MAX5986A MAXIM [Maxim Integrated Products], MAX5986A Datasheet - Page 13

• Manufacturer Part Number: MAX5986A
• Description: IEEE 802.3af-Compliant, High-Efficiency, Class 1,Powered Devices with Integrated DC-DC Converter
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX5986A/MAX5987A enter frequency foldback
mode when eight consecutive inductor current zero-
crossings occur. The switching frequency is 275kHz
under loads large enough that the inductor current does
not cross zero. In frequency foldback mode, the switch-
ing frequency is reduced to 137.5kHz to increase power
conversion efficiency. The device returns to normal
mode when the inductor current does not cross zero for
eight consecutive switching periods. Frequency foldback
mode is forced during startup until 50% of the soft-start
is completed.
The MAX5986A/MAX5987A include a hiccup protection
feature. When hiccup protection is triggered, the devices
turn off the high-side and turn on the low-side MOSFET
until the inductor current reaches the valley current limit.
The control logic waits 120ms until attempting a new soft-
start sequence. Hiccup mode is triggered if the current
in the high-side MOSFET exceeds the runaway current-
limit threshold, both during soft-start and during normal
operating mode. Hiccup mode can also be triggered in
normal operating mode in the case of an output under-
voltage event. This happens if the regulated feedback
voltage drops below 60% (typ).
The MAX5987A features an open-drain RESET output
that indicates if either the LDO or the switching regula-
tor drop out of regulation. The RESET output goes low if
either regulator drops below 92% of its regulated feed-
back value. RESET goes high impedance 100µs after
both regulators are above 95% of their value.
For applications where an auxiliary power source such
as a wall power adapter is used to power the PD, the
MAX5986A features wall power adapter detection. The
device gives priority to the WAD supply over V
and smoothly switches the power supply to WAD when
it is detected. The wall power adapter is connected from
WAD to PGND. The MAX5986A detects the wall power
adapter when the voltage from WAD to PGND is greater
than 8.8V. When a wall power adapter is detected, the
internal isolation MOSFET is turned off, classification
current is disabled and the device draws power from the
Powered Devices with Integrated DC-DC Converter
IEEE 802.3af-Compliant, High-Efficiency, Class 1,
Applications Information
High-Efficiency Light-Load Operation
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Frequency Foldback Protection for
Operation with Wall Adapter
RESET Output (MAX5987A)
Hiccup Mode
DD
supply,
auxiliary power source through V
iary power source to WAD, connect a diode from WAD to
V
An uncommitted LDO regulator is available to provide
a supply voltage to external circuits. A preset voltage
of 3.3V is set by connecting LDO_FB directly to VDRV.
For different output voltages connect a resistor divider
from LDO_OUT and LDO_FB to GND. The total feedback
resistance should be in the range of 100kω. The maxi-
mum output current is 85mA and thermal considerations
must be taken to prevent triggering thermal shutdown.
The LDO regulator can be powered by VOUT, a differ-
ent power supply, or grounded when not used. The LDO
is enabled once the buck converter has reached the
regulation voltage. The LDO is disabled when the buck
converter is turned off or not regulating.
The buck converter output voltage is set by changing the
feedback resistor-divider ratio. The output voltage can
be set from 3.0V to 5.6V. The FB voltage is regulated to
1.225V. Keep the trace from the FB pin to the center of
the resistive divider short, and keep the total feedback
resistance around 100kω.
Choose an inductor with the following equation:
where LIR is the ratio of the inductor ripple current to
full load current at the minimum duty cycle. Choose LIR
between 20% to 40% for best performance and stability.
Use an inductor with the lowest possible DC resistance
that fits in the allotted dimensions. Powdered iron ferrite
core types are often the best choice for performance.
With any core material, the core must be large enough
not to saturate at the current limit of the MAX5986A.
The input capacitor reduces the current peaks drawn
from the input power supply and reduces switching noise
in the IC. The total input capacitance must be equal or
greater than the value given by the following equation
to keep the input-ripple voltage within specification and
minimize the high-frequency ripple current being fed
back to the input source:
CC
Adjusting LDO Output Voltage (MAX5987A)
. See the typical application circuit in Figure 2.
MAX5986A/MAX5987A
Adjusting Buck Converter Output Voltage
V
CC
Input Capacitor Selection
Inductor Selection
CC
. Connect the auxil-