MAX887HESA Maxim Integrated Products, MAX887HESA Datasheet - Page 10

• Manufacturer Part Number: MAX887HESA
• Description: DC/DC Switching Controllers 100% Duty Step-Down DC/DC Converter
• Manufacturer: Maxim Integrated Products

Specifications of MAX887HESA

• Output Voltage: 1.25 V to 10.5 V
• Output Current: 600 mA
• Input Voltage: 3.5 V to 11 V
• Supply Current: 2.7 mA
• Switching Frequency: 300 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C

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.
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
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).
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
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-
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
Table 1. Inductor and Output Filter
vs. Sync Frequency
10
________________Design Information
RANGE (kHz)
LIM
______________________________________________________________________________________
300–400
200–300
150–200
100–150
75–100
SYNC
current limit. Table 2 lists component suppliers.
FB
= 1.25V. A small ceramic capacitor (C1)
R1
R2
Output Voltage Selection
(µH)
100
150
V
L1
33
47
68
V
OUT
FB
Capacitor Selection
Inductor Selection
1
C
(µF)
100
150
OUT
33
47
68
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
AVX
Nihon
Siliconix
Sumida
United
Chemi-Con
COMPANY
Coilcraft
Coiltronics
Dale
International
Rectifier
Motorola
Nichicon
Sanyo
Sprague
USA
USA
USA
USA
USA
USA
USA
Japan
USA
Japan
USA
Japan
USA
USA
USA
Japan
USA
[phone]
[phone]
[phone]
[phone]
[phone]
[phone]
[phone]
[phone]
81-7-5231-8461
[phone]
81-3-3494-7411
[phone]
81-7-2070-6306
[phone]
[phone]
[phone]
[phone]
81-3-3607-5111
[phone]
PHONE
[phone]
[phone]
[phone]
[phone]
[phone]
[phone]
[phone]
81-7-5256-4158
[phone]
81-3-3494-7414
[phone]
81-7-2070-1174
[phone]
[phone]
[phone]
81-3-3607-5144
[phone]
FAX