LTC3805-5 Linear Technology, LTC3805-5 Datasheet - Page 15

• Manufacturer Part Number: LTC3805-5
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
trip threshold from 100mV. This section describes how
to pick R
discussion that follows be careful to distinguish between
“current limit” where the converter continues to run with
the I
while the output voltage falls below the regulation point
and “overcurrent protection” where the OC pin senses an
overcurrent and shuts down the converter for a timeout
period before attempting an automatic restart.
One overcurrent protection strategy is for the converter
to never enter current limit but to maintain output volt-
age regulation up to the point of tripping the overcurrent
protection. Operation at minimum input voltage V
hits current limiting for the smallest output current and
is the design point for this strategy.
First, for operation at V
Cycle V
whether the converter is a boost, fl yback or SEPIC. Then
use Duty Cycle V
using the formula in the prior section. For overcurrent
protection to trip at exactly the point where current limit-
ing would begin set:
To fi nd the actual output current that trips overcurrent
protection, calculate the peak switch current I
from:
Then calculate the converter output current that corre-
sponds to I
both on converter type and the details of converter design
R
I
PK VIN MIN
SENSE
OC CRIT
(
IN(MIN)
(
OC
(
pin limiting current on a cycle-by-cycle basis
PK
)
to achieve the desired performance. In the
=
using the appropriate formula depending on
(
)
VIN(MIN)
)
Δ
=
V
IN(MIN)
100
SENSE VIN MIN
IN(MIN)
10μ
) . Again, the calculation depends
mV
(
to calculate ΔV
A
−
, calculate the duty cycle Duty
Δ
(
R
V
SENSE
SENSE VIN MIN
)
)
(
SENSE
(
PK
)
(
(
)
VIN(MIN)
VIN(MIN)
IN(MIN)
)
)
including inductor current ripple. For minimum input volt-
age, R
current just before loss of output voltage regulation and
the onset of current limiting. Note that the output current
that causes an overcurrent trip is higher for higher input
voltages but that an overcurrent trip will always occur
before loss of output voltage regulation. If desired to
meet a specifi c design target, an increase in R
R
make the converter trip for a lower output current.
This calculation is based on steady-state operation. De-
pending on design, overcurrent protection can also be
triggered during a start up transient, particularly if large
output fi lter capacitors are being charged as output voltage
rises. If that is a problem, output capacitor charging can
be slowed by using a larger value of SSFLT capacitor. It is
also possible to trip overcurrent protection during a load
step especially if the trip threshold is lowered by making
R
Another overcurrent protection strategy is keep the con-
verter running as current limiting reduces the duty cycle
and the output voltage sags. In this case, the goal is often
keep the converter in normal operation over as wide a range
as possible, including current limiting, and to trigger the
overcurrent trip only to prevent damage. To implement
this strategy use a value of R
This also reduces sensitivity to overcurrent trips caused by
transient operation. In the limit, set R
the OC pin directly to R
trip near minimum duty cycle or around 6%.
In some cases it may be desirable to increase the trip
threshold even further. In this strategy, the converter is
allowed to operate all the way down to minimum duty
cycle at which point the cycle-by-cycle current limit of
OC(CRIT)
OC
> R
OC(CRIT)
OC(CRIT)
can be used to reduce the trip threshold and
produces an overcurrent trip at an output
.
SENSE
. This causes an overcurrent
OC
smaller than R
LTC3805-5
OC
= 0 and connect
OC
OC(CRIT)
15
above
38055fa
.