LT3799
OPERATION
ꢀhe V
pin supplies a 2V reference voltage to be used
with AC, the following equation should be used with the
correction factor:
REF
with the control pins. ꢀo set an output current, a resistor
divider is used from the 2V reference to one of the control
pins. ꢀhe following equation sets the output current with
a resistor divider:
N
IOUꢀ = CꢀRL •
42 •RSENSE − CF
2N
2N
• RSENSE
R1= R2
− 1
R1= R2
− 1
(42 • I
• RSENSE • CF)
OUꢀ
42 • I
OUꢀ
where CR is the output current correction factor on the
Y-axis in Figure 3.
where R1 is the resistor connected to the V pin and the
CꢀRL pin and R2 is the resistor connected to the CꢀRL
REF
pin and ground.
Setting Control Voltages for LED Over Temperature
and Brownout Conditions
When used with an AC input voltage, the Lꢀ3799 senses
when the V
goes below 6±mV and above 6±mV
IN_SENSE
Critical Conduction Mode Operation
for detecting when the ꢀRIAC is off. During this low input
voltagetime,theoutputcurrentregulationloopisoffbutthe
partstillswitches.ꢀhishelpswithoutputcurrentregulation
with a ꢀRIAC but introduces a line regulation error. When
Criticalconductionmodeisavariablefrequencyswitching
scheme that always returns the secondary current to zero
with every cycle. ꢀhe Lꢀ3799 relies on boundary mode
and discontinuous mode to calculate the critical current
because the sensing scheme assumes the secondary
current returns 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 tripping due to leakage inductance ringing,
V
is low, very little power is being delivered to
IN_SENSE
the output and since the output current regulation loop
is off, this time period needs to be accounted for in set-
ting the output current. ꢀhis time period slightly varies
with line voltage. Figure 2 shows the correction factor
in selecting the resistor divider resistors. When used
1.16
1.14
1.12
1.10
1.08
1.06
1.04
1.02
0.5
PEAK V
1
1.5
0
IN_SENSE
3799 F03
Figure 2. Correction Factor in Selecting the
Resistor Divider Resistors
Figure 3. Output Current Correction Factor
3799p
12
Linear [ Linear ]
