L6986I datasheet(28/60 Pages) STMICROELECTRONICS

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Application notes

6.1

Output voltage adjustment

Primary output voltage

The error amplifier reference voltage is 0.85 V typical. The output voltage is adjusted accordingly to equation

below:

VOUT_pri= 0.85∙1+R1R2

(23)

where R1 and R2 are the resistors used in the output divider (see figure below).

Figure 31. L6986I application circuit

signal GND

power GND

CCOMP

COUT_pri

CIN

VOUT_sec+

CSS

VIN

GND

CIN

L6986I

RST

VCC

SS/INH

SYNCH

FSW

MLF

COMP

DELAY

SGND

PGND

VIN

BIAS

VOUT_pri

RCOMP

CVCC

COUT_sec

VOUT_sec-

The integration of a P-channel MOSFET as high side switch theoretically allows duty cycle up to 100%.

Nevertheless, in an iso-buck architecture some restrictions should be considered. A general recommendation

is to keep the duty-cycle below 50-60%. A higher duty cycle would limit off time, limiting the time in which the

energy transfer to the secondary side takes place.

Other restrictions in the duty cycle selection arise due to the minimum on time (see Section 6.2 ), generally

summarized by the following equation:

D ≥ DMIN = TON MIN∙fSW

(24)

Secondary output voltage

In the iso-buck converter, the secondary output voltage is not included in the regulation loop. Nevertheless, a

good regulation of the secondary output voltage is achieved by relying on the primary output voltage regulation,

the selection of a proper turn ratio of the transformer as well as considering the voltage drops due to the

secondary winding resistance and the Schottky diode. That can be summarized by the following equation:

VOUT_sec=N∙VOUT_pri+IOUTpri∙RDSonLS+Rwind_pri ‐Rwind_sec∙IOUT_sec‐VFD1

(25)

Where N is the turn ratio, Rwind_prim and Rwind_sec are the winding resistances of the primary and secondary

side respectively and VFD1 is the forward voltage of the Schottky diode.

If no current is drawn from the primary output, the equation (25) can be simplified as follows:

VOUT_sec=N∙VOUTpri−Rwind_sec∙IOUT_sec−VFD1

(26)

Equation (26) emphasizes how the selection of the transformer and, to some extent, the Schottky diode plays a

crucial role in the accurate regulation of the secondary output voltage. The figure below shows instead the effect

of the current drawn from the primary output on the isolated voltage, as described by the eq. (9).

L6986I

Application notes

DS13647 - Rev 1

page 28/60

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