lt3825 Linear Technology Corporation, lt3825 Datasheet - Page 19

• Manufacturer Part Number: lt3825
• Description: Isolated No-opto Synchronous Flyback Controller With Wide Input Supply Range
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The bias current on this pin depends on the pin volt-
age and UVLO state. The change provides the user with
adjustable UVLO hysteresis. When the pin rises above
the UVLO threshold a small current is sourced out of the
pin, increasing the voltage on the pin. As the pin voltage
drops below this threshold, the current is stopped, further
dropping the voltage on UVLO. In this manner, hysteresis
is produced.
Referring to Figure 3, the voltage hysteresis at V
equal to the change in bias current times R
procedure is to select the desired V
hysteresis, V
where:
R
If we wanted a V
(5%) of hysteresis (on at 36V, off at 34.2V):
V
(3a) UV Turning ON
B
IN
R
R
I
R
R
R
R
UVLO
is then selected with the desired turn-on voltage:
A
B
B
B
A
A
=
=
=
= 1.8V
UVLO
= I
3.4μA
V
LT3825
V
1.23V
I
V
UVHYS
UVLO
IN(ON)
36V
UVLOL
UVLO
523k
I
UVLO
UVHYS
R
A
= 529k, use 523k
– 1
– 1
IN
– I
. Then:
-referred trip point of 36V, with 1.8V
(3b) UV Turning OFF
V
UVLOH
IN
= 18.5k, use 18.7k
R
R
A
B
Figure 3
UVLO
is approximately 3.4μA
LT3825
I
UVLO
IN
referred voltage
(3c) UV Filtering
A
C
. The design
UVLO
V
IN
R
R
R
3825 F03
A1
A2
B
IN
UVLO
is
Even with good board layout, board noise may cause
problems with UVLO. You can fi lter the divider but keep
large capacitance off the UVLO node because it will slow
the hysteresis produced from the change in bias current.
Figure 3c shows an alternate method of fi ltering by split-
ting the R
put more of the resistance on the UVLO side.
Converter Start-Up
The standard topology for the LT3825 utilizes a third
transformer winding on the primary side that provides both
feedback information and local V
(see Figure 4). This power “bootstrapping” improves
converter effi ciency but is not inherently self-starting.
Start-up is affected with an external “trickle-charge” re-
sistor and the LT3825’s internal V
circuit. The V
to facilitate start-up.
In operation, the “trickle charge” resistor R
to V
of 1mA to charge C
only its start-up current. When C
threshold voltage the LT3825 turns on abruptly and draws
its normal supply current.
IN
and supplies a small current, typically on the order
+
A
V
IN
Figure 4. Typical Power Bootstrapping
resistor with the capacitor. The split should
CC
R
C
TR
V
TR
ON
undervoltage lockout has wide hysteresis
THRESHOLD
TR
. Initially the LT3825 is off and draws
V
LT3825
I
V
VCC
VCC
GND
V
PG
0
CC
I
VCC
PG
TR
CC
CC
reaches the V
power for the LT3825
undervoltage lockout
V
IN
3825 F04
•
•
TR
LT3825
is connected
•
CC
turn-on
19
3825fa