24
LTC1736
V
IN
= 12V
V
OUT
= 1.5V
1.5V
100mV/DIV
15A
0A
10A/DIV
OUTPUT
VOLTAGE
LOAD
CURRENT
50
µ
s/DIV
1736 F07
Figure 7. Normal Transient Response (Without R1, R5)
V
IN
= 12V
V
OUT
= 1.5V
1.582V
1.5V
1.418V
100mV/DIV
15A
0A
10A/DIV
50
µ
s/DIV
1736 F08
Figure 8. Transient Response with Active Voltage Positioning
OUTPUT
VOLTAGE
LOAD
CURRENT
To calculate the resistor values, first determine the ratio
between them:
k
V
V
V
V
V
V
INTVCC
ITH NOM
ITH NOM
=
=
=
–
.
– .
.
.
(
)
(
)
5 2
1 085
1 085
3 79
V
INTVCC
is equal to V
EXTVCC
or 5.2V if EXTVCC is not used.
Resistor R5 is:
R
k
R
k
k
ITH
4
1
3 79 1 17 54
84 0
= +
=
+
=
(
) •
( .
) •
.
.
Resistor R1 is:
R
k
R
k
k
k
ITH
1
1
3 79 1 17 54
3 79
22 17
= +
=
+
=
(
) •
( .
) •
.
.
.
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 7 and 8 show the transient response before and
after active voltage positioning is implemented. Notice
that the output voltage droop and overshoot levels don’t
change but the peak-to-peak output voltage reduces con-
siderably with active voltage positioning.
Refer to Design Solutions 10 for more information about
active voltage positioning.
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 automobile
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 power 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 9 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 LTC1736 has a maximum input voltage of
36V, most applications will be limited to 30V by the
MOSFET BV
DSS
.
APPLICATIO S I FOR ATIO
W
U
U
U
FIGURE 6 CIRCUIT
FIGURE 6 CIRCUIT
