MAX5090ASLASHBSLASHC datasheet(12/17 Pages) MAXIM

12 / 17 page

calculate the input capacitance and the ESR required for

a specified ripple using the following equations:

ESR

OUT

ESR

D(1 D)

∆







×−

∆×

where:

OUT

) V

−×

∆=

××

IOUT is the maximum output current of the converter and

fSW is the oscillator switching frequency (127kHz). For

example, at VIN = 48V, VOUT = 3.3V, the ESR and input

capacitance are calculated for the input peak-to-peak

ripple of 100mV or less, yielding an ESR and capacitance

value of 40mΩ and 100µF, respectively.

Low-ESR ceramic multilayer chip capacitors are

recommended for size-optimized application. For ceramic

capacitors, assume the contribution from ESR and capac-

itor discharge is equal to 10% and 90%, respectively.

The input capacitor must handle the RMS ripple current

without significant rise in the temperature. The maximum

capacitor RMS current occurs at approximately 50% duty

cycle. Ensure that the ripple specification of the input

capacitor exceeds the worst-case capacitor RMS ripple

current. Use the following equations to calculate the input

capacitor RMS current:

CRMS

PRMS

AVGin

−

where:

PK DC

PRMS

OUT

AVGin

OUT

)

+×

×η

∆

−

IPRMS is the input switch RMS current, IAVGin is the input

average current, and h is the converter efficiency.

The ESR of the aluminum electrolytic capacitor increases

significantly at cold temperatures. Use a 1µF or greater value

ceramic capacitor in parallel with the aluminum electrolytic

input capacitor, especially for input voltages below 8V.

Output Filter Capacitor

The output capacitor (COUT) forms double pole with

the inductor and a zero with its ESR. The MAX5090’s

internal fixed compensation is designed for a 100µF

capacitor, and the ESR must be from 20mΩ to 100mΩ. The

use of an aluminum or tantalum electrolytic capacitor is

recommended. See Table 2 to choose an output capacitor

for stable operation.

The output ripple is comprised of ∆VOQ (caused by the

capacitor discharge), and ∆VOESR (caused by the ESR

of the capacitor). Use low-ESR tantalum or aluminum

electrolytic capacitors at the output. Use the following

equations to calculate the contribution of output capaci-

tance and its ESR on the peak-to-peak output rip voltage:

OESR

OUT

ESR

∆

= ∆×

∆

≈

××

The MAX5090 has a programmable soft-start time (tSS).

The output rise time is directly proportional to the output

capacitor, output voltage, and the load. The output rise time

also depends on the inductor value and the current-limit

threshold. It is important to keep the output rise time at

startup the same as the soft-start time (tSS) to avoid output

Figure 3. Forward-Voltage Drop vs. Temperature of the Internal

Body Diode of MAX5090

100

200

300

400

500

600

700

800

-40

100

125

150

TEMPERATURE (°C)

MAX5090A/B/C

2A, 76V, High-Efficiency MAXPower

Step-Down DC-DC Converters

www.maximintegrated.com

Maxim Integrated │ 12

MAX5090A/B/C датащи(PDF) 12 Page - Maxim Integrated Products

MAX5090A/B/C датащи(HTML) 12 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

датащи скачать

ссылки URL