MAX5090ASLASHBSLASHC datasheet(13/17 Pages) MAXIM

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overshoot. Large output capacitors take longer than the

programmed soft-start time (tSS) and cause error-amplifier

saturation. This results in output overshoot. Use greater

than 2ms soft-start time for a 100µF output capacitor.

In a dynamic-load application, the allowable deviation of

the output voltage during the fast transient load dictates

the output capacitance value and the ESR. The output

capacitors supply the step-load current until the controller

responds with a greater duty cycle. The response time

(tRESPONSE) depends on the closed-loop bandwidth

of the converter. The resistive drop across the capaci-

tor ESR and capacitor discharge cause a voltage droop

during a step-load. Use a combination of low-ESR

tantalum and ceramic capacitors for better transient

load and ripple/noise performance. Use the following

equations to calculate the deviation of output voltage due

to the ESR and capacitance value of the out capacitor:

OESR

STEP

OUT

STEP

RESPONSE

OUT

ESR

∆=

∆

where ISTEP is the load step and tRESPONSE is the

response time of the controller. Controller response time

is approximately one-third of the reciprocal of the closed-

loop unity-gain bandwidth, 20kHz typically.

Board Layout Guidelines

1) Minimize ground noise by connecting the anode of the

Schottky rectifier, the input bypass capacitor ground

lead, and the output filter capacitor ground lead to a

large PGND plane.

2) Minimize lead lengths to reduce stray capacitance,

trace resistance, and radiated noise. In particular,

place the Schottky rectifier diode right next to the

device. Also, place the BST and VD bypass capacitors

very close to the device.

3) Connect the exposed pad of the IC to the SGND plane.

Do not make a direct connection between the exposed

pad plane and SGND (pin 7) under the IC. Connect

the exposed pad and pin 7 to the SGND plane sepa-

rately. Connect the ground connection of the feedback

resistive divider, ON/OFF threshold resistive divider,

and the soft-start capacitor to the SGND plane.

Connect the SGND plane and PGND plane at one

point near the input bypass capacitor at VIN.

4) Use large SGND plane as a heatsink for the MAX5090.

Use large PGND and LX planes as heatsinks for the

rectifier diode and the inductor.

MAX5090A/B/C

2A, 76V, High-Efficiency MAXPower

Step-Down DC-DC Converters

www.maximintegrated.com

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