Rainbow Electronics MAX887 - Manual (Page 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 5k
Ω
to 100k
Ω
range, since FB input leakage is
±100nA max. R1 is then given by:
where V FB = 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 0.25
Ω
.
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
V
V
1
OUT
FB
= −

 

Table 1. Inductor and Output Filter
vs. Sync Frequency
Table 2. Component Suppliers
L1
(µH)
C 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
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