34921 datasheet(21/36 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34921

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

Figure 7. Internal Block Diagram

5.0 V AND 3.3 V REGULATORS

The 34921 5.0 V regulators have two operating modes—

switching and linear—that share a dedicated input terminal,

as illustrated in Figure 8 and Figure 9. The 5.0 V switching

regulator operates off B+ directly. The 5.0 V linear regulator

is only used when B+ < 20 V, and the dedicated input

terminal is connected to B+ through an external power

resistor to dissipate some power off-chip. The regulator that

is used depends on the power requirement and B+NOM of the

application. The designer is able to trade off power versus

overall system cost for each particular application. The linear

regulator mode is a low-current mode and has much less

external component cost.

Figure 8. 5.0 V Switching Regulator Mode

Figure 9. 5.0 V Linear Regulator Mode

The 5V SELECT terminal must be tied to ground for

switching regulator mode. An internal pull-up is incorporated

in the 34921 sufficient to avoid any problems owing to

switching noise on this terminal.

The 5.0 V switching and linear regulators may supply

external logic components of the overall assembly,

depending on the application. For the 5.0 V linear regulator,

an external capacitor on the output should be used for

filtering.

5.0 V AND 3.3 V SWITCHING REGULATORS

The 5.0 V and 3.3 V switching regulators are implemented

as constant ripple buck regulators. These regulators operate

in both discontinuous and continuous mode. The clock

source is the on-board 200 kHz master oscillator. The actual

frequency of the switch terminal can vary owing to cycle

skipping. The switch MOSFET is internal to the 34921 IC, but

the remaining components—recovery diode, inductor, and

output capacitor—must be externally supplied. The input

voltage to the regulators is B+, and the regulators perform

within specifications over the range of I5.0 ≤ 0.6 A for the

5.0 V switching regulator and I3.3 ≤ 2.5 A for the 3.3 V

switching regulator. Each has cycle-by-cycle current limiting.

5.0 V Regulator

Switching and

Charge

Pump

DC Motor

Drivers

Timing

Logic

Step

Motor Driver

V Core

Linear Regulator

A/D

Convertor

Linear

3.3 V Regulator

Switching

Supervisory

RST Function

34921

5V SUPPLY

5V SWITCH

5V SELECT

B+

34921

5V SUPPLY

5V SWITCH

5V SELECT

B+

R =

B+ - 9.0 V

IOUT

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