Detailed Description
The MAX17681/MAX17681A is a high-voltage, high-
efficiency, iso-buck DC-DC converter designed to provide
isolated power up to 5W. The device operates over a
wide 4.5V to 42V input and uses primary side feedback to
regulate the output voltage.
The MAX17681/MAX17681A uses peak-current-mode
control. The low-resistance, on-chip MOSFETs ensure
high efficiency at full load while simplifying the PCB layout.
The programmable soft-start feature allows users to
reduce input inrush current. The device also incorporates
an output enable/undervoltage lockout pin (EN/UVLO)
that allows the user to turn on the part at the desired
input-voltage level. An open-drain RESET pin provides a
delayed power-good signal to the system upon achieving
successful regulation of the primary output voltage.
The device operates over the -40°C to +125°C industrial
temperature range and is available in a compact 10-pin
(3mm x 2mm) TDFN package.
Linear Regulator (VCC)
An internal linear regulator (VCC) provides a 5V nominal
supply to power the internal blocks and the low-side
MOSFET driver. The output of the VCC linear regulator
should be bypassed with a 1μF ceramic capacitor to
GND. The device employs an undervoltage-lockout circuit
that disables the internal linear regulator when VCC falls
below 3.7V (typ). The internal VCC linear regulator can
source up to 40mA (typ) to supply the device and to power
the low-side gate driver.
Enable Input (EN/UVLO) and Soft-Start (SS)
When the EN/UVLO voltage increases above 1.218V (typ),
the device initiates a soft-start sequence with the duration of
the soft-start being dependent on the value of the capacitor
connected from SS to GND. A 5μA current source charges
the capacitor and ramps up the SS pin voltage. The SS pin
voltage is used as reference for the internal error amplifier. The
reference ramp-up allows the output voltage to increase
monotonically from zero to the target value.
The EN/UVLO can be used as an input-voltage UVLO-
adjustment input. An external voltage-divider between VIN
and EN/UVLO to GND adjusts the input voltage at which
the device turns on or turns off. See the Setting the Input
Undervoltage Lockout Level section for details. If input
UVLO programming is not desired, connect the EN/UVLO
to VIN (see the Electrical Characteristics table for the EN/
UVLO rising and falling-threshold voltages). Driving the
EN/UVLO low disables both power MOSFETs as well as
other internal circuitry and reduces VIN quiescent current
to 0.9μA (typ). The SS capacitor is discharged with an
internal pulldown resistor when the EN/UVLO is low. If
the EN/UVLO pin is driven from an external signal source,
a series resistance of minimum 1kΩ is recommended to
be placed between the signal source output and the EN/
UVLO pin to reduce voltage ringing on the line.
Overcurrent Protection/HICCUP Mode
The MAX17681/MAX17681A are provided with an
overcurrent-protection scheme that protects the device
under overload and output short-circuit conditions. A cycle-
by-cycle peak current limit turns off the high-side MOSFET
whenever the switch current exceeds the internal limit of
1.65A (typ). Additionally, the sink current limit turns off
the low-side switch when the low side MOSFET negative
current exceeds 1.25A (typ). A runaway current limit on
the highside MOSFET current at 1.7A (typ) protects the
devices under high input voltage, short-circuit conditions.
The MAX17681 enters hiccup mode, either on one occurrence
of the runaway current limit or when the primary output
voltage (VPRI) drops to 70.5% (typ) of its nominal value
after the soft-start is completed. In the MAX17681, when
hiccup is triggered, the converter is protected by suspending
switching for a hiccup timeout period of 32,768 clock
cycles. Once the hiccup timeout period expires, soft-start
is attempted again. This behaviour works well for primary
output over-current events. However, when secondary-
side overcurrent events occur, additional measures are
required for the Iso-Buck topology to enter into hiccup
mode reliably, and also support robust output voltage
recovery after overcurrent removal. These measures are
not supported by MAX17681.
MAX17681 4.5V to 42V Input, High-Eciency,
Iso-Buck DC-DC Converter
www.maximintegrated.com Maxim Integrated
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