LTC3803 Datasheet, PDF (9/12 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803
APPLICATIO S I FOR ATIO
CURRENT SENSE RESISTOR CONSIDERATIONS
The external current sense resistor (R_SENSE in Figure 2)
allows the user to optimize the current limit behavior for
the particular application. As the current sense resistor is
varied from several ohms down to tens of milliohms, peak
switch current goes from a fraction of an ampere to several
amperes. Care must be taken to ensure proper circuit
operation, especially with small current sense resistor
values.
For example, a peak switch current of 5A requires a sense
resistor of 0.020Ω. Note that the instantaneous peak
power in the sense resistor is 0.5W and it must be rated
accordingly. The LTC3803 has only a single sense line to
this resistor. Therefore, any parasitic resistance in the
ground side connection of the sense resistor will increase
its apparent value. In the case of a 0.020Ω sense resistor,
one milliohm of parasitic resistance will cause a 5%
reduction in peak switch current. So the resistance of
printed circuit copper traces and vias cannot necessarily
be ignored.
PROGRAMMABLE SLOPE COMPENSATION
The LTC3803 injects a ramping current through its SENSE
pin into an external slope compensation resistor (R_SL in
Figure 2). This current ramp starts at zero right after the
NGATE pin has been high for the LTC3803’s minimum
duty cycle of 6%. The current rises linearly towards a peak
of 5µA at the maximum duty cycle of 80%, shutting off
once the NGATE pin goes low. A series resistor (R_SL)
connecting the SENSE pin to the current sense resistor
(R_SENSE) thus develops a ramping voltage drop. From the
perspective of the SENSE pin, this ramping voltage adds
to the voltage across the sense resistor, effectively reduc-
ing the current comparator threshold in proportion to duty
cycle. This stabilizes the control loop against subharmonic
oscillation. The amount of reduction in the current com-
parator threshold (∆V_SENSE) can be calculated using the
following equation:
∆V_SENSE
=
Duty
Cycle
74%
–
6%
•
5µA
•
R_SL
Note: LTC3803 enforces 6% < Duty Cycle < 80%.
A good starting value for R_SL is 5.9k, which gives a 30mV
drop in current comparator threshold at 80% duty cycle.
Designs not needing slope compensation may replace R_SL
with a short circuit.
INTERNAL WIDE HYSTERESIS UNDERVOLTAGE
LOCKOUT
The LTC3803 is designed to implement DC/DC converters
operating from input voltages of typically 48V or more.
The standard operating topology employs a third trans-
former winding (L_BIAS in Figure 2) on the primary side that
provides power for the LTC3803 via its V_CC pin. However,
this arrangement is not inherently self-starting. Start-up is
affected by the use of an external “trickle-charge” resistor
(R_START in Figure 2) and the presence of an internal wide
hysteresis undervoltage lockout circuit that monitors V_CC
pin voltage. Operation is as follows:
“Trickle charge” resistor R_START is connected to V_IN and
supplies a small current, typically on the order of 100µA,
to charge C_VCC. After some time, the voltage on C_VCC
reaches the V_CC turn-on threshold. The LTC3803 then
turns on abruptly and draws its normal supply current. The
NGATE pin begins switching and the external MOSFET
(Q1) begins to deliver power. The voltage on C_VCC begins
to decline as the LTC3803 draws its normal supply cur-
rent, which exceeds that delivered by R_START. After some
time, typically tens of milliseconds, the output voltage
approaches its desired value. By this time, the third
transformer winding is providing virtually all the supply
current required by the LTC3803.
One potential design pitfall is undersizing the value of
capacitor C_VCC. In this case, the normal supply current
drawn by the LTC3803 will discharge C_VCC too rapidly;
before the third winding drive becomes effective, the V_CC
turn-off threshold will be reached. The LTC3803 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.
3803i
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