LTC1735-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1735-1' PDF : 28 Pages View PDF
LTC1735-1
APPLICATIO S I FOR ATIO
Resistor R1 is:
R1= (k + 1) • RITH = (3.79 + 1) • 17.54k = 22.17k
k
3.79
Unfortunately, PCB noise can add to the voltage developed
across the sense resistor, R6, causing the ITH pin voltage
to be slightly higher than calculated for a given output
current. The amount of noise is proportional to the output
current level. This PCB noise does not present a serious
problem but it does change the effective value of R6 so the
calculated values of R1 and R5 may need to be adjusted to
achieve the required results. Since PCB noise is a function
of the layout, it will be the same on all boards with the same
layout.
Figures 9 and 10 show the transient response before and
after active voltage positioning is implemented. Notice
that active voltage positioning reduced the transient re-
sponse from almost 200mVP-P to a little over 100mVP-P.
Refer to Design Solutions 10 for more information about
active voltage positioning.
1.582V
100mV/DIV 1.50V
VIN = 12V
VOUT = 1.5V
1.418V
15A
LOAD
CURRENT
FIGURE 8 CIRCUIT
OUTPUT
VOLTAGE
5A/DIV
0.2A
0A
50µs/DIV
1735-1 F09
Figure 9. Transient Response Without Active Voltage Positioning
1.582V
100mV/DIV 1.50V
1.418V
15A
VIN = 12V
VOUT = 1.5V
OUTPUT
VOLTAGE
LOAD
CURRENT
FIGURE 8 CIRCUIT
Automotive Considerations: Plugging Into the
Cigarette Lighter
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserve or even recharge battery packs during operation.
But before you connect, be advised: you are plugging into
the supply from hell. The main power line in an auto is the
source of a number of nasty potential transients, including
load dump, reverse battery and double battery.
Load dump is the result of a loose battery cable. When the
cable breaks connection, the field collapse in the alternator
can cause a positive spike as high as 60V which takes
several hundred milliseconds to decay. Reverse battery is
just what it says, while double battery is a consequence of
tow-truck operators finding that a 24V jump start cranks
cold engines faster than 12V.
The network shown in Figure 11 is the most straight
forward approach to protect a DC/DC converter from the
ravages of an automotive power line. The series diode
prevents current from flowing during reverse battery,
while the transient suppressor clamps the input voltage
during load dump. Note that the transient suppressor
should not conduct during double-battery operation, but
must still clamp the input voltage below breakdown of the
converter. Although the LTC1735-1 has a maximum input
voltage of 36V, most applications will be limited to 30V by
the MOSFET BVDSS.
50A IPK RATING
VIN
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC1735-1
1735-1 F11
Figure 11. Plugging Into the Cigarette Lighter
5A/DIV
0.2A
0A
50µs/DIV
1735-1 F10
Figure 10. Transient Response with Active Voltage Positioning
23
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