FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
values for V1 and V2 voltage regulators on page 6 of the
Static Electrical Characteristics table.
phase from the V1 regulator to minimize ripple current on
VB+.
This converter is designed so that the current limit
threshold is lowered during the power-on period to allow for a
“softer” start-up, thereby reducing electrical stress in the
external components. This limiting is required for their safe
operation.
POWER-SAVING MODE OF OPERATION
The V2 voltage regulator can be disabled via the serial
interface by setting the V2_Enable bit (bit 1–LSB) to a value
of 0. This provides a reduction in the bias current provided by
the V1 supply.
The output voltage is variable with ±2% precision, with a
VVB+ supply voltage range of 21 V to 42 V. The exact voltage
V1 VOLTAGE REGULATOR
will be set externally with a resistor (1% tolerance) divider
network. The V2_FB input voltage should be chosen, using
external voltage divider resistors, so as to provide a regulator
feedback voltage, for the required output regulated voltage,
to equal the internal regulator reference voltages of 2.50 V
±2%.
Implementation of the V1 switching voltage regulator is
accomplished through the use of an internal switch MOSFET,
internal MOSFET current sense resistor, external Schottky
diode, external inductor, and filter capacitor. The frequency of
operation of this regulator is controlled by the internal clock,
which is 200 kHz ±25 kHz. The duty cycle (on-time) for this
internal regulator clock is a fixed 37.5%. This regulator
switches out of phase from the V2 regulator to minimize ripple
Output current sensing is implemented by sensing the
voltage across an internal sense resistor connected between
VB+ and the drain of the internal MOSFET. Current is
measured on a cycle-by-cycle basis. The purpose of this
current sense is to prevent any damage to the 34920 and its
associated external components.
current on VB+. The line regulation range is 21 V < VVB+
<
42 V. The load side regulation is specified on page 6 of the
Static Electrical Characteristics table.
This converter is designed so that the current limit
threshold is lowered during the power-on period to allow for a
“softer” start-up, thereby reducing electrical stress in the
external components. This limiting is required for their safe
operation.
Note There is a V2_Enable bit in the Serial
Communication Input register (bit 1). When this bit is set to
logic [1], the V2 voltage regulator is enabled. When this bit =
logic [0], the V2 voltage regulator is disabled. Refer to
Tables 5 through 9, pp. 13–17, for a description of this bit.
The V2_Enable bit will also disable the motor drivers.
The voltage is set externally with a resistor (1% tolerance)
divider network. The V1_FB input voltage should be chosen,
using external voltage divider resistors, so as to provide a
regulator feedback voltage, for the required output regulated
voltage, to equal the internal regulator reference voltage of
2.50 V ±2%. The V1 regulator is ideal for providing either
3.3 V or 5.0 V with a precision of +5%/-4%.
VB GENERATOR
The boost voltage generator circuit is a charge pump
circuit using two external capacitors to provide the necessary
voltage to drive internal 34920 loads. This circuit is driven at
a frequency of 200 kHz ±25 kHz.
Output current sensing is implemented by sensing the
voltage across an internal sense resistor connected between
VB+ and the drain of the internal MOSFET. Current is
measured on a cycle-by-cycle basis. The purpose of this
current sense is to prevent damage to the 34920 and its
associated external components.
The VB generator is utilized exclusively by the 34920.
There is no provision for external loading. Also, there is no
disable feature for the VB generator.
MOTOR DRIVE SYSTEMS
The 34920 provides two motor drivers. Both drivers are
mode selectable to be either a multi-current level bi-
directional driver for bipolar step motors or a bi-directional DC
motor driver with PWM control. The DR1_MODE (Mode1)
and DR2_MODE (Mode2) pins select whether the
appropriate motor driver will drive a step motor (pin = 1) or DC
motor (pin = 0). Figures 7 and 8 depict the two motor
configurations.
V2 VOLTAGE REGULATOR
The V2 switching voltage regulator is implemented as a
buck regulator with an internal switch MOSFET, internal
MOSFET current sense resistor, external Schottky diode,
external inductor, and filter capacitor. The frequency of
operation of this regulator is controlled by the internal clock,
which is 200 kHz ±25 kHz. This regulator switches out of
34920
Analog Integrated Circuit Device Data
Freescale Semiconductor
19
FREESCALE [ Freescale ]
