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参数资料
| 型号： | LTC3805IMSE-5#TRMPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 700 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | LEAD FREE, PLASTIC, MSOP-10 |
| 文件页数： | 6/20页 |
| 文件大小： | 203K |
| 代理商： | LTC3805IMSE-5#TRMPBF |
LTC3805-5
14
38055fb
APPLICATIONS INFORMATION
For example, with the peak current sense voltage of 100mV
on the ISENSE pin, a peak switch current of 5A requires
a sense resistor of 0.020
Ω. Note that the instantaneous
peak power in the sense resistor is 0.5W and it must be
rated accordingly. The LTC3805-5 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
Ω 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 LTC3805-5 injects a ramping current through its ISENSE
pin into an external slope compensation resistor RSLOPE.
This current ramp starts at zero right after the GATE pin
has been high for the LTC3805-5’s minimum duty cycle
of 6%. The current rises linearly towards a peak of 10μA
at the maximum duty cycle of 80%, shutting off once the
GATE pin goes low. A series resistor RSLOPE connecting the
ISENSE pin to the current sense resistor RSENSE develops a
ramping voltage drop. From the perspective of the ISENSE
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 (
ΔVSENSE)
can be calculated using the following equation:
ΔV
Duty Cycle
AR
SENSE
SLOPE
=
6
80
10
%
μ
Note: LTC3805-5 enforces 6% < Duty Cycle < 80%. A good
starting value for RSLOPE is 3k, which gives a 30mV drop
in current comparator threshold at 80% duty cycle.
Designs that do not operate at greater than 50% duty cycle
do not need slope compensation and may replace RSLOPE
with a direct connection.
Overcurrent Threshold Adjustment
Figure 5 shows the connection of the overcurrent pin OC
along with the ISENSE pin and the current sense resistor
RSENSE located in the source circuit of the power NMOS
which is driven by the GATE pin. The internal overcurrent
threshold on the OC pin is set at VOCT = 100 mV which is the
same as the peak current sense voltage VI(MAX)=100mVon
the ISENSE pin. The role of the slope compensation adjust-
ment resistor RSLOPE and the slope compensation current
ISLOPE isdiscussedinthepriorsection.Incombinationwith
the overcurrent threshold adjust current IOC = 10μA, an
external resistor ROC can be used to lower the overcurrent
Figure 5. Circuit to Decrease Overcurrent Threshold
Figure 4. Circuit for Isolated Feedback
LTC3805-5
ITH
FB
VCC
ISOLATION
BARRIER
GND
38055 F04
LTC3805-5
OC
GATE
ISENSE
RSENSE
RSLOPE
IOC = 10μA
ISLOPE
GND
38055 F05
ROC
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