MAX887 데이터 시트보기 (PDF) - Maxim Integrated
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MAX887 Datasheet PDF : 12 Pages
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100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
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:
R1
=
R2
VOUT
VFB
−
1
where VFB = 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 ILIM 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-
Table 1. Inductor and Output Filter
vs. Sync Frequency
SYNC
RANGE (kHz)
300–400
200–300
150–200
100–150
75–100
L1
(µH)
33
47
68
100
150
COUT
(µF)
33
47
68
100
150
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.
Table 2. Component Suppliers
COMPANY
AVX
USA
Coilcraft
Coiltronics
Dale
International
Rectifier
Motorola
Nichicon
Nihon
Sanyo
Siliconix
USA
USA
USA
USA
USA
USA
Japan
USA
Japan
USA
Japan
USA
Sprague
Sumida
United
Chemi-Con
USA
USA
Japan
USA
PHONE
(803) 946-0690
(800) 282-4975
(847) 639-6400
(561) 241-7876
(605) 668-4131
(310) 322-3331
FAX
(803) 626-3123
(847) 639-1469
(561) 241-9339
(605) 665-1627
(310) 322-3332
(602) 303-5454
(847) 843-7500
81-7-5231-8461
(805) 867-2555
81-3-3494-7411
(619) 661-6835
81-7-2070-6306
(408) 988-8000
(800) 554-5565
(603) 224-1961
(847) 956-0666
81-3-3607-5111
(714) 255-9500
(602) 994-6430
(847) 843-2798
81-7-5256-4158
(805) 867-2698
81-3-3494-7414
(619) 661-1055
81-7-2070-1174
(408) 970-3950
(603) 224-1430
(847) 956-0702
81-3-3607-5144
(714) 255-9400
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