LT1432 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1432' PDF : 28 Pages View PDF
LT1432
APPLICATI S I FOR ATIO
100mV/DIV
100mV/DIV
50µs/DIV
Figure 13
TO VLIM
PIN
FROM
INDUCTOR
“L” IS MEASURED FROM
POINT “X” TO POINT “Y”
“X”
“W”
TO VOUT
PIN
“Y”
} KEEP THIS DISTANCE SHORT
FOR BEST LOAD REGULATION.
TO LOAD
LT 432 F14
Figure 14. PC trace Current Limit Sense Resistor
with Kelvin Contacts
Time scale has been expanded to 50µs/DIV. The spike
consists of several switching cycles of the LT1271 as
shown in the lower trace. In the upper trace, the output
filter has smoothed the switching frequency content of the
spike, but the actual spike amplitude is only modestly
reduced. Increasing the output filter constants from 10µH
and 220µF to 20µH and 330µF would eliminate most of
the spike.
Current Limiting
The LT1432 has true switching current limit with a sense
voltage of 60mV. This low sense voltage is used to
maintain high efficiency with normal loads and to make it
possible to use the printed circuit board trace material as
the sense resistor. The sense resistor value must take
ripple current into account because the LT1432 limits on
the peak of the inductor ripple current. Errors in the sense
resistor must also be allowed for.
( ) RSENSE
=
IMAX
VSENSE
1.2 * +
IRIP
2
RSENSE = Required sense resistor
VSENSE = 60mV
IMAX = Maximum load current, including any surge
longer than 50µs
* 1.2 is a fudge factor for errors in RSENSE and VSENSE.
IRIP = 1/2 Peak to Peak Inductor Ripple Current
2
( ) = VOUT VIN – VOUT
2VIN(f)(L)
f = Frequency
L = Inductance
Use VIN maximum
Example: IMAX = 2A, f = 60kHz, maximum VIN = 15V,
L = 50µH;
IRIP =
5(15 – 5)
= 0.55A
( ) 2
2
15
60E3
50E–6
( ) RSENSE
=
2A
60mV
1.2 + 0.55A
=
0.02Ω
The formula for RSENSE shows a 1.2 multiplier term in the
denominator which makes typical current limit 20% above
full load current. This accounts for small errors in the PCB
trace resistance. Trace resistance errors are kept to a
minimum by using internal traces (on multilayer boards)
13
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