MPZ2012S221A datasheet(31/56 Pages) STMICROELECTRONICS

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Application notes

9.1

Output voltage adjustment

Primary output voltage

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

below:

VOUT_prim=0.85∙1+RFB1RFB2

(22)

where RFB1 and RFB2 are the resistors used in the output divider (see figure below).

Figure 37. Primary output voltage regulation

PGOOD

BOOT

VBIAS

PGND

GND

VOUT_prim

VOUT_iso

GND_iso

L6983I

RFB1

RFB2

CSEC

COUT

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 9.2 Switching

frequency), generally summarized by the following equation:

D≥DMIN=TONMIN∙fSW

(23)

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,

and 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_prim+IOUT

pri ∙

(RDS(onLS+Rwind_pri)]∙ Lm

Lm+LLEAK

- Rwind_sec∙I

OUT_sec-VFD1

(24)

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

side respectively, VFD1 is the forward voltage of the Schottky diode, Lm is the magnetizing inductance of the

transformer and LLEAK the leakage inductance of the transformer.

If no current is drawn from the primary output, equation 24 can be simplified as follows:

VOUT_sec=N∙VOUTprim∙ Lm

Lm+LLEAK‐Rwind_sec∙IOUT_sec‐VFD1

(25)

L6983I

Application notes

DS14040 - Rev 1

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