LTC3803
10
3803fc
APPLICATIONS INFORMATION
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 0.020. 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 0.020 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
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 – 6%
74%
•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 120µA, 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 cur-
rent. 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 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.
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,
