MC33394DH Freescale Semiconductor, MC33394DH Datasheet - Page 30

• Manufacturer Part Number: MC33394DH
• Description: IC POWER SUPPLY MULT-OUT 44-HSOP
• Manufacturer: Freescale Semiconductor

Specifications of MC33394DH

• Applications: Motorola MPC55x, MPC56x Microprocessors
• Interface: SPI Serial
• Voltage - Supply: 4 V ~ 26.5 V
• Package / Case: 44-BSOP (0.433", 11.00mm Width) Exposed Pad
• Mounting Type: Surface Mount
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

5.2.1.4. Selecting the Power MOSFET Q2
voltage drop across the output schottky rectifier D2 gives the
MOSFET’s maximum drain–source voltage stress:
breakdown voltage parameter is not critical.
capability. The external power MOSFET has to withstand
higher currents than the upper current limit of the 33394:
I DQ2 >3A
converter to its minimum, a very low R DSon power MOSFET
has to be selected. Moreover, due to the fact that the 33394
external MOSFET gate driver is supplied from VPRE, in
order to assure proper switching of Q2 a logic level device
has to be selected.
harsh automotive environment with low thermal resistance.
MTD20N03HDL power MOSFET from ON Semiconductor.
5.2.1.5. Selecting the Boost Converter Output
Rectifier D2
used to select the boost converter output rectifier. Its reverse
breakdown voltage is not a critical parameter:
V rD2 >V o =6 V
is the 33394 internal switch upper current limit I lim(max) .
which has to be minimal. This parameter is also crucial for the
proper 33394 switcher functionality, and especially for proper
transition between the buck and boost modes.
parameters play a significant role when selecting the output
rectifier D2.
schottky rectifier from Microsemi, Inc.
30
The boost converter maximum output voltage plus the
BV dsQ2 >V o +V fwd2 = 6 V+0.5 V, as can be seen, the
The more important in our case is the Q2 current handling
In order to keep the power dissipation of the 33394 boost
Last but not least, the Q2 package has to suitable for the
These requirements are met, for example by the
Criteria similar to that of selecting the power MOSFET was
The D2 rectifier has to withstand higher peak current than
The most important parameter is its forward voltage drop,
Finally, its switching speed, forward and reverse recovery
These requirements are met, for example by the HSM350
V in
+
S
–
V out /V in = G/(1 + GH)
(Modulator)
Gain Block
Feedback
Block
G
H
Freescale Semiconductor, Inc.
For More Information On This Product,
Figure 17. The Buck Converter Control Loop
V out
Go to: www.freescale.com
33394
MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA
5.2.2. Input Filter Selection
cold crank condition, the 33394 EMC (electromagnetic
compatibility) performance is not of concern during this mode
of operation. Therefore, only the Buck mode of operation is
important for selecting the appropriate input filter. For the
Buck converter topology (see Figure 13) the low impedance
3rd order filter (C3, L2, C4 and C26 in the Application
Schematic Diagram Figure 20) offers a good solution. It can
be seen from the Buck converter current waveforms that
comparatively high current pulses are drawn from the
converter’s input source. The filter inductance must be kept
minimal and the capacitor, which is placed right next to the
power switch, must be sized large enough to provide
sufficient energy reservoir for proper switcher operation.
to be sufficiently low to reduce the switching ripple of the
switcher input node VBAT. There are three main reasons to
keep the voltage ripple of the VBAT pin at its minimum. First,
it is the EMC (electromagnetic compatibility) performance of
the switcher in the normal operating mode (buck mode).
Second, it allows a smooth transition between the boost and
buck mode of operation. Third, it helps to avoid entering an
undervoltage condition too early. A practical way to achieve
sufficiently low ESR of the switcher input capacitor, even at
low temperature extremes, is to use several high value
ceramic capacitors in parallel with a large electrolytic
capacitor. These capacitors should be physically placed as
close to the VBAT pins as possible.
5.2.3. Buck Converter Feedback Compensation
Figure 17. The loop consists of a power processing block —
the modulator in series with an error–detecting block — the
Error (Feedback) Amplifier. In principle, a portion of the
output voltage (VPRE of the 33394 switcher) is compared to
a reference voltage (V bg ) in the Error Amplifier and the
difference is amplified and inverted and used as a control
input for the modulator to keep the controlled variable (output
voltage VPRE) constant.
Since the switcher will work in the Boost mode only during
The ESR of this input capacitor combination C4, C26 has
A typical control loop of the buck regulator is shown in
V in
Ramp
Reference
Voltage
ERROR FEEDBACK AMPLIFIER
–
+
Signal
PWM
Z f
–
+
MODULATOR
Z in
+
V out
To Load