3.3V/5V or Adjustable-Output,

Step-Up DC-DC Converters

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9

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

Since the LBI current is less than 100nA, large resistor

values (typically 10k

Ω to 300kΩ) 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,

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

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.