Rainbow-electronics MAX887 Bedienungsanleitung PDF herunterladen (Seite 10)

MAX887

100% Duty Cycle, Low-Noise,

Step-Down, PWM DC-DC Converter

10 ______________________________________________________________________________________

when to turn off this synchronous rectifier. A fourth

comparator (ILIM) is used at the P-channel MOSFET

switch for overcurrent detection. This protects the sys-

tem, external components, and internal MOSFETs

under overload conditions.

________________Design Information

Output Voltage Selection

To select an output voltage between 1.25V and 10.5V,

connect FB to a resistor voltage divider between the

output and GND (Figure 2). Select feedback resistor R2

in the 5kΩ to 100kΩ range, since FB input leakage is

±100nA max. R1 is then given by:

where V

= 1.25V. A small ceramic capacitor (C1)

around 100pF to 470pF should be added in parallel

with R1 to compensate for stray capacitance at the FB

pin, and output capacitor equivalent series resistance

(ESR).

Inductor Selection

A 1.3A inductor with the value recommended in Table 1

is sufficient for most applications. However, the exact

inductor value is not critical, and values within 50% of

those in Table 1 are acceptable. For best efficiency, the

inductor’s DC resistance should be less than 0.25Ω.

The inductor saturation current rating must exceed the

1A I

LIM

current limit. Table 2 lists component suppliers.

Capacitor Selection

Input and output filter capacitors should be chosen to

service inductor currents with acceptable voltage rip-

ple. The input filter capacitor also reduces peak cur-

rents and noise at the voltage source. See Table 1 for

suggested values. The MAX887’s loop gain is relatively

low, to enable the use of small, low-valued output filter

capacitors. Higher values provide improved output rip-

ple and transient response. Lower oscillator frequen-

cies require a larger-value output capacitor. When Idle

Mode is used, verify capacitor selection with light loads

during PFM operation, since output ripple is higher

under these conditions.

Low-ESR capacitors are recommended. Capacitor ESR

is a major contributor to output ripple (usually more

than 60%). Ordinary aluminum-electrolytic capacitors

have high ESR and should be avoided. Low-ESR alu-

minum-electrolytic capacitors are acceptable and rel-

atively inexpensive. Low-ESR tantalum capacitors

are better and provide a compact solution for space-

constrained surface-mount designs. Do not exceed

the ripple current ratings of tantalum capacitors.

Ceramic capacitors have the lowest ESR overall, and

OS-CON capacitors have the lowest ESR of the high-

value electrolytic types. It is generally not necessary to

use ceramic and OS-CON capacitors for the MAX887;

they need only be considered in very compact, high-

reliability, or wide-temperature applications, where the

expense is justified. When using very-low-ESR capaci-

tors, such as ceramic or OS-CON, check for stability

while examining load-transient response, and increase

the compensation capacitor C1 if needed. Table 2 lists

suppliers for the various components used with the

MAX887.

R1 R2

OUT

=−













Table 1. Inductor and Output Filter

vs. Sync Frequency

Table 2. Component Suppliers

(µH)

OUT

(µF)

300–400 33 33

200–300 47 47

150–200 68 68

100–150 100 100

75–100 150 150

SYNC

RANGE (kHz)

COMPANY PHONE FAX

AVX USA (803) 946-0690 (803) 626-3123

(800) 282-4975

Coilcraft USA (847) 639-6400 (847) 639-1469

Coiltronics USA (561) 241-7876 (561) 241-9339

Dale USA (605) 668-4131 (605) 665-1627

International USA (310) 322-3331 (310) 322-3332

Rectifier

Motorola USA (602) 303-5454 (602) 994-6430

Nichicon USA (847) 843-7500 (847) 843-2798

Japan 81-7-5231-8461 81-7-5256-4158

Nihon USA (805) 867-2555 (805) 867-2698

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

Siliconix USA (408) 988-8000 (408) 970-3950

(800) 554-5565

Sprague USA (603) 224-1961 (603) 224-1430

Sumida USA (847) 956-0666 (847) 956-0702

Japan 81-3-3607-5111 81-3-3607-5144

United USA (714) 255-9500 (714) 255-9400

Chemi-Con

1 2 ... 5 6 7 8 9 10 11 12

100% Duty Cycle, Low-Noise 1

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