the tangent to the forward

characteristic in the specified operating point. Then Equation 17 can be used to find the

suitable value for Co and its ESR.

Equation 17

The maximum allowed ripple current for Co must be:

Equation 18

4.1.3

Half-bridge operating parameters

In order to generate a proper oscillation into the LC resonant tank, the half-bridge must

generate a square wave having a 50% of duty cycle. Any unbalancing of this duty cycle may

cause the loss of zero voltage switching condition and, as a consequence, increases the

risk of components overstress.

The adoption of the same conduction time (TON) for both the halfbridge MOSFETs ensures

this condition. When one MOSFET is turned off then the middle point of the half-bridge,

thanks to resonance, moves to the opposite side of the half-bridge, in a time that depends

on the overall capacitance (real or parasitic) connected to this node.

In order to avoid undesired switching losses, a deadtime (see “hf” command), longer than

this transition time, shall be applied between the time one side of the half-bridge switches off

and the time the opposite side turns on.

In order to correctly regulate the LED current, it is important to accurately detect the instant

of the peak of the resonant current. The peak is detected by the STLUX385A via a shunt

resistor placed between the source of the low-side MOSFET and ground. The sampling time

( )

(

)

Vdout

VLED

min

Vpfc,

ILED

Lres

min

hb,

⋅

min

hb,

min

Vpfc,

ILED

Cres

⋅

≥

min

hb,

ESR

RLED

ILED

⋅













⋅

≥

⋅

∆

ILED

∆ICo ≥

AN4461 Datasheet(PDF) 20 Page - STMicroelectronics

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