SQM48T/S20 DC-DC Converter Data Sheet
36-75 VDC Input; 1.2-3.3 VDC @ 20A Outputs
Operations
Input and Output Impedance
These power converters have been designed to be
stable with no external capacitors when used in low
inductance input and output circuits.
In many applications, the inductance associated with
the distribution from the power source to the input of
the converter can affect the stability of the converter.
The addition of a 33 μF electrolytic capacitor with an
ESR < 1 ? across the input helps ensure stability of
the converter. In many applications, the user has to
use decoupling capacitance at the load. The power
converter will exhibit stable operation with external
load capacitance up to 20,000 μF on 3.3 – 1.2 V
of ? 0.8 V. An external voltage source ( ± 20 V
maximum) may be connected directly to the ON/OFF
input, in which case it must be capable of sourcing or
sinking up to 1 mA depending on the signal polarity.
See the Start-up Information section for system timing
waveforms associated with use of the ON/OFF pin.
Remote Sense (Pins 5 and 7)
The remote sense feature of the converter
compensates for voltage drops occurring between the
output pins of the converter and the load. The
SENSE(-) (Pin 5) and SENSE(+) (Pin 7) pins should
be connected at the load or at the point where
regulation is required (see Fig. B).
Semi Q Family
outputs.
Additionally, see the EMC section of this data sheet
for discussion of other external components which
may be required for control of conducted emissions
Vin
Vin (+)
ON/OFF
TM
(Top View)
Converter Vout (+)
100
SENSE (+)
TRIM
SENSE (-)
Rw
Rload
10
ON/OFF (Pin 2)
Vin (-)
Vout (+)
Rw
The ON/OFF pin is used to turn the power converter
on or off remotely via a system signal. There are two
remote control options available, positive logic and
negative logic and both are referenced to Vin(-). A
typical connection is shown in Fig. A.
Fig. B: Remote sense circuit configuration.
If remote sensing is not utilized, the SENSE(-) pin must be
connected to the Vout(-) pin (Pin 4), and the SENSE(+) pin
must be connected to the Vout(+) pin (Pin 8) to ensure the
converter will regulate at the specified output voltage. If these
connections are not made, the converter will deliver an output
voltage that is slightly higher than the specified data sheet
value.
Semi Q Family
Vin
Vin (+)
ON/OFF
TM
Converter
(Top View)
Vout (+)
SENSE (+)
TRIM
Rload
Because the sense leads carry minimal current, large
traces on the end-user board are not required.
However, sense traces should be run side by side and
located close to a ground plane to minimize system
SENSE (-)
Vin (-) Vout (-)
CONTROL
INPUT
Fig. A: Circuit configuration for ON/OFF function.
The positive logic version turns on when the ON/OFF
pin is at a logic high and turns off when at a logic low.
The converter is on when the ON/OFF pin is left
open. See table, page 2 for logic high/low definitions.
The negative logic version turns on when the pin is at
a logic low and turns off when the pin is at a logic
high. The ON/OFF pin can be hard wired directly to
Vin(-) to enable automatic power up of the converter
without the need of an external control signal.
The ON/OFF pin is internally pulled-up to 5V through
a resistor. A properly debounced mechanical switch,
open collector transistor, or FET can be used to drive
the input of the ON/OFF pin. The device must be
capable of sinking up to 0.2mA at a low level voltage
noise and insure optimum performance.
When using the remote sense function, the
converter’s output overvoltage protection (OVP)
senses the voltage across Vout(+) and Vout(-), and
not across the sense lines, so the resistance (and
resulting voltage drop) between the output pins of the
converter and the load should be minimized to
prevent unwanted triggering of the OVP.
When utilizing the remote sense feature, care must be
taken not to exceed the maximum allowable output
power capability of the converter, equal to the product
of the nominal output voltage and the allowable output
current for the given conditions.
When using remote sense, the output voltage at the
converter can be increased by as much as 10%
above the nominal rating in order to maintain the
required voltage across the load. Therefore, the
designer must, if necessary, decrease the maximum
MCD10208 Rev. 1.0, 08-Jul-10
Page 3 of 33
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