MC34921 Datasheet, PDF (21/36 Pages) List of Unclassifed Manufacturers – Configurable Motor Driver IC with Power Supplies

FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
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 7 and Figure 8. 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.
34921
B+
5V SUPPLY
5V
5V SWITCH
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 I_5.0 ≤ 0.6 A for the
5.0 V switching regulator and I_3.3 ≤ 2.5 A for the 3.3 V
switching regulator. Each has cycle-by-cycle current limiting.
The power up sequence of the 5.0 V and 3.3 V switching
regulators is controlled such that -1.0v <= V5 - V3.3 <= 2.6v.
Figure 9 depicts the power-up sequence for the 5.0 V and
3.3 V regulators.
V_5.0 < V_{5_3.3t}
5V SELECT
Figure 7. 5.0 V Switching Regulator Mode
34921
5V SUPPLY
5V
B+
R
5V SWITCH
5V SELECT
B+ - 9.0 V
R=
I_OUT
Figure 8. 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,
V_5.0 ≥ V_{5_3.3t}
3.3 V ≠ Positive
Threshold
3.3 V Start
3.3 V Reg ON
Ref = 2.5 V
5.0 V Reg ON
3.3 V ≥ Positive
Threshold
Power-Up
5.0 V Reg ON
Ref = 2.5 V
3.3 V Reg OFF
5.0 V ≥ 3.3 V
Undervoltage (UV)
5.0 V Fault
5.0 V Reg ON
3.3 V Reg OFF
V_5.0 ≥ V_{5_3.3t}
& 3.3 V > 3.3 V UV
Full Start
Ref = 5.0 V
3.3 V Reg ON
5.0 V Reg ON
V_5.0 ≤ V_{5_3.3t}
Figure 9. Power-Up Sequencing
VCORE Linear Regulator
The output voltage of the VCORE linear regulator is
selectable for different applications. The output is selected
with an external pull-up or pull-down, which instructs internal
logic to select the appropriate regulator set-point (refer to
Table 5). The VCORE linear regulator is available whenever
the 3.3 V supply is in stable operation.
Current limiting is implemented to provide short circuit
protection. The VCORE linear regulator is shut off by the local
thermal shutdown sensor, thus protecting the 34921 IC from
an over-temperature condition resulting from a VCORE short
Analog Integrated Circuit Device Data
Freescale Semiconductor
34921
21