L6986ITR datasheet(10/60 Pages) STMICROELECTRONICS

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4.1.2

Switchover feature

The switchover scheme of the pre-regulator block is used to derive the main contribution of the supply current for

the internal circuitry from an external voltage (3 V < VBIAS < 5.5 V is typically connected to the primary regulated

output voltage). This helps to decrease the equivalent quiescent current seen at VIN.

4.1.3

Voltage monitor

An internal block continuously senses the VCC, VBIAS and VBG (bandgap). If the monitored voltages are good, the

regulator starts operating. There is also a hysteresis on the VCC (UVLO).

4.1.4

Error amplifier

The voltage error amplifier is the core of the loop regulation of the primary output voltage. It is a transconductance

operational amplifier whose non inverting input is connected to the internal voltage reference (0.85 V), while the

inverting input (FB) is connected to the external divider or directly to the output voltage.

Table 8. Uncompensated error amplifier characteristics

Description

Value

Transconductance

155 µS

Low frequency gain

100 dB

The error amplifier output is compared with the inductor current sense information to perform PWM control.

Figure 6. Internal circuit

STARTER

IC BIAS

PREREGULATOR

BANDGAP

VREG

VREF

VCC

4.2

Transformer

The transformer is the key component for the iso-buck, ensuring the desired isolation as well as allowing the

energy transfer to the secondary side, hence generating the secondary isolated output voltage.

More details about the transformer selection are provided in the dedicated section (see Section 4.4 .)

4.3

Secondary side

The secondary side includes an LC filter (secondary winding of the transformer and secondary output capacitor)

and the rectifying element (Schottky diode).

L6986I

Transformer

DS13647 - Rev 1

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