MAX1836 데이터 시트보기 (PDF) - Maxim Integrated

부품명

상세내역

제조사

MAX1836

24V Internal Switch, 100% Duty Cycle, Step-Down Converters

Maxim Integrated 

MAX1836/MAX1837

24V Internal Switch, 100% Duty Cycle,

Step-Down Converters

The inductor’s saturation current rating must be greater

than the peak switching current, which is determined

by the switch current limit plus the overshoot due to the

300ns current-sense comparator propagation delay:

IPE= AK

ILIM

+

(VIN

−

VOUT

L

)

300ns

where the switch current-limit (ILIM) is typically 312mA

(MAX1836) or 625mA (MAX1837). Saturation occurs

when the inductor’s magnetic flux density reaches the

maximum level the core can support, and the inductance

starts to fall.

Inductor series resistance affects both efficiency and

dropout voltage. See the Input-Output (Dropout) Voltage

section. High series resistance limits the maximum current

available at lower input voltages and increases the drop-

out voltage. For optimum performance, select an inductor

with the lowest possible DC resistance that fits in the

allotted dimensions. Typically, the inductor’s series resis-

tance should be significantly less than that of the internal

P-channel MOSFET’s on-resistance (1.1Ω typ). Inductors

with a ferrite core, or equivalent, are recommended.

The maximum output current of the MAX1836/MAX1837

current-limited converter is limited by the peak inductor

current. For the typical application, the maximum output

current is approximately:

IOUT(MAX) IPEAK

Output Capacitor

Choose the output capacitor to supply 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) = IPEAKESR

VRIPPLE(C)

=

L(IPEAK − IOUT) 2

2COUT VOUT







VIN

VIN

− VOUT







where IPEAK is the peak inductor current. See the Inductor

Selection section. These equations are suitable for initial

capacitor selection, but final values should be set by test-

ing a prototype 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 but decreases with lower input voltages.

With low-cost aluminum electrolytic capacitors, the ESR-

induced ripple can be larger than that caused by the

current into and out of the capacitor. Consequently, high-

quality low-ESR aluminum-electrolytic, tantalum, polymer,

or ceramic filter capacitors are required to minimize out-

put ripple. Best results at reasonable cost are typically

achieved with an aluminum-electrolytic capacitor in the

100μF range, in parallel with a 0.1μF ceramic capacitor.

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 currents defined by the

following equation:

IRMS = ILOAD

VOUT(VIN − VOUT)

VIN

For most applications, nontantalum chemistries (ceramic,

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

their robustness with high inrush currents typical of sys-

tems with low-impedance battery inputs. Alternatively,

two (or more) smaller-value low-ESR capacitors can be

connected in parallel for lower cost. Choose an input

capacitor that exhibits < +10°C temperature rise at the

RMS input current for optimal circuit longevity.

Diode Selection

The current in the external diode (D1) changes abruptly

from zero to its peak value each time the LX switch turns

off. To avoid excessive losses, the diode must have a

fast turn-on time and a low forward voltage. Use a diode

with an RMS current rating of 0.5A or greater, and with a

breakdown voltage > VIN. Schottky diodes are preferred.

For high-temperature applications, Schottky diodes may

be inadequate due to their high leakage currents. In

such cases, ultra-high-speed silicon rectifiers are recom-

mended, although a Schottky diode with a higher reverse

voltage rating can often provide acceptable performance.

www.maximintegrated.com

Maxim Integrated │  10

Share Link: