LTC3805 Linear Technology, LTC3805 Datasheet - Page 15

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

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APPLICATIONS INFORMATION
Overcurrent Threshold Adjustment
Figure 6 shows the connection of the overcurrent pin OC
along with the I
R
which is driven by the GATE pin. The internal overcurrent
threshold on the OC pin is set at V
the same as the peak current sense voltage V
mV on the I
adjustment resistor R
tion current I
combination with the overcurrent threshold adjust cur-
rent I
lower the overcurrent trip threshold from 100mV. This
section describes how to pick R
performance. In the discussion that follows be careful to
distinguish between “current limit” where the converter
continues to run with the I
on a cycle-by-cycle basis 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
SENSE
OC
IN(MIN)
located in the source circuit of the power NMOS
= 10μA, an external resistor R
SENSE
SLOPE
using the appropriate formula depending on
SENSE
pin. The role of the slope compensation
is discussed in the prior section. In
IN(MIN)
pin and the current sense resistor
SLOPE
, calculate the duty cycle Duty
SENSE
and the slope compensa-
OC
OCT
to achieve the desired
Figure 6. Circuit to Decrease Overcurrent Threshold
pin limiting current
= 100 mV which is
OC
can be used to
I(MAX)
LTC3805
I
SENSE
GATE
GND
IN(MIN)
OC
= 100
R
R
SLOPE
OC
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
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
I
OC(CRIT)
OC
I
I
R
SLOPE
PK VIN(MIN)
= 10μA
OC(CRIT)
(
3805 F06
OC(CRIT)
can be used to reduce the trip threshold and
R
SENSE
PK
=
(
produces an overcurrent trip at an output
VIN(MIN)
) =
ΔV
IN(MIN)
100mV − ΔV
SENSE VIN(MIN)
10µA
) . Again, the calculation depends
(
to calculate ΔV
R
SENSE
SENSE VIN(MIN)
)
(
LTC3805
SENSE
PK
(
(
OC
)
VIN(MIN)
VIN(MIN)
15
above
3805fb
)
)