LTC1980
13
1980f
the transformer which can reduce the leakage inductance,
reduce the need for aggressive snubber design and for this
reason improve efficiency.
Avoid transformer saturation
under all operating condi-
tions and combinations (usually the biggest problems
occur at high output currents and extreme duty cycles.
Also
check these conditions for battery charging and
regulation modes.
Finally, in low voltage applications, select a transformer
with low winding resistance. This will improve efficiency at
heavier loads.
Capacitors
Check the RMS current rating on your capacitors on both
sides of your circuit. Low ESR and ESL is recommended
for lowest ripple. OS-CON capacitors (from Sanyo) work
very well in this application.
Diodes
In low voltage applications, Schottky diodes should be
placed in parallel with the drain and source of the MOSFETs
in the PWM supply. This prevents body diode turn on and
improves efficiency by eliminating loss from reverse re-
covery in these diodes. It also reduces conduction loss
during the RGTDR/BGTDR break interval.
The LTC1980 can operate to voltages as low as 2.8V.
Suitable Schottky diodes include the ZHCS1000 (V
F
=
420mV at I
F
= 1A) and SL22/23 (V
F
= 440mV at I
F
= 2A) for
most 500mA to 1A output current applications.
Vendor List
VENDOR COMPONENTS TELEPHONE
BH Electronics Transformers 952-894-9590
Coiltronics/Cooper Electronic Transformers 561-752-5000
Fairchild Semiconductor MOSFETs
Schottky Rectifiers 800-341-0392
Vishay (General Semiconductor) MOSFETs
Schottky Rectifiers 631-847-3000
Sanyo OS-CON Capacitors 408-749-9714
Sumida Electric USA Transformers 847-956-0666
Vishay (Siliconix) MOSFETs 408-988-8000
Current Sense Resistor
Voltage drop in the current sense resistor should be
limited to approximately ±100mV with respect to ground
at max load currents in all modes. This value strikes a
reasonable balance between providing an adequate low
current signal, while keeping the losses from this resistor
low. For applications where the inputs and output voltages
may be low, a somewhat lower drop can be used (in order
to reduce conduction losses slightly).
The LTC1980 has several features, such as leading-edge
blanking, which make application of this part easier to use.
However for best charge current accuracy, the current
sense resistor should be Kelvin sensed.
MOSFETs
The LTC1980 uses low side MOSFET switches. There are
two very important advantages. First, N-channel MOSFETs
are used—this generally means that efficiency will be
higher than a comparable on-resistance P-channel device
(because less gate charge is required). Second, low V
T
(‘logic-level’) MOSFETs with relatively low absolute maxi-
mum V
GS
ratings can be used, even in higher voltage
applications. Refer to Application Note 19 for information
on determining MOSFET voltage and current ratings.
Transformer
Turns ratio affects the duty factor of the power converter
which impacts current and voltage stress on the power
MOSFETs, input and output capacitor RMS currents and
transformer utilization (size vs power). Using a 50% duty
factor under nominal operating conditions usually gives
reasonable results. For a 50% duty factor, the turns ratio
is:
N = V
REG
/V
BAT
N should be calculated for the design operating as a DC/DC
converter and as a battery charger. The final turns ratio
should be chosen so that it is approximately equal to the
average of the two calculated values for N. In addition
choose a turns ratio which can be made from the ratio of
small integers. This allows bifilar windings to be used in
APPLICATIO S I FOR ATIO
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