MC34708 datasheet(63/159 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

MC34708

Functional Block Description

Figure 16. Voltage Stepping with DVS

7.5.5

Boost Switching Regulator

SWBST is a boost switching regulator with a programmable output, which defaults to 5.0 V on power up, operating at 2.0 MHz.

SWBST supplies the VUSB regulator for the USB PHY in OTG mode, as well as the VBUS voltage. Note that the parasitic

leakage path for a boost regulator will cause the output voltage SWBSTOUT and SWBSTFB to sit at a Schottky drop below the

battery voltage whenever SWBST is disabled. The switching NMOS transistor is integrated on-chip. An external fly back Schottky

diode, inductor, and capacitor are required.

Figure 17. Boost Regulator Architecture

SWBST output voltage programmable via the SWBST[1:0] SPI bits as shown in Table 51.

Table 51. SWBST Voltage Programming

Parameter

Voltage

SWBST Output Voltage

SWBST[1:0]

5.000 (default)

5.050

5.100

5.150

Actual

Output Voltage

Example

Actual Output

Voltage

Possible

Output Voltage

Window

Internally

Controlled Steps

Output Voltage

with light Load

Initial

Set Point

Voltage

Change

Request

Internally

Controlled Steps

Output

Voltage

Requested

Set Point

Initiated by SPI Programming,Standby Control

Request for

Higher Voltage

Request for

Lower Voltage

SWxP WGD

22uF

2.2uH

Output

Drive

SWBSTLX

GNDSWBST

SWBSTIN

Boosted Output

Voltage SWBST

SWBST

SWBSTIN

SWBSTFB

SPI

Registers

32KHz

Switcher

Core

Control

= Package Pin

4.7u

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