Datasheet

42681fc
LTC4268-1
35
Converter Start-Up
The standard topology for the LTC4268-1 utilizes a third
transformer winding on the primary side that provides
both feedback information and local V
CC
power for the
LTC4268-1 (see Figure 16). This power “bootstrapping”
improves converter efficiency but is not inherently self-
starting. Start-up is affected with an external “trickle charge”
resistor and the LTC4268-1’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 connected
to V
IN
and supplies a small current, typically on the order
of 1mA to charge C
TR
. Initially the LTC4268-1 is off and
draws only its start-up current. When C
TR
reaches the
V
CC
turn-on threshold voltage the LTC4268-1 turns on
abruptly and draws its normal supply current.
Switching action commences and the converter begins to
deliver power to the output. Initially the output voltage is
low and the flyback voltage is also low, so C
TR
supplies
most of the LTC4268-1 current (only a fraction comes
from R
TR
.) V
CC
voltage continues to drop until after some
time, typically tens of milliseconds, the output voltage
approaches its desired value. The
flyback winding
then
provides the LTC4268-1 supply current and the V
CC
voltage stabilizes.
applicaTions inForMaTion
If C
TR
is undersized, V
CC
reaches the V
CC
turn-off threshold
before stabilization and the LTC4268-1 turns off. The V
CC
node then begins to charge back up via R
TR
to the turn-on
threshold, where the part again turns on. Depending upon
the circuit, this may result in either several on-off cycles
before proper operation is reached, or permanent relaxation
oscillation at the V
CC
node.
R
TR
is selected to yield a worst-case minimum charging
current greater than the maximum rated LTC4268-1 start-
up current, and a worst-case maximum charging current
less than the minimum rated LTC4268-1 supply current.
R
TR(MAX)
<
V
IN(MIN)
− V
CC(ON _ MAX)
I
CC(ST _ MAX)
and
R
TR(MIN)
>
V
IN(MAX)
− V
CC(ON _ MIN)
I
CC(MIN)
Make C
TR
large enough to avoid the relaxation oscillatory
behavior described above. This is complicated to deter-
mine theoretically as it depends on the particulars of the
secondary circuit and load behavior. Empirical testing is
recommended. Note that the use of the optional soft-start
function lengthens the power-up timing and requires a
correspondingly larger value for C
TR
.
+
I
VCC
42681 F16
R
TR
C
TR
V
IN
V
IN
I
VCC
V
VCC
V
ON
THRESHOLD
0
V
PG
V
CC
LTC4268-1 PG
GND
•
•
•
Figure 16. Typical Power Bootstrapping