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| 型号： | MC33394 |
| 厂商： | 飞思卡尔半导体(中国)有限公司 |
| 元件分类： | CAN |
| 英文描述： | Switch Mode Power Supply with Multiple Linear Regulators and High Speed CAN Transceiver(带有多个线性稳压器和高速CAN收发器的开关方式电源) |
| 中文描述： | 开关电源的多线性稳压器和高速CAN收发器（带有多个线性稳压器和高速的CAN收发器的开关方式电源） |
| 文件页数： | 16/44页 |
| 文件大小： | 641K |
| 代理商： | MC33394 |
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Go to: www.freescale.com
4. FUNCTIONAL DESCRIPTION
The 33394 is an integrated buck regulator/linear supply
specifically designed to supply power to the Motorola
MPC55x/MPC56x microprocessors. A detailed functional
description of the Buck Regulator, Linear Regulators, Power
Up/Down Sequences, Thermal Shutdown Protection, Can
Transceiver Reset Functions and Reverse Battery Function
are given below. Block diagram of the 33394 is given in Figure
1. The 33394 is packaged in a 44 pin HSOP, 54 pin SOICW
and the 44 pin QFN.
4.1. Input Power Source (VBAT, KA_VBAT & VIGN)
The VBAT and KA_VBAT pins are the input power source
for the 33394. The VBAT pins must be externally protected
from vehicle level transients greater than +45 V and reverse
battery. See typical application diagram in Figure 1. The VBAT
pins directly supply the pre–regulator switching power supply.
All power to the linear regulators (except VKAM in the power
down mode) is supplied from VBAT through the switching
regulator. VKAM power is supplied through VBAT input pins
and switching regulator when the 33394 is awake. When the
microprocessor is in a power down mode (no VDDH or VDDL
supply), the current requirement on VKAM falls to less than 12
mA. During this period the VKAM current is supplied from the
reverse battery protected KA_VBAT input.
The KA_VBAT supply pin is the power source to the Keep
Alive Memory regulator (VKAM) in power down mode. Power
is continuously supplied regardless of the state of the ignition
switch (VIGN input). The KA_VBAT input is reverse battery
protected but requires external load dump protection (refer to
Figure 1).
The VIGN pin is used as a control input to the 33394. The
regulation circuits will function and draw current from VBAT
when VIGN is high (active) or REGON is high (active) or on
CAN bus activity
(WAKEUP active).
To keep the VIGN input
from floating, a 10k pull–down resistor to GND should be
used. The VIGN pin has a 3.0 V threshold and 1.0 volt of
hysteresis. VIGN is designed to operate up to +26.5 volt
battery while providing reverse battery and +45 volt load dump
protection. The input requires ESD, and transient protection.
See Figure 1 for external component required.
4.2. Switching Regulator Functional Description
A block diagram of the internal switching regulator is shown
in Figure 4. The switching regulator incorporates circuitry to
implement a Buck or a Buck/Boost regulator with additional
external components. A high voltage, low RDS(on) power
MOSFET is included on chip to minimize the external
components required to implement a Buck regulator. The
power MOSFET is a sense FET to implement current limit. For
low voltage operation, a low side driver is provided that is
capable of driving external logic level MOSFETs. This allows
a switching regulator utilizing Buck/Boost topology to be
implemented. Two independent control schemes are utilized
in the switching regulator.
In Buck mode, voltage mode pulse–width modulation
(PWM) control is used. The switcher output voltage divided by
an internal resistor divider is sensed by an Error Amplifier and
compared with the bandgap reference voltage. The PWM
Comparator uses the output signal from the Error Amplifier as
the threshold level. The PWM Comparator compares the
sawtooth voltage from the Ramp Generator with the output
signal from the Error Amplifier thus creating a PWM signal to
the control logic block. The Error Amplifier inverting input and
output are brought out to enable the control loop to be
externally compensated. The
compensation technique is
described in paragraph
5.2.3. Buck Converter Feedback
Compensation
in the Application Information section
.
In
order to improve line rejection, feed forward is implemented in
the ramp generator. The feed forward modifies the ramp slope
in proportion to the VBAT voltage in a manner to keep the loop
gain constant, thus simplifying loop compensation. At startup,
a soft start circuit lowers the current limit value to prevent
potentially destructive in–rush current.
In Boost mode, pulse–frequency modulation (PFM) control
is utilized. The duty cycle is set to 75% and the switching
action is stopped either by the Boost Comparator, sensing the
switcher output voltage VPRE, or by the Current Limit circuit
when the switching current reaches its predetermined limit
value
.
This control method requires no external components.
The selection of the control method is determined by the
control logic based on the VBAT input voltage.
4.2.1. Switching Transistor (SW1)
The internal switching transistor is an n–channel power
MOSFET. The RDS(on) of this internal power FET is
approximately 0.25 ohm at +125 C. The 33394 has a nominal
instantaneous current limit of 3.0 A (well below the saturation
current of the MOSFET and external surface mounted
inductor) in order to supply 1.2 A of current for the linear
regulators that are connected to the VPRE pin (see Figure 1).
The input to the drain of the internal N—channel MOSFET
must be protected by an external series blocking diode, for
reverse battery protection (see Figure 1).
4.2.2. Bootstrap Pin (BOOT)
An external bootstrap 0.1
μ
F capacitor connected between
SW1 and the BOOT pin is used to generate a high voltage
supply for the high side driver circuit of the buck controller. The
capacitor is pre charged to approximately 10V while the
internal FET is off. On switching, the SW1 pin is pulled up to
VBAT, causing the BOOT pin to rise to approximately
VBAT+10V — the highest voltage stress on the 33394.
4.2.3. External MOSFET Gate Drive (SW2G)
This is an output for driving an external FET for boost mode
operation. Due to the fact that the gate drive supply voltage is
VPRE the external power MOSFET should be a logic level
device. It also has to have a low RDS(on) for acceptable
efficiency. During buck mode, this gate output is held low.
4.2.4. Compensation (INV, VCOMP)
The PWM error amplifier inverting input and output are
brought out to allow the loop to be compensated. The
recommended compensation network is shown in Figure 18
and its Bode plot is in
Figure 19. The use of external
compensation components allows optimization of the buck
converter control loop for the maximum bandwidth. Refer to
the
paragraph
5.2.3.
Buck
Compensation
in the Application Information section for
further details of the buck controller compensation.
Converter
Feedback
4.2.5. Switching Regulator Output Voltage (VPRE)
The output of the switching regulator is brought into the chip
at the VPRE pin. This voltage is required for both the switching
regulator control and as the supply voltage for all the linear
regulators.
F
Freescale Semiconductor, Inc.
For More Information On This Product,
n
.
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