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.

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

(‘logic-level’) MOSFETs with relatively low absolute maxi-

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 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