LTC3803-3 Datasheet, PDF (11/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803-3
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 LTC3803-3 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 LTC3803-3 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 LTC3803-3’s
minimum duty cycle of 8%. 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 reducing the current comparator threshold in
proportion to duty cycle. This stabilizes the control loop
against subharmonic oscillation. The amount of reduction
in the current comparator threshold (ΔV_SENSE) can be
calculated using the following equation:
ΔV_SENSE
=
Duty
Cycle
80%
–
8%
•
5μA
•
R_SL
Note: LTC3803-3 enforces 8% < 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 LTC3803-3 is designed to implement DC/DC convert-
ers 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 LTC3803-3 via its V_CC pin. How-
ever, 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 LTC3803-3 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 LTC3803-3 draws its normal supply
current, 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 LTC3803-3.
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