SABRE-LL-I datasheet(38/141 Pages) STMICROELECTRONICS

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Main switching regulator

SABRE-LL-I

38/141

Main switching regulator

12.1

Overview

Main switching regulator is an asynchronous switching regulator intended to be the source

of the main voltage in the system. It implements a soft start strategy and could be a system

regulator so even if its output voltage VMAIN_SW is not used to power any internal circuit,

S.A.B.Re will check that it is in the good value range before enabling all its internal

functions. When S.A.B.Re detects a system regulator under-voltage event with a duration

longer than the period defined by the deglitch filter (Tprim_uv), it will enter in reset state

signaling this event to the microcontroller by pulling low the nRESET pin and disabling most

of its internal block (e.g. bridges, GPIOs, …).

The output voltage will be externally set by a divider network connected to feedback pin. To

reduce as much as possible the regulation voltage error S.A.B.Re has the possibility to

choose between four feedback voltage references (and, as a consequence, four under-

voltage thresholds) using the serial interface. The feedback reference voltage selection is

made by writing the SelFBRef bits in the MainSwCfg register according to the table here

below:

Reference voltage range can be changed by using a metal layer change in order to adapt

them to customer system.

Here after are summarized the primary features of this regulator:

–

Internal power switch.

–

Soft start circuitry to limit inrush current flow from primary supply.

–

Internally generated PWM (250kHz switching frequency).

–

Nonlinear pulse skipping control.

–

Protected against load short circuit.

–

Cycle by cycle current limiting using internal current sensor.

–

Under voltage signal (both continuous and latched) accessible through SPI.

When S.A.B.Re is in “Low Power mode”, this regulator will be disabled.

In order to save external components and power when using two or more S.A.B.Re IC’s on

the same board, the primary switching regulator can be disabled by serial interface. Care

must be paid using this function because an under-voltage on this regulator, as previously

seen, will be read as a fault condition by S.A.B.Re.

Table 18.

Switching regulator controller PWM specification

MainSwCfg register

Reference voltage (VFBREF)

Unit

Comments

SelFBref[1]

SelFBref[0]

Min

Typ

Max

0.776

0.8

0.824

0.97

1.03

Default state

2.425

2.5

2.575

2.910

3.09

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