Rainbow Electronics MAX859 - Manual (Page 9)

MAX856–MAX859
3.3V/5V or Adjustable-Output,
Step-Up DC-DC Converters
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Low-Battery Detection
The MAX856 series contains an on-chip comparator for
low-battery detection. If the voltage at LBI falls below
the regulator’s internal reference voltage (1.25V), LBO
(an open-drain output) sinks current to GND. The low-
battery monitor’s threshold is set by two resistors, R3
and R4 (Figure 2). Set the threshold voltage using the
following equation:
VLBI
R3 = R4 ( ______ - 1)
VREF
where V LBI is the desired threshold of the low-battery
detector and V REF is the internal 1.25V reference.
Since the LBI current is less than 100nA, large resistor
values (typically 10k
Ω
to 300k
Ω
) can be used for R3
and R4 to minimize loading of the input supply.
When the voltage at LBI is below the internal threshold,
LBO sinks current to GND. Connect a pull-up resistor of
10k
Ω
or more from LBO to OUT when driving CMOS
circuits. When LBI is above the threshold, the LBO out-
put is off. If the low-battery comparator is not used,
connect LBI to V
IN and leave LBO open.
Inductor Selection
An inductor value of 47µH performs well in most
MAX856–MAX859 applications. However, the inductance
value is not critical, and the MAX856–MAX859 will work
with inductors in the 10µH to 100µH range. Smaller
inductance values typically offer a smaller physical size
for a given series resistance, allowing the smallest
overall circuit dimensions. However, due to higher peak
inductor currents, the output voltage ripple (I
PEAK x
output filter capacitor ESR) also tends to be higher.
Circuits using larger inductance values exhibit higher
output current capability and larger physical dimen-
sions for a given series resistance.
The inductor’s incremental saturation current rating
should be greater than the peak switch-current limit,
which is 500mA for the MAX856/MAX857, and 125mA
for the MAX858/MAX859. However, it is generally
acceptable to bias the inductor into saturation by as
much as 20%, although this will slightly reduce
efficiency.
The inductor’s DC resistance significantly affects effi-
ciency. See the Efficiency vs. Load Current for Various
Inductors graph in the
Typical Operating Characteristics.
See Tables 1 and 2 for a list of suggested inductor
suppliers.
Capacitor Selection
A 68µF, 10V, 0.85 Ω , surface-mount tantalum (SMT)
output filter capacitor typically provides 50mV output
ripple when stepping up from 2V to 5V at 100mA
(MAX856/ MAX857). Smaller capacitors (down to 10µF
with higher ESRs) are acceptable for light loads or in
applications that can tolerate higher output ripple.
Values in the 10µF to 47µF range are recommended for
the MAX858/MAX859.
The equivalent series resistance (ESR) of both bypass
and filter capacitors affects efficiency and output rip-
ple. The output voltage ripple is the product of the peak
inductor current and the output capacitor’s ESR. Use
low-ESR capacitors for best performance, or connect
two or more filter capacitors in parallel. Low-ESR, SMT
tantalum capacitors are currently available from
Sprague (595D series) and AVX (TPS series). Sanyo
OS-CON organic-semiconductor through-hole capaci-
tors also exhibit very low ESR, and are especially useful
for operation at cold temperatures. See Table 1 for a list
of suggested capacitor suppliers.
Rectifier Diode
For optimum performance, a switching Schottky diode
(such as the 1N5817) is recommended. Refer to Table
1 for a list of component suppliers. For low output
power applications, a PN-junction switching diode
(such as the 1N4148) will also work well, although its
greater forward voltage drop will reduce efficiency.
PC Layout and Grounding
The MAX856 series’ high-frequency operation makes
PC layout important for minimizing ground bounce and
noise. Keep the IC’s GND pin and the ground leads of
C1 and C2 (Figure 1) less than 0.2in (5mm) apart. Also
keep all connections to the FB and LX pins as short as
possible. To maximize output power and efficiency and
minimize output ripple voltage, use a ground plane and
solder the IC’s GND (pin 7) directly to the ground
plane.