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參數(shù)資料
| 型號(hào)： | MAX887 |
| 廠商： | Maxim Integrated Products, Inc. |
| 元件分類： | DC/DC變換器 |
| 英文描述： | 100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter |
| 中文描述： | 100%占空比、低噪聲、降壓型、PWM DC-DC轉(zhuǎn)換器 |
| 文件頁數(shù)： | 10/12頁 |
| 文件大?。?/td> | 146K |
| 代理商： | MAX887 |
M
100% Duty Cyc le, Low-Noise,
S tep-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 V oltage S elec tion
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
to 100k
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).
Induc tor S elec tion
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
.
The inductor saturation current rating must exceed the
1A I
LIM
current limit. Table 2 lists component suppliers.
Capac itor S elec tion
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
200–300
150–200
100–150
75–100
33
47
68
100
150
33
47
68
100
150
SYNC
RANGE (kHz)
COMPANY
AVX
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(803) 946-0690
(800) 282-4975
(847) 639-6400
(561) 241-7876
(605) 668-4131
(310) 322-3331
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| MAX8873SEUK+ | 制造商:Maxim Integrated Products 功能描述:LDO REG 1.25V TO 6.5V/2.84V 0.12A 5PIN SOT-23 - Rail/Tube |
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