MAX5988BEVKIT# Maxim Integrated, MAX5988BEVKIT# Datasheet - Page 14

• Manufacturer Part Number: MAX5988BEVKIT#
• Description: Power Management IC Development Tools MAX5988B Eval Kit
• Manufacturer: Maxim Integrated

Specifications of MAX5988BEVKIT#

• Rohs: yes

The devices enter frequency foldback mode when eight
consecutive inductor current zero-crossings occur. The
switching frequency is 215kHz under loads large enough
that the inductor current does not cross zero. In frequen-
cy foldback mode, the switching frequency is reduced to
107.5kHz to increase power conversion efficiency. The
device returns to normal mode when the inductor current
does not cross zero for eight consecutive switching peri-
ods. Frequency foldback mode is forced during startup
until 50% of the soft-start is completed.
The devices 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 154ms 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 devices feature an open-drain RESET output that
indicates if either the LDO or the switching regulator drop
out of regulation. The RESET output goes low if either
regulator drops below 90% of its regulated feedback
value. RESET goes high impedance 4.8ms after both
regulators are above 95% of their value.
The devices feature the MPS to comply with the IEEE
802.3af standard. It is able to maintain a minimum current
(10mA) of the port to avoid the power disconnection from
the PSE. The devices enter MPS mode when the port
current is lower than 14mA and also exit the MPS mode
when the port current is greater than 40mA. The feature is
enabled by connecting the MPS pin to VDRV, or disabled
by connecting the MPS pin to GND.
For applications where an auxiliary power source such
as a wall power adapter is used to power the PD, the
devices feature wall power adapter detection. The device
Maxim Integrated
IEEE 802.3af-Compliant, High-Efficiency, Class 1/Class 2,
Powered Devices with Integrated DC-DC Converter
Maintain Power Signature (MPS)
Applications Information
High-Efficiency Light-Load Operation
Frequency Foldback Protection for
Operation with Wall Adapter
RESET Output
Hiccup Mode
gives priority to the WAD supply over V
smoothly switches the power supply to WAD when it is
detected. The wall power adapter is connected from
WAD to PGND. The devices detect the wall power adapt-
er 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 auxiliary
power source through V
source to WAD, connect a diode from WAD to V
the typical application circuit in
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 feed-
back resistance should be in the range of 100kω. The
minimum output current capability is 85mA and thermal
considerations must be taken to prevent triggering ther-
mal shutdown. The LDO regulator can be powered by
VOUT, a different 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 (MAX5988A) or 5.4V to
14V (MAX5988B). The FB voltage is regulated to 1.227V.
Keep the trace from the FB pin to the center of the resis-
tive divider short, and keep the total feedback resistance
around 10kω.
Choose an inductor with the following equation:
where L
full load current at the minimum duty cycle. Choose L
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 devices.
MAX5988A/MAX5988B
IR
is the ratio of the inductor ripple current to
Adjusting Buck Converter Output Voltage
L
=
f
S
Adjusting LDO Output Voltage
×
V
OUT
V
CC
CC
×
×
. Connect the auxiliary power
(
L
V
IR
CC
×
Figures 3
Inductor Selection
−
I
OUT MAX
V
OUT
(
)
DD
and 4.
)
supply, and
CC
. See
14
IR