LXE1684
Evaluation Board Installation Guide
Copyright 2001
Rev. 0.1 9/11/01
Page 3
New Product Information
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ESCRIPTION
With Both 5V and 12V power supplies readily available on an AT or ATX desktop power supply, the
LX1684 provides a flexible option for choosing either 5V or 12V as power source. Differential current
sensing allows more accurate current sensing from 12V which is unavailable in the earlier
LX1681/LX1682 controllers. The LXE1684 provides lossless HICCUP mode current limiting eliminating
the need for an expensive current sense resistor. The unit can survive an output short circuit condition
indefinitely without overstressing onboard power MOSFETS and inductor. Power consumption during
output short-circuit and during Hiccup mode is less than 10% of 10A full current load.
Utilizing synchronous Logic Level MOSFET rectification, the LXE1684 features high efficiency
conversion (typically 87% at full load of 10A for 12V to 3.3V application). With a switching frequency
of about 200KHz, it provides an optimum efficiency and low ripple output voltage over a full range of
output current load.
Features
High efficiency switching 3.3V power supply
Up to 92% efficiency for 12V to 3.3V application.
Internally compensated 16Mhz bandwidth opamp provides fast transient response
Lossless Hiccup R
DS_ON
sensing current limit protection
Voltage mode opamp requires no external compensation
Built in MOSFET drivers reduce overall cost.
Short Circuit protection: Can sustain output short circuit without overstressing MOSFET
and inductors on board,
Differential Kelvin current sensing provides superior performance than LX1681/LX1682
in terms of ease of use, accuracy and noise immunity.
Capacitor selection
The LXE1684 are shipped from factory with aluminum electrolytic capacitors in order to minimize
overall cost. The designer should choose long life, 105°C high temperature and high maximum ripple
current electrolytic capacitors in order to meet the demand of high ripple currents inherent to DC/DC high
current switching power supplies. Traditional aluminum electrolytic capacitors however are bulky in
comparison to newer type of special polymer dielectric or organic dielectric capacitors.
The LXE1684 PCB layout therefore is designed with both surface mount pads and through hole pads for
its power components such as FETs, inductors and capacitors, can be easily reconfigured to use all
surface mount components for high performance and low profile space constrained applications such as
video cards, memory power supply and other PCI bus and AGP bus multimedia interface cards.
The major advantage of LX1684 over LX1681 is that with differential current sensing with both positive
current sense and negative current sense, the topside power MOSFET can be far away from LX1684
and/or have different voltage and still provide superior performance in comparison with the LX1682. The
LX1684 does not limit the power input option to 5V, if current sensing is used. With 5V readily available
on desktop AT and ATX power supplies, the LXE1684 can provide a DC to DC power supply which can
draw power from 12V, 5V or 3.3V to provide output voltages as low as 1.25V while providing lossless
R
DS_ON
sensing for all power input options. Ripple voltage can be lower than 10mV if high performance
Poly capacitors or OSCON capacitors are used. The LX1684 provides stable operation with total output
capacitor ESR as low as 5m
when used in a DC to DC buck configuration.
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