Power Transistor Selection

Use an N-channel MOSFET power transistor with the

MAX608.

Use logic-level or low-threshold N-FETs to ensure the

external N-channel MOSFET (N-FET) is turned on com-

pletely and that start-up occurs. N-FETs provide the

highest efficiency because they do not draw any DC

gate-drive current.

When selecting an N-FET, some important parameters

results; the maximum value is usually grossly over-

specified since it is a guaranteed limit and not the mea-

sured value. The typical total gate charge should be

50nC or less. With larger numbers, the EXT pins may

not be able to adequately drive the gate. The EXT

rise/fall time varies with different capacitive loads as

shown in the

Typical Operating Characteristics.

The two most significant losses contributing to the

Determine the maximum required gate-drive current

voltage.

When using a power supply that decays with time

(such as a battery), the N-FET transistor will operate in

its linear region when the voltage at EXT approaches

the threshold voltage of the FET, dissipating excessive

power. Prolonged operation in this mode may damage

(such as the MAX8211) to put the IC in shutdown mode

once the input supply voltage falls below a predeter-

mined minimum value. Excessive loads with low input

voltages can also cause this condition.

The MAX608’s maximum allowed switching frequency

during normal operation is 300kHz. However, at start-

up, the maximum frequency can be 500kHz, so the

maximum current required to charge the N-FET’s gate

transistor data sheet. For example, the MMFT3055EL

rent required to charge the gate is:

Figure 2a’s application circuit uses a 4-pin MMFT3055EL

surface-mount N-FET that has 150m

Ω on-resistance with

2c’s application circuit uses an Si6426DQ logic-level N-

Diode Selection

The MAX608’s high switching frequency demands a

high-speed rectifier. Schottky diodes such as the

1N5817–1N5822 are recommended. Make sure the

Schottky diode’s average current rating exceeds the

applications, Schottky diodes may be inadequate due

to their high leakage currents; high-speed silicon

diodes such as the MUR105 or EC11FS1 can be used

instead. At heavy loads and high temperatures, the

benefits of a Schottky diode’s low forward voltage may

outweigh the disadvantage of high leakage current.

Capacitor Selection

Output Filter Capacitor

The primary criterion for selecting the output filter capac-

itor (C4) is low effective series resistance (ESR). The

product of the peak inductor current and the output filter

capacitor’s ESR determines the amplitude of the ripple

seen on the output voltage. Two OS-CON 100µF, 16V

output filter capacitors in parallel with 35m

Ω of ESR each

typically provide 75mV ripple when stepping up from 2V

to 5V at 500mA (Figure 2a). Smaller-value and/or higher-

ESR capacitors are acceptable for light loads or in appli-

cations that can tolerate higher output ripple.

Since the output filter capacitor’s ESR affects efficien-

cy, use low-ESR capacitors for best performance. See

Table 1 for component selection.

Input Bypass Capacitors

The input bypass capacitor (C1) reduces peak currents

drawn from the voltage source and also reduces noise

caused by the switching action of the MAX608 at the

voltage source. The input voltage source impedance

determines the size of the capacitor required at the

OUT input. As with the output filter capacitor, a low-ESR

capacitor is recommended. For output currents up to

1A, 150µF (C1) is adequate, although smaller bypass

capacitors may also be acceptable.

Bypass the IC with a 0.1µF ceramic capacitor (C2)

placed as close as possible to the OUT and GND pins.

Reference Capacitor

Bypass REF with a 0.1µF capacitor (C3). REF can

source up to 100µA of current for external loads.

5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

10

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