LT3798
OPERATION
Programming Output Current
pins. ꢀhe following equation sets the output current with
a resistor divider:
ꢀhe maximum output current depends on the supply volt-
age and the output voltage in a flyback topology. With the
⎛
⎞
2NPS
•RSENSE
R1=R2
– 1
⎜
⎝
⎟
V
pinconnectedto100μAcurrentsourceandaDC
IN_SENSE
42 •I
⎠
OUꢀ
supplyvoltage,themaximumoutputcurrentisdetermined
at the minimum supply voltage, and the maximum output
voltage using the following equation:
where R1 is the resistor connected to the V pin and the
REF
CꢀRL pin and R2 is the resistor connected to the CꢀRL
pin and ground.
NPS
42 •RSENSE
IOUꢀ(MAX) = 2•(1– D)•
Setting V
Resistor
IN_SENSE
where
ꢀheV
resistorsetsthecurrentfeedingtheinternal
IN_SENSE
VOUꢀ •NPS
VOUꢀ •NPS + V
multiplierthatmodulatesthecurrentlimitforpowerfactor
correction.Atthemaximumlinevoltage,V ,thecurrent
D =
MAX
IN
is set to 360μA. Under this condition, the resistor value is
ꢀhe maximum control voltage to achieve this maximum
output current is 2V • (1-D).
equal to (V /360μA).
MAX
For DC input or non-PFC AC input applications, connect
It is suggested to operate at 9±5 of these values to give
margin for the part’s tolerances.
a 2±k resistor from V
AC line voltage.
to INꢀV instead of the
IN_SENSE
CC
When designing for power factor correction, the output
currentwaveformisgoingtohaveahalfsinewavesquared
shape and will no longer be able to provide the above
currents. By taking the integral of a sine wave squared
over half a cycle, the average output current is found to
be half the value of the peak output current. In this case,
the recommended maximum average output current is
as follows:
Critical Conduction Mode Operation
Criticalconductionmodeisavariablefrequencyswitching
scheme that always returns the secondary current to zero
witheverycycle.ꢀheLꢀ3798reliesonboundarymodeand
discontinuousmodetocalculatethecriticalcurrentbecause
thesensingschemeassumesthesecondarycurrentreturns
to zero with every cycle. ꢀhe DCM pin uses a fast current
input comparator in combination with a small capacitor to
detect dv/dt on the third winding. ꢀo eliminate false trip-
ping due to leakage inductance ringing, a blanking time of
between600nsand2ꢁsisappliedaftertheswitchturnsoff,
depending on the current limit shown in the Leakage In-
ductanceBlankingꢀimevsSENSECurrentLimitꢀhreshold
curve in the ꢀypical Performance Characteristics section.
ꢀhe detector looks for 80ꢁA of current through the DCM
pin due to falling voltage on the third winding when the
secondarydiodeturnsoff.ꢀhisdetectionisimportantsince
the output current is calculated using this comparator’s
output. ꢀhis is not the optimal time to turn the switch on
NPS
42 •RSENSE
IOUꢀ(MAX) = 2•(1−D) •
• 47.±5
where
VOUꢀ •NPS
VOUꢀ •NPS + V
D =
IN
ꢀhe maximum control voltage to achieve this maximum
output current is (1-D) • 47.±5.
For control voltages below the maximum, the output cur-
rent is equal to the following equation:
NPS
42 •RSENSE
because the switch voltage is still close to V
+ V
• N
IOUꢀ = CꢀRL•
IN
OUꢀ
PS
and would waste all the energy stored in the parasitic ca-
pacitanceontheswitchnode.Discontinuousringingbegins
when the secondary current reaches zero and the energy
in the parasitic capacitance on the switch node transfers
ꢀhe V
pin supplies a 2V reference voltage to be used
REF
with the control pins. ꢀo set an output current, a resistor
divider is used from the 2V reference to one of the control
3798f
11
Linear [ Linear ]
