MAX1776 查看數據表（PDF） - Maxim Integrated

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MAX1776

24V, 600mA Internal Switch, 100% Duty Cycle, Step-Down Converter

Maxim Integrated 

24V, 600mA Internal Switch, 100% Duty Cycle,

Step-Down Converter

Inductor series resistance affects both efficiency and

dropout voltage (see Input-Output (Dropout) Voltage).

High series resistance limits the maximum current avail-

able at lower input voltages, and increases the dropout

voltage. For optimum performance, select an inductor

with the lowest possible DC resistance that fits in the

allotted dimensions. Some recommended component

manufacturers are listed in Table 2.

Maximum Output Current

The MAX1776 converter’s output current determines

the regulator’s switching frequency. When the convert-

er approaches continuous mode, the output voltage

falls out of regulation. For the typical application, the

maximum output current is approximately:

ILOAD(MAX) = 1/2 ILX (PEAK)(MIN)

For low-input voltages, the maximum on-time may be

reached and the load current is limited by:

ILOAD = 1/2 (VIN - VOUT) ✕ 10µs / L

Output Capacitor

Choose the output capacitor to service the maximum

load current with acceptable voltage ripple. The output

ripple has two components: variations in the charge

stored in the output capacitor with each LX pulse, and

the voltage drop across the capacitor’s equivalent

series resistance (ESR) caused by the current into and

out of the capacitor:

VRIPPLE ≅ VRIPPLE(ESR) + VRIPPLE(C)

The output voltage ripple as a consequence of the ESR

and output capacitance is:

VRIPPLE(ESR) = ESR × IPEAK

( ) VRIPPLE(C)

=

L

× IPEAK -IOUTPUT

2COUT × VOUTPUT

2





VIN

VIN

- VOUTPUT





where IPEAK is the peak inductor current (see Inductor

Selection). The worst-case ripple occurs at no-load.

These equations are suitable for initial capacitor selec-

tion, but final values should be set by testing a proto-

type or evaluation circuit. As a general rule, a smaller

amount of charge delivered in each pulse results in

less output ripple. Since the amount of charge deliv-

ered in each oscillator pulse is determined by the

inductor value and input voltage, the voltage ripple

increases with larger inductance, and as the input volt-

age decreases. See Table 3 for recommended capaci-

tor values and Table 2 for recommended component

manufacturers.

Input Capacitor

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching.

The input capacitor must meet the ripple-current

requirement (IRMS) imposed by the switching current

defined by the following equation:

IRMS

=

ILOADVOUTPUT

VIN





4

3

×

VIN

VOUTPUT

−



1

For most applications, nontantalum chemistries (ceram-

ic, aluminum, polymer, or OS-CON) are preferred due to

their robustness to high inrush currents typical of sys-

tems with low-impedance battery inputs. Alternatively,

connect two (or more) smaller value low-ESR capacitors

in parallel to reduce cost. Choose an input capacitor

that exhibits less than +10°C temperature rise at the

RMS input current for optimal circuit longevity.

Table 2. Component Suppliers

SUPPLIER

DIODES

Central Semiconductor

Fairchild

General Semiconductor

International Rectifier

Nihon

On Semi

Vishay-Siliconix

Zetex

CAPACITORS

AVX

Kemet

Nichicon

Sanyo

Taiyo Yuden

INDUCTORS

Coilcraft

Coiltronics

Pulse Engineering

Sumida USA

Toko

WEBSITE

www.centralsemi.com

www.fairchildsemi.com

www.gensemi.com

www.irf.com

www.niec.co.jp/engver2/

niec.co.jp_eg.htm

www.onsemi.com

www.vishay.com/brands/siliconix/

main.html

www.zetex.com

www.avxcorp.com

www.kemet.com

www.nichicon-us.com

www.sanyo.com

www.t-yuden.com

www.coilcraft.com

www.cooperet.com

www.pulseeng.com

www.sumida.com

www.tokoam.com

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