The two most significant losses contributing to the

Determine the maximum required gate-drive current

The MAX773’s maximum allowed switching frequency

during normal operation is 300kHz; but at start-up the

maximum frequency can be 500kHz, so the maximum

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

transistor data sheet. For example, the Si9410DY has a

required to charge the gate is:

The bypass capacitor on V+ (C2) must instantaneously

furnish the gate charge without excessive droop (e.g.,

less than 200mV):

∆V+ = ——

C2

Continuing with the example,

∆V+ = 17nC/0.1µF = 170mV.

resistor. See the

Shunt Regulator Operation section.

Figure 2a’s application circuit uses an MTD3055EL

logic-level N-FET with a guaranteed threshold voltage

8-pin Si9410DY surface-mount N-FET that has 50m

Ω

less than 3V.

NPN Transistors

The MAX773 can drive NPN transistors, but be

extremely careful when determining the base-current

requirements. Too little base current can cause exces-

sive power dissipation in the transistor; too much base

current can cause the base to oversaturate, so the tran-

sistor remains on continually. Both conditions can dam-

age the transistor.

When using the MAX773 with an NPN transistor, con-

between EXTH and the base (Figure 8c).

To determine the required peak inductor current,

efficiency graphs and the theoretical output current

capability vs. input voltage graphs to determine a

sense resistor that will allow the desired output current.

Divide the 170mV worst-case (smallest) voltage across

Use the worst-case (lowest) value for ß given in the

transistor’s electrical specification, where the collector

It may be necessary to use even higher base currents

operation by extending the transistor’s turn-off time.

minimum base current that forces the transistor into

saturation. This equation reduces to (V+ - 700mV -

sible, but small enough to ensure the transistor is

always driven near saturation. Highest efficiency is

obtained with a fast-switching NPN transistor

voltage and a high current gain. A good transistor to

use is the Zetex ZTX694B.

Diode Selection

The MAX770–MAX773’s high switching frequency

demands a high-speed rectifier. Schottky diodes such

as the 1N5817–1N5822 are recommended. Make sure

that the Schottky diode’s average current rating

ature applications, Schottky diodes may be inadequate

due to their high leakage currents; high-speed silicon

diodes may be used instead. At heavy loads and high

temperatures, the benefits of a Schottky diode’s low for-

ward voltage may outweigh the disadvantages of its

high leakage current.

Capacitor Selection

Output Filter Capacitor

The primary criterion for selecting the output filter

capacitor (C2) 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. An OS-CON 300µF,

6.3V output filter capacitor has approximately 50m

Ω of

ESR and typically provides 180mV ripple when

stepping up from 3V to 5V at 1A (Figure 2a).

5V/12V/15V or Adjustable, High-Efficiency,

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