33710 datasheet(11/17 Pages) FREESCALE

11 / 17 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

34710

TYPICAL APPLICATIONS

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

LDO

B+

13 V - 32 V

RST

MODE0

MODE1

MODE2

GND

CP1

CP2

B+

VSW

VFB

LINB

VCORE

SUPERVISORY

& SHUTDOWN

BUCK

REG

CHARGE

PUMP

330 µF

1 K

0.1 mF

0.1 µF

10 µF

RSERIES

1.8

330 µF

V I/O

VCORE

SW1

MBRS130LT3

MC34710

100 µH

33710 Datasheet(PDF) 11 Page - Freescale Semiconductor, Inc

33710 Datasheet(HTML) 11 Page - Freescale Semiconductor, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL