ltc3803es6-3 Linear Technology Corporation, ltc3803es6-3 Datasheet - Page 11

• Manufacturer Part Number: ltc3803es6-3
• Description: Constant Frequency Current Mode Flyback Dc/dc Controller In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
CURRENT SENSE RESISTOR CONSIDERATIONS
The external current sense resistor (R
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
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
resistor (R
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
SL
SL
) connecting the SENSE pin to the current sense
in Figure 2). This current ramp starts at zero right
SENSE
) thus develops a ramping voltage drop.
U
U
W
SENSE
in Figure 2)
U
in the current comparator threshold (ΔV
calculated using the following equation:
Note: LTC3803-3 enforces 8% < Duty Cycle < 80%.
A good starting value for R
drop in current comparator threshold at 80% duty cycle.
Designs not needing slope compensation may replace R
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
provides power for the LTC3803-3 via its V
ever, this arrangement is not inherently self-starting.
Start-up is affected by the use of an external “trickle-
charge” resistor (R
an internal wide hysteresis undervoltage lockout circuit
that monitors V
“Trickle charge” resistor R
supplies a small current, typically on the order of 100μA,
to charge C
reaches the V
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
to decline as the LTC3803-3 draws its normal supply
current, which exceeds that delivered by R
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.
Δ
V
SENSE
VCC
=
CC
Duty Cycle
CC
. After some time, the voltage on C
turn-on threshold. The LTC3803-3 then
BIAS
pin voltage. Operation is as follows:
START
80
in Figure 2) on the primary side that
%
in Figure 2) and the presence of
– %
SL
START
8
is 5.9k, which gives a 30mV
•
is connected to V
5
μ
A R
LTC3803-3
•
SL
SENSE
CC
START
VCC
pin. How-
) can be
11
begins
. After
IN
38033fa
and
VCC
SL