LT1173CS8 datasheet(9/16 Pages) LINER

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LT1173

keep VFB at the internal reference voltage of 1.245V. R1

and R2 set the output voltage according to the formula

OUT =+







 ()

( )

1 245

Step-Down (Buck Mode) Operation

A step-down DC-DC converter converts a higher voltage

to a lower voltage. The usual hookup for an LT1173 based

step-down converter is shown in Figure 6.

LT1173 • TA11

GND

SW2

SW1

LIM

100

VOUT

1N5818

VIN

Ω

LT1173

Figure 6. Step-Down Mode Hookup

When the switch turns on, SW2 pulls up to VIN– VSW. This

puts a voltage across L1 equal to VIN – VSW – VOUT,

causing a current to build up in L1. At the end of the switch

ON time, the current in L1 is equal to

PEAK

OUT

−−

()

.15

When the switch turns off, the SW2 pin falls rapidly and

actually goes below ground. D1 turns on when SW2

reaches 0.4V below ground.

D1 MUST BE A SCHOTTKY

DIODE. The voltage at SW2 must never be allowed to go

below –0.5V. A silicon diode such as the 1N4933 will allow

SW2 to go to – 0.8V, causing potentially destructive power

dissipation inside the LT1173. Output voltage is deter-

mined by

OUT =+







 ()

( )

1 245

R3 programs switch current limit. This is especially im-

portant in applications where the input varies over a wide

range. Without R3, the switch stays on for a fixed time

each cycle. Under certain conditions the current in L1 can

build up to excessive levels, exceeding the switch rating

and/or saturating the inductor. The 100

Ω resistor pro-

grams the switch to turn off when the current reaches

approximately 800mA. When using the LT1173 in step-

down mode, output voltage should be limited to 6.2V or

less. Higher output voltages can be accommodated by

inserting a 1N5818 diode in series with the SW2 pin

(anode connected to SW2).

Inverting Configurations

The LT1173 can be configured as a positive-to-negative

converter (Figure 7), or a negative-to-positive converter

(Figure 8). In Figure 7, the arrangement is very similar to

a step-down, except that the high side of the feedback is

referred to ground. This level shifts the output negative.

As in the step-down mode, D1 must be a Schottky

diode, and

VOUTshould be less than 6.2V. More nega-

tive output voltages can be accomodated as in the prior

section.

Figure 7. Positive-to-Negative Converter

In Figure 8, the input is negative while the output is

positive. In this configuration, the magnitude of the input

voltage can be higher or lower than the output voltage. A

level shift, provided by the PNP transistor, supplies proper

polarity feedback information to the regulator.

APPLICATI

I FOR ATIO

LT1173 • F07

–VOUT

1N5818

+VIN

GND

SW2

SW1

LIM

LT1173

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