Rainbow Electronics MAX5090C - Manual (Page 13)

2A, 76V, High-Efficiency MAXPower Step-Down
DC-DC Converters
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MAX5090A/B/C
In a dynamic load application, the allowable deviation
of the output voltage during the fast transient load dic-
tates 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 (t
RESPONSE ) depends on the closed-
loop bandwidth of the converter. The resistive drop
across the capacitor ESR and capacitor discharge
cause a voltage droop during a step-load. Use a com-
bination 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 output capacitor:
where I STEP is the load step and t RESPONSE 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 capaci-
tors 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 separately. 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 V
IN
.
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.
∆
∆
V I x ESR
V
I x t
C
OESR STEP OUT
OQ
STEP RESPONSE
OUT
=
=