MC34921 Datasheet, PDF (22/36 Pages) List of Unclassifed Manufacturers – Configurable Motor Driver IC with Power Supplies

FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
circuit, but otherwise allowing VCORE to follow the 3.3 V
switching regulator.
The VCORE SUPPLY terminal is the drain or collector of
the linear regulator transistor and must be tied to the 3.3V
terminal to use the internal regulator. This allows the option
of using an external regulator if the internal 3.3 V regulator
cannot supply enough current for a particular application.
Use of an external regulator requires leaving this terminal
open, thus disabling the internal regulator. The output of the
external regulator is then connected to the VCORE terminal
for under-voltage monitoring.
Table 5. V_CORE Regulator Output Voltage Select
V_{CORE SELECT}
V_{CORE_NOM}
(Volts)
Tied to Ground terminal
1.5
Tied to 3.3V terminal
1.8
Floating
2.5
DC Motor Drivers
There are two DC motor drivers on the 34921 IC: if used in
a printer application, for example, they might be the carriage
motor driver and the paper motor driver. A third drive, Motor
Driver C, can be configured as a DC motor driver or, when
B+_NOM = 18 V, as a step motor driver (refer to succeeding
paragraph Step Motor Driver). Configuration bit 13
determines the mode: 0 = step mode, 1 = DC mode. A step
motor driver can only be used in B+ = 12 V to 20 V
applications. Step motor outputs are suppressed by the
internal supervisor for B+ > 20 V. The ability to use the low-
side MOSFETs for general purpose low-side outputs is
included when the system is in DC motor mode (LSOUTx).
Alternatively, the ability to use the high-side MOSFETs for
general purpose high-side outputs (HSOUTx) has been
included when the system is in step mode. (Refer to
Table 20, page 31, and Table 21, page 32.)
The DC motor drivers are pulse width modulated (PWM’d)
via inputs from the digital subsystem on the APWM and
BPWM terminals, respectively. This signal is approximately
20 kHz to 40 kHz. The DC motor driver bridge direction may
be reversed while there is significant current flowing in the
motor. The purpose of this action is to brake the motor by
rapidly lowering the current. There are pull-downs on the
PWM input terminals so that, in the event of a connection
failure, the driver will default to a safe condition.
The DC motor drivers provide high-side and low-side
current limiting. The current limits have a 0.5 µs to 6.0 µs
deglitch filter, followed by an off-timer. The off-timer shuts off
the bridge long enough to meet the 4% duty cycle goal. The
motor drivers also have thermal shutdown protection.
Step Motor Driver C
Step motor driver C can be configured as a DC motor
driver or, when B+_NOM = 18 V, as a step motor driver (refer to
succeeding paragraph Step Motor Driver). Configuration
bit 13 determines the mode: 0 = step mode, 1 = DC mode.
The ability to use the low-side MOSFETs for general purpose
low-side outputs is included when the system is in DC motor
mode (LSOUTx). Alternatively, the ability to use the high-side
MOSFETs for general purpose high-side outputs (HSOUTx)
has been included when the system is in step mode. (Refer
to Table 20, page 31, and Table 21, page 32.)
Step motor driver C is PWM’d via an input from the digital
subsystem on the CPWMA/CDCPWM terminal. This signal is
approximately 20 kHz to 40 kHz. There are pull-downs on the
PWM input terminals so that DC motor drive C will default to
a safe condition in the event of a connection failure.
Step Motor Driver
A step motor driver can be configured as a DC motor driver
(refer to preceding paragraph DC Motor Drivers) or, when
B+_NOM ≤ 20 V, as a unipolar step driver. Serial input
configuration frame bit 4 determines the mode: 0 = step
mode, 1 = DC mode. A step motor driver will only be used in
B+ = 12 V to 20 V applications.
Note It is possible to use the step motor driver with
B+ > 20 V if the step motor is driven from a separate supply
that is ≤ 20 V.
The step motor driver on the 34921 is a unipolar, voltage-
mode wave drive circuit employing synchronous rectification.
The centertap of each phase-counterphase pair is connected
to B+.
Two PWM signals are sent directly from the digital
subsystem. The CPWMA/CDCPWM terminal provides the
PWM signal for the A and A outputs. The CPWMB terminal
provides the PWM signal for the B and B outputs. The step
motor driver employs synchronous rectification to control
substrate currents. In synchronous rectification, when an
output is turned off, the counterphase output MOSFET is
turned on to maintain current continuity. In order to avoid a
large shoot-through current, there is a dead time delay
(t_delay) between phase off and counterphase on. Refer to
Figure 5, Step Motor Crossover Delay Timing, page 16.
VBOOST Charge Pump
The high-side MOSFETs in the DC motor H-bridges and
the external GATEOUT switch need a gate voltage in excess
of B+, which is provided by the VBOOST supply. The
VBOOST regulator is a charge pump, switching directly off
the B+ supply and operating at 200 kHz.
34921
22
Analog Integrated Circuit Device Data
Freescale Semiconductor