Input Bypass Capacitor

The input bypass capacitor reduces peak currents

drawn from the voltage source, and also reduces the

amount of noise at the voltage source caused by the

switching action of the MAX1649/MAX1651. The input

voltage source impedance determines the size of the

capacitor required at the V+ input. As with the output fil-

ter capacitor, a low-ESR capacitor is recommended.

Bypass the IC separately with a 0.1µF ceramic capac-

itor placed close to the V+ and GND pins.

Reference Capacitor

Bypass REF with a 0.1µF or larger capacitor.

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

______________________________________________________________________________________

11

___________________Chip Topography

TRANSISTOR COUNT: 428

SUBSTRATE CONNECTED TO V+

0.106"

(2.692mm)

0.081"

(2.057mm)

OUT

GND

CS

EXT

V+

FB

SHDN

REF

Table 2. Component Suppliers

COMPANY

PHONE

FAX

(207) 282-5111

AVX

USA

or

(207) 283-1941

(800) 282-4975

Coiltronics

USA

(407) 241-7876

(407) 241-9339

CoilCraft

USA

(708) 639-6400

(708) 639-1469

Dale

USA

(402) 564-3131

(402) 563-1841

International

USA

(310) 322-3331

(310) 322-3332

Rectifier

IRC

USA

(512) 992-7900

(512) 992-3377

(602) 244-3576

Motorola

USA

or

(602) 244-4015

(602) 244-5303

Nichicon

USA

(708) 843-7500

(708) 843-2798

Japan

81-7-5231-8461

81-7-5256-4158

Nihon

USA

(805) 867-2555

(805) 867-2556

Japan

81-3-3494-7411

81-3-3494-7414

Sanyo

USA

(619) 661-6835

(619) 661-1055

Japan

81-7-2070-6306

81-7-2070-1174

(408) 988-8000

Siliconix

USA

or

(408) 970-3950

(800) 554-5565

Sprague

USA

(603) 224-1961

(603) 224-1430

Sumida

USA

(708) 956-0666

(708) 956-0702

Japan

81-3-3607-5111

81-3-3607-5144

United

USA

(714) 255-9500

(714) 255-9400

Chemi-Con

Layout Considerations

Proper PC board layout is essential because of high

current levels and fast switching waveforms that radi-

ate noise. Minimize ground noise by connecting the

anode of the rectifier, the input bypass capacitor

ground lead, and the output filter capacitor ground

lead to a single point (“star” ground configuration). A

ground plane is recommended. Also minimize lead

lengths to reduce stray capacitance, trace resistance,

and radiated noise. In particular, the traces connected

to FB (if an external resistor divider is used) and EXT

must be short. Place the 0.1µF ceramic bypass capac-

itor as close as possible to the V+ and GND pins.

MAX1649/MAX1651 vs. MAX649/MAX651

The MAX1649 and MAX1651 are pin compatible with

the MAX649 and MAX651, but have been optimized for

improved dropout performance and efficiency—partic-

ularly with low input voltages. The MAX1649/MAX1651

feature increased maximum switch duty cycle (96.5%)

and reduced current-limit sense voltage (110mV).

Their predecessors, the MAX649/MAX651, use a high-

er two-step (210mV/110mV) current-limit sense voltage

to provide tighter current-sense accuracy and reduced

inductor peak current at light loads.