MAX749CSLASHD datasheet(10/12 Pages) MAXIM

10 / 12 page

The Sumida CD54-470N (47µH, 720mA, 370m

Ω) is suit-

able for a wide range of applications, and the larger

CD105-470N (47µH, 1.17A, 170m

Ω) permits higher cur-

rent levels and efficiencies.

Diode Selection

The MAX749’s high switching frequency demands a high-

speed rectifier. Schottky diodes such as the 1N5817-

1N5822 family are recommended. Choose a diode with an

average current rating approximately equal to the peak

current, as determined by 180mV/RSENSE and a break-

down voltage greater than V+ +

I-VOUTMAXI.

External Switching Transistor

The MAX749 can drive a PNP transistor or a P-channel

logic-level MOSFET. The choice of a power switch is

dictated by the input voltage range, cost, and efficiency.

MOSFETs provide the highest efficiency because they

do not draw any DC gate-drive current (see

Typical

Operating Characteristics graphs). However, a gate-

source voltage of several volts is needed to turn on a

MOSFET, so a 5V or greater input supply is required

(although this restriction may change as lower-thresh-

old P-channel MOSFETs become available). PNP tran-

sistors, meanwhile, may be used over the entire 2V to

6V operating voltage range of the MAX749.

When using a MOSFET, connect DHI and DLOW to its

gate (see

Typical Operating Circuit). When using a PNP

transistor, connect DHI to its base, and connect a resis-

tor between the base and DLOW (RBASE) (Figure 1). The

PNP transistor is turned off quickly by the direct pull-up

of DHI, and turned on by the base current provided

through RBASE. This resistor limits the transistor’s base-

drive current to (VIN - 140mV - VBE)/RBASE, where VIN is

the input voltage, 140mV is the drop across RSENSE, VBE

is the transistor’s base-emitter voltage, and RBASE is the

current-limiting resistor. For maximum efficiency, make

RBASE as large as possible, but small enough so that the

transistor is always driven into saturation.

Highest efficiency with a PNP transistor comes from

using a device with a low collector-emitter saturation

voltage and a high current gain. Use a fast-switching

type. For example the Zetex ZTX792A has switching

speeds of 40ns (tON) and 500ns (tOFF).

The transistor must have a collector-to-emitter (PNP) or

drain-to-source (MOSFET) voltage rating greater than the

input-to-output voltage differential (VIN - VOUT). In either

case the transistor must have a current rating that exceeds

the peak current set by the current-sense resistor.

PNP transistors are generally less expensive than P-

channel MOSFETs. Table 2 lists some suppliers of

switching transistors suitable for use with the MAX749.

Table 2. Component Suppliers

Base Resistor

The base resistor, RBASE in Figure 1, controls the amount of

base current in the PNP transistor. A low value for RBASE

increases base drive, which provides higher output cur-

rents and compensates for lower input voltages, but

decreases efficiency. Conversely, a high RBASE value

increases efficiency but reduces the output capability,

especially at low voltages. When using high-gain transis-

tors, e.g. the Zetex ZTX750 or ZTX792, typical values for

RBASE are in the 150Ω to 510Ω range, but will depend on

the required input voltage range and output current (see

Typical Operating Characteristics). Lower-gain transistors

require lower values for RBASE and are less efficient. Larger

RBASE values are suitable if less output power is required.

Digitally Adjustable LCD Bias Supply

_____________________________________________________________________________________

SUPPLIER

INDUCTORS

Coiltronics

(305) 781-8900

(305) 782-4163

Gowanda

(716) 532-2234

(716) 532-2702

Sumida USA

(708) 956-0666

(708) 956-0702

Sumida Japan

81-3-3607-511

81-3-3607-5428

CAPACITORS

Kemet

(803) 963-6300

(803) 963-6322

Matsuo

(714) 969-2491

(714) 960-6492

Nichicon

(708) 843-7500

(708) 843-2798

Sprague

(603) 224-1961

(603) 224-1430

Sanyo USA

(619) 661-6322

Sanyo Japan

81-3-3837-6242

United Chemi-Con

(714) 255-9500

(714) 255-9400

DIODES

Motorola

(800) 521-6274

Nihon USA

(805) 867-2555

(805) 867-2698

Nihon Japan

81-3-3494-7411

81-3-3494-7414

POWER TRANSISTORS - MOSFETS

Harris

(407) 724-3739

(407) 724-3937

International

Rectifier

(213) 772-2000

(213) 772-9028

Siliconix

(408) 988-8000

(408) 727-5414

POWER TRANSISTORS - PNP TRANSISTORS

Zetex USA

(516) 543-7100

(516) 864-7630

Zetex UK

44 (61) 727 5105

44 (61) 627 5467

CURRENT-SENSE RESISTORS

IRC

(512) 992-7900

(512) 992-3377

FAX

PHONE

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