MAX5090A/B/C
2A, 76V, High-Efficiency MAXPower
Step-Down DC-DC Converters
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:
∆VOESR =ISTEP × ESR OUT
∆VOQ
= ISTEP
× t RESPONSE
C OUT
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.
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