Datasheet

LTC3803-3
11
38033fd
APPLICATIONS INFORMATION
One potential design pitfall is undersizing the value of
capacitor C
VCC
. In this case, the normal supply current
drawn by the LTC3803-3 will discharge C
VCC
too rapidly;
before the third winding drive becomes effective, the V
CC
turn-off threshold will be reached. The LTC3803-3 turns
off, and the V
CC
node begins to charge via R
START
back up
to the V
CC
turn-on threshold. Depending on the particular
situation, this may result in either several on-off cycles
before proper operation is reached or permanent relaxation
oscillation at the V
CC
node.
Component selection is as follows:
Resistor R
START
should be made small enough to yield a
worst-case minimum charging current greater than the
maximum rated LTC3803-3 start-up current, to ensure
there is enough current to charge C
VCC
to the V
CC
turn-
on threshold. It should be made large enough to yield
a worst-case maximum charging current less than the
minimum rated LTC3803-3 supply current, so that in
operation, most of the LTC3803-3’s supply current is
delivered through the third winding. This results in the
highest possible effi ciency.
Capacitor C
VCC
should then be made large enough to avoid
the relaxation oscillation behavior described above. This
is complicated to determine theoretically as it depends on
the particulars of the secondary circuit and load behavior.
Empirical testing is recommended.
The third transformer winding should be designed so that
its output voltage, after accounting for the D2’s forward
voltage drop, exceeds the maximum V
CC
turn-off threshold.
Also, the third winding’s nominal output voltage should be
at least 0.5V below the minimum rated V
CC
clamp voltage
to avoid running up against the LTC3803-3’s V
CC
shunt
regulator, needlessly wasting power.
V
CC
SHUNT REGULATOR
In applications including a third transformer winding,
the internal V
CC
shunt regulator serves to protect the
LTC3803-3 from overvoltage transients as the third wind-
ing is powering up.
In applications where a third transformer winding is
undesirable or unavailable, the shunt regulator allows
the LTC3803-3 to be powered through a single dropping
resistor from V
IN
to V
CC
, in conjunction with a bypass
capacitor, C
VCC
, that closely decouples V
CC
to GND (see
Figure 3). This simplicity comes at the expense of reduced
effi ciency due to the static power dissipation in the R
VCC
dropping resistor.
The shunt regulator can draw up to 25mA through the
V
CC
pin to GND to drop enough voltage across R
VCC
to
regulate V
CC
to around 9.5V. For applications where V
IN
is low enough such that the static power dissipation in
R
VCC
is acceptable, using the V
CC
shunt regulator is the
simplest way to power the LTC3803-3.
LTC3803-3
V
CC
R
VCC
C
VCC
38033 F03
V
IN
GND
Figure 3. Powering the LTC3803-3 Via the
Internal Shunt Regulator
EXTERNAL PREREGULATOR
The circuit in Figure 4 shows a third way to power the
LTC3803-3. An external series preregulator consisting of
series pass transistor Q1, Zener diode D1, and bias resistor
R
B
brings V
CC
to at least 7.6V nominal, well above the V
CC
turn-off threshold. Resistor R
START
momentarily charges
the V
CC
node up to the V
CC
turn-on threshold, enabling
the LTC3803-3.
LTC3803-3
V
CC
R
START
R
B
D1
8.2V
Q1
C
VCC
38033 F04
V
IN
GND
Figure 4. Powering the LTC3803-3 with an External Preregulator