LTC1736 datasheet(24/28 Pages) LINER

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LTC1736

VIN = 12V

VOUT = 1.5V

1.5V

100mV/DIV

15A

10A/DIV

OUTPUT

VOLTAGE

LOAD

CURRENT

µs/DIV

1736 F07

Figure 7. Normal Transient Response (Without R1, R5)

VIN = 12V

VOUT = 1.5V

1.582V

1.5V

1.418V

100mV/DIV

15A

10A/DIV

µs/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:

INTVCC

ITH NOM

–

.– .

()

1085

1 085

379

VINTVCC is equal to VEXTVCC or 5.2V if EXTVCC is not used.

Resistor R5 is:

ITH

3 79 1 17 54

84 0

() •

( .

) •

Resistor R1 is:

ITH

3 79 1 17 54

379

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 BVDSS.

APPLICATIO S I FOR ATIO

FIGURE 6 CIRCUIT

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