33710 Просмотр технического описания (PDF) - Freescale Semiconductor
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33710 Datasheet PDF : 17 Pages
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TYPICAL APPLICATIONS
TYPICAL APPLICATIONS
B+
13 V - 32 V
C1
330 µF
SW1
R1
1K
C1
0.1 mF
1
2
3
4
RST
MODE0
MODE1
MODE2
SUPERVISORY
& SHUTDOWN
CHARGE
PUMP
BUCK
REG
LDO
CT
CP1
CP2
32
31
30
VB
B+
VSW
VFB
LINB
29
28
27
26
25
24
VCORE 23
C5
0.1 µF
C6
10 µF
RSERIES
1.8
13 GND
MC34710
L1
100 µH
C8
330 µF
D1
MBRS130LT3
1 V I/O
2 VCORE
Figure 4. Typical Application Diagram
The MC34710 provides both a buck converter and an LDO
regulator in one IC. Figure 4 above shows a typical
application schematic for the MC34710. L1 is the buck
converter's inductor. The buck inductor is a key component
and must not only present the required reactance, but do so
at a dc resistance of less than 20 milliohms in order to
preserve the converter's efficiency. Also important to the
converter's efficiency is the utilization of a low Vf Schottky
diode for D1.
Note that a 0.1uF capacitor is connected between CT and
the reset pins; this prevents any possibility of oscillations
occurring on the reset line during transitions by allowing the
CT terminal to discharge to ground potential via the RST pin,
and then charge when RST returns to a logic high. The
capacitor between the CP1 and CP2 pins is the charge
pump's “bucket capacitor”, and sequentially charges and
discharges to pump up the reservoir capacitor connected to
the VB pin. Note that the reservoir capacitor's cathode is
connected to B+ rather than ground. Also note that the
charge pump is intended only to provide gate-drive potential
for the buck regulator's internal power MOSFET, and
therefore connecting external loads to the VB pin is not
recommended.
The IC's internal VCORE LDO regulator can provide up to
500 mA of current as long as the operating junction
temperature is maintained below 105 degrees C. The heat-
generating power dissipation of the LDO is primarily a
function of the Volt x Amp product across the LINB+ and
VCORE terminals. Therefore, if the LINB+ voltage is >> than
the selected VCORE voltage + 0.8 V, it is recommended to use
a power resistor in series with the LINB+ input to drop the
voltage and dissipate the heat externally from the IC. For
example, if the output of the buck regulator (V I/O on the
schematic) is used as the input to LINB+, and the mode
switches are set such that V I/O = 5 V and VCORE = 3.3 V,
then a series resistance of 1.8 ohms at the LINB+ pin would
provide an external voltage drop at 500 mA while still leaving
the minimum required headroom of 0.8 V. Conversely, if the
mode switches are set such that V I/O = 3.3 V and VCORE =
2.5 V, then no series resistance would be required, even at
the maximum output current of 500 mA.
Designing a power supply circuit with the MC34710, like all
dc-dc converter ICs, requires special attention not only to
component selection, but also to component placement (i.e.,
printed circuit board layout). The MC34710 has a nominal
switching frequency of 200 kHz, and therefore pcb traces
between the buck converter discrete component terminals
and the IC should be kept as short and wide as possible to
keep the parasitic inductance low. Likewise, keeping these
Analog Integrated Circuit Device Data
Freescale Semiconductor
34710
11
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