34701 datasheet(18/38 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34701

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

INTRODUCTION

The 34701 incorporates a high-performance synchronous

buck regulator, supplying the microprocessor’s core, and a

low dropout (LDO) linear regulator providing the

microprocessor I/O and bus voltages. This device

incorporates many advanced features; e.g., precisely

maintained up/down power sequencing, ensuring the proper

operation and protection of the CPU and power system.

Figure 11. 34701 Functional Internal Block Diagram

BOOST REGULATOR

A boost regulator provides a high voltage necessary to

properly drive the buck regulator power MOSFETs,

especially during the low input voltage condition. The LDO

regulator external N-channel MOSFET gate is also powered

from the boost regulator. In order to properly enhance the

high-side MOSFETs when only a +3.3 V supply rail powers

the integrated circuit, the boost regulator provides an output

voltage of 7.75 V nominal value.

The 34701 boost regulator uses a simple hysteretic

current control technique, which allows fast power-up and

does not require any compensation. When the boost

regulator main power switch (low side) is turned on, the

current in the inductor starts to ramp up. After the inductor

current reaches the upper current limit (nominally set at

1.0 A), the low-side switch is turned off and the current

charges the output capacitor through the internal rectifier.

When the inductor current falls below the valley current limit

value (nominally 600 mA), the low-side switch is turned on

again, starting the next switching cycle. After the boost

regulator output capacitor reaches approximately 6.0 volts,

the peak and valley current limit levels are proportionally

scaled down to approximately one fifth of their original values.

When the boost regulator reaches its regulation limit (7.75 V

typical), the low-side switch is turned off until the output

voltage falls below the regulation limit again.

The higher current limit values in the beginning of the

boost regulator start-up sequence allow fast power up of the

whole IC, while the normal operation with reduced current

limit greatly reduces the switching noise and therefore

improves the overall EMC performance. See Figure 12 for

the boost regulator output voltage and inductor current

waveforms (picture not to scale).

Boost Regulator

Reset Control

VDDI

Power Sequencing

I2C Interface

Voltage Margining

Internal

Bandgap

Voltage

Thermal Shutdown

Switcher

UVLO

Buck Control Logic

Linear Regulator

ILIM

Buck HS and

LS Driver

Control

Oscillator

300 kHz

Watchdog Timer

Reference

Supply

POR Timer

Bandgap

Voltage

Reference

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