The maximum output current should be limited to less
than 15A. The usable output voltage range for the EV
kit is +2.6V to +4.0V. Additionally, U3, U2, and resistor
R19 limit the minimum output voltage (VOUT) to +2.6V.
Current Limiting
The EV kit features cycle-by-cycle current limiting of the
transformer primary current. The MAX5051 turns off
both external switching transistors (N1, N2) when the
voltage at the CS pin of the MAX5051 reaches 154mV
(typ). Current-sense resistor R17 (= 0.027Ω) limits the
peak primary current to approximately 5.7A
(154mV/0.027Ω≈5.7A). This limits short-circuit current
on the secondary output (VOUT) to 20.3A peak typical-
ly. Under short-circuit conditions, the average output
current is only 473mA typically due to hiccup-mode
fault protection. To evaluate lower current limits, cur-
rent-sense resistor R17 must be replaced with a differ-
ent value surface-mount resistor (1206 size) as deter-
mined by the following equation:
where V
SENSE
= 0.154V, N
s
= 2, N
p
= 8 and I
OUTMAX
=
maximum DC output current (15A or less). Note that
some fine tuning may be required when selecting the
current-limit resistor. There are errors introduced as a
result of the presence of the transformer and output
inductor ripple current.
Soft-Start
The MAX5051 EV kit limits the output voltage rate of
rise with a soft-start feature. Capacitor C5 sets the
ramp time to 91ms. To evaluate other soft-start ramp
times, replace capacitor C5 with another surface-mount
capacitor (0603 size) as determined by the following
equation:
where softstart_time is the desired soft-start time in
seconds.
Undervoltage Lockout (UVLO)
The MAX5051 EV kit features a UVLO circuit that pre-
vents operation below the programmed input supply
start voltage. Resistors R5 and R6 set the input voltage
brownout UVLO of the EV kit. To evaluate other input
UVLO voltages, replace resistor R6 with another sur-
face-mount resistor (0805 size). Using the desired start-
up voltage, resistor R6 is then found by using the fol-
lowing equation:
where VIN
STARTUP
is the desired startup voltage at
which the EV kit starts and resistor R5 is typically 38.3kΩ.
Payton Planar Magnetics Ltd. 561-969-9585 561-989-9587 www.paytongroup.com
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