LTC3803-3 Datasheet, PDF (12/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803-3
APPLICATIO S I FOR ATIO
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 relax-
ation 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 efficiency.
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 thresh-
old. 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
winding is powering up.
In applications where a third transformer winding is unde-
sirable 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
efficiency 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.
V_IN
R_VCC
C_VCC
LTC3803-3
V_CC
GND
38033 F03
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 resis-
tor R_B brings V_CC to at least 7.6V nominal, well above the
maximum rated V_CC turn-off threshold of 6.8V. Resistor
R_START momentarily charges the V_CC node up to the V_CC
turn-on threshold, enabling the LTC3803-3.
V_IN
R_B
Q1
D1
8.2V
R_START
C_VCC
LTC3803-3
V_CC
GND
38033 F04
Figure 4. Powering the LTC3803-3 with an External Preregulator
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