MAX1795 View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX1795 Datasheet PDF : 13 Pages
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MAX1795/MAX1796/
MAX1797
Low-Supply Current, Step-Up DC-DC Converters
with True Shutdown
VTRIP is the input voltage where the low-battery detector
output goes high impedance.
For single-cell applications, LBI may be connected to the
battery. When VBATT <1.0V>, the LBI threshold increases to
0.925V (see the Typical Operating Characteristics section).
Connect a pullup resistor of 100kΩ or greater from LBO to
OUT for a logic output. LBO is an open-drain output and
can be pulled as high as 6V regardless of the voltage at
OUT. When LBI is below the threshold, the LBO output
is high impedance. If the low-battery comparator is not
used, ground LBI and LBO.
Applications Information
Inductor Selection
An inductor value of 22μH performs well in most appli-
cations. The MAX1795/MAX1796/MAX1797 will also
work with inductors in the 10μH to 47μH range. Smaller
inductance values typically offer a smaller physical size
for a given series resistance, allowing the smallest overall
circuit dimensions, but have lower output current capabil-
ity. Circuits using larger inductance values exhibit higher
output current capability, but are physically larger for the
same series resistance and current rating.
The inductor’s incremental saturation current rating should
be greater than the peak switch-current limit, which is
0.25A for the MAX1795, 0.5A for the MAX1796, and 1A for
the MAX1797. However, it is generally acceptable to bias
the inductor into saturation by as much as 20% although
this will slightly reduce efficiency. Table 1 lists some sug-
gested components for typical applications.
The inductor’s DC resistance significantly affects efficien-
cy. Calculate the maximum output current (IOUT(MAX))
as follows, using inductor ripple current (IRIP) and duty
cycle (D):
IRIP
=
VOUT
+ ILIM × (RPFET + LESR ) − VBATT
L
t
OFF
+
(
R PFET
+
2
L
ESR
)
D
=
VOUT
+
ILIM
−
IRIP
2
× (RPFET
+
L ESR
)
−
VBATT
VOUT
+
ILIM
−
IRIP
2
× (RPFET
− RNFET
+ LESR )
and
IOUT(M= AX )
ILIM
+
IRIP
2
where: IRIP = Inductor ripple current (A)
VOUT = Output voltage (V)
ILIM = Device current limit (0.25A, 0.5A, or 1A)
RPFET = On-resistance of P-channel MOSFET
(Ω) (typ 0.27Ω)
LESR = ESR of Inductor (Ω) (typ 0.095Ω)
VBATT = Input voltage (V)
L = Inductor value in μH
tOFF = LX switch’s off-time (μs) (typ 1μs)
D = Duty cycle
RNFET = On-resistance of N-channel MOSFET
(Ω) (typ 0.17Ω)
IOUT(MAX) = Maximum output current (A)
Capacitor Selection
Table 1 lists suggested tantalum or polymer capacitor
values for typical applications. The ESR of both input
bypass and output filter capacitors affects efficiency and
output ripple. Output voltage ripple is the product of the
peak inductor current and the output capacitor ESR. High-
frequency output noise can be reduced by connecting a
0.1μF ceramic capacitor in parallel with the output filter
capacitor. See Table 2 for a list of suggested component
suppliers.
PC Board Layout and Grounding
Careful printed circuit layout is important for minimizing
ground bounce and noise. Keep the IC’s GND pin and the
ground leads of the input and output filter capacitors less
than 0.2in (5mm) apart. In addition, keep all connections
to the FB and LX pins as short as possible. In particular,
when using external feedback resistors, locate them
as close to FB as possible. To maximize output power
and efficiency and minimize output ripple voltage, use a
ground plane and solder the IC’s GND pin directly to the
ground plane.
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