MAX770EPA View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX770EPA

5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers

Maxim Integrated 

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

Low IQ, Step-Up DC-DC Controllers

The two most significant losses contributing to the

N-FET’s power dissipation are I2R losses and switching

losses. Select a transistor with low rDS(ON) and low

CRSS to minimize these losses.

Determine the maximum required gate-drive current

from the Qg specification in the N-FET data sheet.

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

f(max) x Qg(typ). Use the typical Qg number from the

transistor data sheet. For example, the Si9410DY has a

Qg(typ) of 17nC (at VGS = 5V), therefore the current

required to charge the gate is:

IGATE (max) = (500kHz) (17nC) = 8.5mA.

The bypass capacitor on V+ (C2) must instantaneously

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

less than 200mV):

Qg

∆V+ = ——

C2

Continuing with the example, ∆V+ = 17nC/0.1µF = 170mV.

Use IGATE when calculating the appropriate shunt

resistor. See the Shunt Regulator Operation section.

Figure 2a’s application circuit uses an MTD3055EL

logic-level N-FET with a guaranteed threshold voltage

(VTH) of 2V. Figure 2b’s application circuit uses an

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

on resistance with 4.5V VGS, and a guaranteed VTH of

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-

nect EXTL to the transistor’s base, and connect RBASE

between EXTH and the base (Figure 8c).

To determine the required peak inductor current,

IC(PEAK), observe the Typical Operating Characteristics

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

the current-sense amplifier VCS(max) by the sense-

resistor value. To determine IB, set the peak inductor

current (ILIM) equal to the peak transistor collector cur-

rent IC(PEAK). Calculate IB as follows:

IB = ILIM/ß

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

transistor’s electrical specification, where the collector

current used for the test is approximately equal to ILIM.

It may be necessary to use even higher base currents

(e.g., IB = ILIM /10), although excessive IB may impair

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

RBASE is determined by:

( ) VEXTH - VBE - VCS(min )

RBASE

=

————————————–

IB

Where VEXTH is the voltage at V+ (in bootstrapped

mode VEXTH is the output voltage), VBE is the 0.7V

transistor base-emitter voltage, VCS(min) is the voltage

drop across the current-sense resistor, and IB is the

minimum base current that forces the transistor into

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

170mV) / IB.

For maximum efficiency, make RBASE as large as pos-

sible, but small enough to ensure the transistor is

always driven near saturation. Highest efficiency is

obtained with a fast-switching NPN transistor

(fT ≥ 150MHz) with a low collector-emitter saturation

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

exceeds the peak current limit set by RSENSE, and that

its breakdown voltage exceeds VOUT. For high-temper-

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 50mΩ of

ESR and typically provides 180mV ripple when

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

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