Datasheet

LT3825
19
3825fe
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
IN
is
equal to the change in bias current times R
A
. The design
procedure is to select the desired V
IN
referred voltage
hysteresis, V
UVHYS
. Then:
R
A
=
V
UVHYS
I
UVLO
where:
I
UVLO
= I
UVLOL
– I
UVLOH
is approximately 3.4µA
R
B
is then selected with the desired turn-on voltage:
R
B
=
R
A
V
IN(ON)
V
UVLO
– 1






If we wanted a V
IN
-referred trip point of 36V, with 1.8V
(5%) of hysteresis (on at 36V, off at 34.2V):
R
A
=
1.8V
3.4µA
= 529k, use 523k
R
B
=
523k
36V
1.23V
– 1






= 18.5k, use 18.7k
Even with good board layout, board noise may cause
problems with UVLO. You can filter 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 filtering by split-
ting the R
A
resistor with the capacitor. The split should
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
CC
power for the LT3825
(see Figure 4). This power “bootstrapping” improves
converter efficiency but is not inherently self-starting.
Start-up is affected with an external “trickle-charge” resis-
tor and the LT3825’s internal V
CC
undervoltage lockout
circuit
. The V
CC
undervoltage lockout has wide hysteresis
to facilitate start-up.
In operation, the “trickle charge” resistor, R
TR
, is con-
nected to V
IN
and supplies a small current, typically on the
order of 1mA to charge C
TR
. Initially the LT3825 is off and
draws only its start-up current. When C
TR
reaches the V
CC
turn-on threshold voltage the LT3825 turns on abruptly
and draws its normal supply current.
APPLICATIONS INFORMATION
V
IN
R
A
LT3825
(3a) UV Turning ON
UVLO
I
UVLO
R
B
V
IN
R
A
LT3825
(3b) UV Turning OFF (3c) UV Filtering
UVLO
UVLO
R
B
V
IN
R
A2
R
A1
C
UVLO
R
B
3825 F03
I
UVLO
Figure 3
+
I
VCC
3825 F04
R
TR
C
TR
V
IN
V
IN
I
VCC
V
VCC
V
ON
THRESHOLD
0
V
PG
V
CC
LT3825 PG
GND
•
•
•
Figure 4. Typical Power Bootstrapping