LT3798
OPERATION
to the input capacitor. ꢀhis is a second-order network
composed of the parasitic capacitance on the switch node
and the magnetizing inductance of the primary winding
of the transformer. ꢀhe minimum voltage of the switch
very high frequency. ꢀhe output voltage sensing circuitry
needs a minimum amount of flyback waveform time to
sense the output voltage on the third winding. ꢀhe time
needed is 3±0ns. ꢀhe minimum current limit allows the
useofsmallertransformerssincethemagnetizingprimary
inductance does not need to be as high to allow proper
time to sample the output voltage information.
node during this discontinuous ring is V – V
• N .
IN
OUꢀ
PS
ꢀhe Lꢀ3798 turns the switch back on at this time, during
the discontinuous switch waveform, by sensing when
the slope of the switch waveform goes from negative to
positive using the dv/dt detector. ꢀhis switching technique
may increase efficiency by ±5.
ꢀo help improve crossover distortion of the line input
current, a second minimum current limit of 65 becomes
activewhentheV
currentislowerthan27μA.Since
IN_SENSE
the off-time becomes very short with this lower minimum
Sense Resistor Selection
current limit, the sample-and-hold is deactivated.
ꢀhe resistor, R
, between the source of the external
SENSE
N-channelMOSFEꢀandGNDshouldbeselectedtoprovide
anadequateswitchcurrenttodrivetheapplicationwithout
exceeding the current limit threshold.
Universal Input
ꢀhe Lꢀ3798 operates over the universal input voltage
range of 90VAC to 26±VAC. In the ꢀypical Performance
For applications without power factor correction, select a
resistor according to:
Characteristics section, the Output Voltage vs V and the
IN
Output Current vs V graphs, show the output voltage
IN
and output current line regulation for the first application
2(1– D)NPS
IOUꢀ • 42
picture in the ꢀypical Applications section.
RSENSE
=
• 9±5
Selecting Winding Turns Ratio
where
Boundarymodeoperationgivesalotoffreedominselecting
the turns ratio of the transformer. We suggest to keep the
VOUꢀ •NPS
VOUꢀ •NPS + V
D =
IN
duty cycle low, lower N , at the maximum input voltage
PS
since the duty cycle will increase when the AC waveform
For applications with power factor correction, select a
resistor according to:
decreases to zero volts. A higher N increases the output
PS
current while keeping the primary current limit constant.
Although this seems to be a good idea, it comes at the
expense of a higher RMS current for the secondary-side
diodewhichmightnotbedesirablebecauseoftheprimary
sideMOSFEꢀ’ssuperiorperformanceasaswitch.Ahigher
2(1– D)NPS
IOUꢀ • 42
RSENSE
=
• 47.±5
where
VOUꢀ •NPS
VOUꢀ •NPS + V
N
PS
does reduce the voltage stress on the secondary-side
D =
diode while increasing the voltage stress on the primary-
side MOSFEꢀ. If switching frequency at full output load is
kept constant, the amount of energy delivered per cycle by
IN
Minimum Current Limit
the transformer also stays constant regardless of the N .
PS
ꢀhe Lꢀ3798 features a minimum current limit of approxi-
mately 185 of the peak current limit. ꢀhis is necessary
when operating in critical conduction mode since low
current limits would increase the operating frequency to a
ꢀherefore, the size of the transformer remains the same at
practical N ’s. Adjusting the turns ratio is a good way to
PS
find an optimal MOSFEꢀ and diode for a given application.
3798f
12
Linear [ Linear ]
