MQHL-28-3R3S Datasheet, PDF (8/17 Pages) SynQor Worldwide Headquarters – HIGH RELIABILITY DC-DC CONVERTER

Technical Specification
MQHL-28-3R3S
Output: 3.3 V
Current: 15 A
BASIC OPERATION AND FEATURES
CONTROL FEATURES
The MQHL DC/DC converter uses a two-stage power
conversion topology. The first, or regulation, stage is a
buck-converter that keeps the output voltage constant over
variations in line, load, and temperature. The second, or
isolation, stage uses transformers to provide the functions of
input/output isolation and voltage transformation to achieve
the output voltage required.
Both the regulation and the isolation stages switch at a fixed
frequency for predictable EMI performance. The isolation
stage switches at one half the frequency of the regulation
stage, but due to the push-pull nature of this stage it
creates a ripple at double its switching frequency. As a
result, both the input and the output of the converter have a
fundamental ripple frequency of about 550 kHz in the free-
running mode.
Rectification of the isolation stage’s output is accomplished
with synchronous rectifiers. These devices, which are
MOSFETs with a very low resistance, dissipate far less energy
than would Schottky diodes. This is the primary reason why
the MQHL converters have such high efficiency, particularly
at low output voltages.
Besides improving efficiency, the synchronous rectifiers
permit operation down to zero load current. There is no
longer a need for a minimum load, as is typical for converters
that use diodes for rectification. The synchronous rectifiers
actually permit a negative load current to flow back into the
converter’s output terminals if the load is a source of short
or long term energy. The MQHL converters employ a “back-
drive current limit” to keep this negative output terminal
current small.
There is a control circuit in the MQHL converter that
determines the conduction state of the power switches.
It communicates across the isolation barrier through a
magnetically coupled device. No opto-isolators are used.
An input under-voltage lockout feature with hysteresis is
provided, as well as an input over-voltage shutdown and an
output over-voltage limit. There is also an output current
limit that is nearly constant as the load impedance decreases
(i.e., there is not fold-back or fold-forward characteristic to
the output current under this condition). When a load fault
is removed, the output voltage rises exponentially to its
nominal value without an overshoot. If a load fault pulls the
output voltage below about 60% of nominal, the converter
will shut down to attempt to clear the load fault. After a
short delay it will try to auto-restart.
The MQHL converter’s control circuit does not implement an
over-temperature shutdown.
The following sections describe the use and operation of
additional control features provided by the MQHL converter.
ENABLE: The MQFL converter has one enable pin, ENA1
(pin 4), which is referenced with respect to the converter’s
input return (pin 2). It must have a logic high level for the
converter to be enabled; a logic low inhibits the converter.
The enable pin is internally pulled high so that an open
connection will enable the converter. Figure A shows the
equivalent circuit looking into the enable pin. It is TTL
compatible and has hysteresis.
5V
82.5K
ENA1
10K
PIN4
TO ENABLE
CIRCUITRY
PIN2 ^{IN RTN}
Figure A: Equivalent circuit looking into the ENA1 pin.
SHUT DOWN: The MQHL converter will shut down in
response to only five conditions: ENA input low, VIN input
below under-voltage lockout threshold, VIN input above
over-voltage shutdown threshold, output voltage below the
output under-voltage threshold, and output voltage above
the output over-voltage threshold. Following any shutdown
event, there is a startup inhibit delay which will prevent the
converter from restarting for approximately 100ms. After
the 100ms delay elapses, if the enable inputs are high and
the input voltage is within the operating range, the converter
will restart. If the VIN input is brought down to nearly 0V
and back into the operating range, there is no startup inhibit,
and the output voltage will rise according to the “Turn-On
Delay, Rising Vin” specification.
REMOTE SENSE: The purpose of the remote sense pins
is to correct for the voltage drop along the conductors that
connect the converter’s output to the load. To achieve
this goal, a separate conductor should be used to connect
the +SENSE pin (pin 10) directly to the positive terminal
of the load, as shown in the connection diagram on Page
2. Similarly, the –SENSE pin (pin 9) should be connected
through a separate conductor to the return terminal of the
load.
NOTE: Even if remote sensing of the load voltage is not
desired, the +SENSE and the -SENSE pins must be connected
to +Vout (pin 7) and OUTPUT RETURN (pin 8), respectively,
to get proper regulation of the converter’s output. If they
are left open, the converter will have an output voltage that
is approximately 200mV higher than its specified value.
Product # MQHL-28-3R3S
Phone [phone]
www.synqor.com
Doc.# 005-0005077 Rev. 1
05/27/09
Page 8