MC34710 Datasheet, PDF(11/18 Page) Motorola, Inc – Adjustable Dual Output Switching Power Supply

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MC34710 Datasheet, PDF (11/18 Pages) Motorola, Inc – Adjustable Dual Output Switching Power Supply

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TYPICAL APPLICATIONS

B+

13- 32V

330Î¼F

0.1mF

SW1

RST

MODE0

MODE1

MODE2

SUPERVISORY

& SHUTDOWN

CHARGE

PUMP

BUCK

REG

LDO

CT 32

CP1

CP2

VB 29

B+ 28

VSW

VFB

LINB

VCORE 23

0.1Î¼F

10Î¼F

RSERIES

1.8

13 GND

MC34710

100Î¼H

330Î¼F

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 pin 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 âbucket

capacitorâ, 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

500mA 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 pins. Therefore, if the LINB+ voltage is >> than the

selected VCORE voltage + 0.8V, 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 = 5V and VCORE = 3.3 , then

a series resistance of 1.8 ohms at the LINB+ pin would

provide an external voltage drop at 500mA while still leaving

the minimum required headroom of 0.8V. Conversely, if the

mode switches are set such that V I/O = 3.3V and VCORE =

2.5V, then no series resistance would be required, even at

the maximum output current of 500mA.

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 200kHz, and therefore pcb traces

between the buck converter discrete component pins and the

IC should be kept as short and wide as possible to keep the

parasitic inductance low. Likewise, keeping these pcb traces

Analog Integrated Circuit Device Data

Freescale Semiconductor

34710

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