LT3798 LINER [Linear Technology], LT3798 Datasheet - Page 10

• Manufacturer Part Number: LT3798
• Description: Isolated No Opto-Coupler Flyback Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
LT3798
fundamental frequency of the supply voltage is 120Hz so
the control loop unity gain frequency needs to be set less
than approximately 12Hz. Without a large amount of energy
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 25k resistor
from V
Startup
The LT3798 uses a hysteretic start-up to operate from
high offline voltages. A resistor connected to the supply
voltage protects the part from high voltages. This resistor
is connected to the V
a capacitor. When the resistor charges the V
turn-on voltage set with the EN/UVLO resistor divider and
the INTV
to switch. The 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
is attached to the V
from allowing V
voltage of the pin. The internal clamp is set at 40V and is
capable of 8mA(typical) of current at room temperature.
Setting the V
A large voltage difference between the V
and the V
third winding to power the part. The EN/UVLO sets these
two voltages. The pin has a 10μA current sink when the
pins voltage is below 1.25V and 0μA when above 1.25V.
The V
Figure 2. The UVLO threshold for V
10
The UVLO Threshold for V
IN
V
V
pin along with the resistor. An internal voltage clamp
IN(UVLO,RISING)
IN(UVLO,FALLING)
IN
IN_SENSE
CC
pin connects to a resistor divider as shown in
IN
turn-off voltage is preferred to allow time for the
pin is at its regulation point, the part begins
IN
to INTV
Turn-On and Turn-Off Voltages
IN
to go above the absolute maximum
IN
=
IN
=
pin to prevent the resistor current
1.25V • R1+ R2
CC
pin on the part and bypassed with
1.25V • R1+ R2
instead of the AC line voltage.
IN
R2
(
R2
Falling is :
(
IN
rising is:
IN
)
turn-on voltage
)
+ 10μA • R1
IN
pin to a
Programming Output Voltage
The output voltage is set using a resistor divider from
the third winding to the FB pin. From the Block Diagram,
the resistors R4 and R5 form a resistor divider from the
third winding. The FB also has an internal current source
that compensates for the diode drop. This current source
causes an offset in the output voltage that needs to be ac-
counted for when setting the output voltage. The output
voltage equation is:
where V
winding ratio between the secondary winding and the third
winding, V
diode, and I
The temperature coefficient of the diode's forward drop
needs to be the opposite of the term, (R4 • I
taking the partial derivative with respect to temperature,
the value of R4 is found to be the following:
where δI
source, and δV
drop of the output rectifying diode.
With R4 set with the above equation, the resistor value
for R5 is found using the following:
V
R4 = N
δI
I
R5 = (V
TC
OUT
TC
= 4.25μA
/δT = 12.4nA/°C
= V
BG
TC
ST
BG
F
Figure 2. Undervoltage Lockout (UVLO)
/δT is the partial derivative of the I
TC
BG
is the internal reference voltage, N
(1/(δI
is the forward drop of the output rectifying
• R4)/(N
is the internal current source for the FB pin.
(R4+R5)/(N
F
/δT is the partial derivative of the forward
TC
/δT)(δV
ST
LT3798
EN/UVLO
(V
OUT
ST
GND
3798
F
/δT))
F02
• R5)–(V
+V
F
V
)+R4 • I
IN
R1
R2
F
+ (R4 • I
TC
-V
TC
BG
TC
)/N
ST
TC
)
current
)/N
ST
is the
. By
ST
3798f
)