LT3798
OPERATION
V
fundamental frequency of the supply voltage is 120Hz so
the control loop unity gain frequency needs to be set less
thanapproximately12Hz.Withoutalargeamountofenergy
storage on the secondary side, the output current will be
affected by the supply voltage changes, but the DC com-
ponent of the output current will be accurate. For DC input
or non-PFC AC input applications, connect a 2±k resistor
IN
R1
R2
EN/UVLO
Lꢀ3798
GND
3798 F02
from V
to INꢀV instead of the AC line voltage.
IN_SENSE
CC
Figure 2. Undervoltage Lockout (UVLO)
Startup
ꢀhe Lꢀ3798 uses a hysteretic start-up to operate from
high offline voltages. A resistor connected to the supply
voltage protects the part from high voltages. ꢀhis resistor
Programming Output Voltage
ꢀhe output voltage is set using a resistor divider from
the third winding to the FB pin. From the Block Diagram,
the resistors R4 and R± form a resistor divider from the
third winding. ꢀhe FB also has an internal current source
that compensates for the diode drop. ꢀhis current source
causes an offset in the output voltage that needs to be ac-
counted for when setting the output voltage. ꢀhe output
voltage equation is:
is connected to the V pin on the part and bypassed with
IN
a capacitor. When the resistor charges the V pin to a
IN
turn-on voltage set with the EN/UVLO resistor divider and
the INꢀV pin is at its regulation point, the part begins
CC
to switch. ꢀhe resistor cannot provide power for the part
in steady state, but relies on the capacitor to start-up the
part, then the third winding begins to provide power to the
V pin along with the resistor. An internal voltage clamp
V
OUꢀ
= V (R4+R±)/(N • R±)–(V + (R4 • I )/N )
BG Sꢀ F ꢀC Sꢀ
IN
is attached to the V pin to prevent the resistor current
IN
where V is the internal reference voltage, N is the
BG
Sꢀ
from allowing V to go above the absolute maximum
IN
windingratiobetweenthesecondarywindingandthethird
voltage of the pin. ꢀhe internal clamp is set at 40V and is
winding, V is the forward drop of the output rectifying
F
capable of 8mA(typical) of current at room temperature.
diode, and I is the internal current source for the FB pin.
ꢀC
Setting the V Turn-On and Turn-Off Voltages
ꢀhe temperature coefficient of the diode's forward drop
IN
needs to be the opposite of the term, (R4 • I )/N . By
ꢀC
Sꢀ
A large voltage difference between the V turn-on voltage
IN
taking the partial derivative with respect to temperature,
the value of R4 is found to be the following:
andtheV turn-offvoltageispreferredtoallowtimeforthe
IN
third winding to power the part. ꢀhe EN/UVLO sets these
two voltages. ꢀhe pin has a 10μA current sink when the
pins voltage is below 1.2±V and 0μA when above 1.2±V.
R4 = N (1/(δI /δꢀ)(δV /δꢀ))
Sꢀ
ꢀC
F
δI /δꢀ = 12.4nA/°C
ꢀC
ꢀhe V pin connects to a resistor divider as shown in
IN
Figure 2. ꢀhe UVLO threshold for V rising is:
I
= 4.2±μA
ꢀC
IN
where δI /δꢀ is the partial derivative of the I current
ꢀC
ꢀC
1.2±V • R1+ R2
(
)
+ 10μA •R1
V
=
source, and δV /δꢀ is the partial derivative of the forward
IN(UVLO,RISING)
F
R2
drop of the output rectifying diode.
ꢀhe UVLO ꢀhreshold for V Falling is :
IN
With R4 set with the above equation, the resistor value
for R± is found using the following:
1.2±V • R1+ R2
(
)
V
=
IN(UVLO,FALLING)
R2
R± = (V • R4)/(N (V +V )+R4 • I -V )
BG
Sꢀ OUꢀ
F
ꢀC BG
3798f
10
Linear [ Linear ]
