FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
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 10 depicts the power-up sequence for the 5.0 V and
3.3 V regulators.
external regulator is then connected to the VCORE terminal
for under-voltage monitoring.
Table 5. VCORE Regulator Output Voltage Select
VCORE_NOM
(Volts)
VCORE SELECT
V
< V
5_3.3
5.0
t
Tied to Ground terminal
Tied to 3.3V terminal
Floating
1.5
1.8
2.5
Power-Up
5.0 V Reg ON
Ref = 2.5 V
3.3 V Reg OFF
V
≥ V
5_3.3t
5.0 V ≥ 3.3 V
Undervoltage (UV)
5.0
3.3 V ≠ Positive
Threshold
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
3.3 V Start
3.3 V Reg ON
Ref = 2.5 V
5.0 V Fault
5.0 V Reg ON
3.3 V Reg OFF
5.0 V Reg ON
V
≥ V
5_3.3t
5.0
& 3.3 V > 3.3 V UV
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.)
Full Start
3.3 V ≥ Positive
Threshold
Ref = 5.0 V
3.3 V Reg ON
V
≤ V
5_3.3t
5.0
5.0 V Reg ON
Figure 10. 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.
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.
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
circuit, but otherwise allowing VCORE to follow the 3.3 V
switching regulator.
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.
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
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.)
34921
Analog Integrated Circuit Device Data
Freescale Semiconductor
22
FREESCALE [ Freescale ]
