34704 datasheet(20/54 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34704

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

Figure 4. MC34704 Functional Internal Block Diagram

INTERNAL BIAS CIRCUIT

Gate Driver Voltage (VG)

REG1/VG is the main regulator of the 34704 IC and will be

used to supply internal circuitry and voltage biases through

the VG output. It also provides the gate drive voltage for the

rest of the regulators and itself.

See Power-Up Sequence on page 28 for more details on

how REG1 is a critical part of powering up the 34704. Based

on this, REG1 will need extra circuitry to help it boot up until

its output voltage is high enough that it can supply internal

circuitry for the main control loop to take over.

REG1 VG starts up in peak current detect PFM mode and

REG1 VG output starts rising. When the appropriate internal

circuitry is alive and the switching frequency FSW1 is

selected, the PWM control of REG1 can take over.

VREF Generator - Internal Reference

Each one of the regulators in the 34704 uses a DAC which

is controlled by the I2C interface to generate a dynamic VREF

voltage for setting the output voltage on each regulator.

VDDI Reference Voltage

The 34704 uses the internal VG voltage to provide a

precise low current 2.5 V voltage that is meant to serve as

reference voltage to derive the FREQ and SS voltage needed

to set the switching frequency 1 (FSW1) and the soft start,

respectively.

FAULT DETECTION AND PROTECTION

Thermal Limit Detection

There is a thermal sensor for each regulator except REG7.

All regulators of the corresponding group will shutdown if at

least one of them reaches the thermal limit. If either REG2,

REG3 or REG4 reaches its thermal limit, the whole part will

shutdown immediately.

Over-Current & Short Circuit Monitoring

The current limit circuitry has two levels of current limiting:

• A soft over-current limit (over-current limit): If the peak

current reaches the typical over-current limit, the switcher

will start a cycle-by-cycle operation to limit the current and

a 10 ms current limit timer starts. The switcher will stay in

this mode of operation until the part regains normal

34704 Datasheet(PDF) 20 Page - Freescale Semiconductor, Inc

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