LT1376CS8 datasheet(24/28 Pages) LINER

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LT1375/LT1376

APPLICATIONS INFORMATION

POSITIVE-TO-NEGATIVE CONVERTER

The circuit in Figure 18 is a classic positive-to-negative

topology using a grounded inductor. It differs from the

standard approach in the way the IC chip derives its

feedback signal, however, because the LT1376 accepts

only positive feedback signals, the ground pin must be tied

to the regulated negative output. A resistor divider to

ground or, in this case, the sense pin, then provides the

proper feedback voltage for the chip.

OUTPUT**

–5V, 0.5A

INPUT

4.5V TO

20V

1375/76 F18

0.1

µF

1N5818

100

µF

10V TANT

µF TO

µF

1N4148

L1*

µH

BOOST

LT1376-5

VIN

VSW

SENSE

GND

* INCREASE L1 TO 10

µH OR 20µH FOR HIGHER CURRENT APPLICATIONS.

SEE APPLICATIONS INFORMATION

** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE

AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION

Figure 18. Positive-to-Negative Converter

Inverting regulators differ from buck regulators in the

basic switching network. Current is delivered to the output

square waves with a peak-to-peak amplitude much

greater than load current. This means that maximum load

current will be significantly less than the LT1376’s 1.5A

maximum switch current, even with large inductor values.

The buck converter in comparison, delivers current to the

output as a triangular wave superimposed on a DC level

equal to load current, and load current can approach 1.5A

with large inductors. Output ripple voltage for the positive-

to-negative converter will be much higher than a buck

converter. Ripple current in the output capacitor will also

be much higher. The following equations can be used to

calculate operating conditions for the positive-to-negative

converter.

Maximum load current:

f L

MAX

OUT

−

()( )

()( )( )













()

−

()

+−

() +

()

IP = Maximum rated switch current

VIN = Minimum input voltage

VOUT = Output voltage

VF = Catch diode forward voltage

0.5 = Switch voltage drop at 1.5A

Example: with VIN(MIN) = 4.7V, VOUT = 5V, L = 10µH, VF =

0.5V, IP = 1.5A: IMAX = 0.52A. Note that this equation does

not take into account that maximum rated switch current

(IP) on the LT1376 is reduced slightly for duty cycles

above 50%. If duty cycle is expected to exceed 50% (input

voltage less than output voltage), use the actual IP value

from the Electrical Characteristics table.

Operating duty cycle:

OUT

−+

(This formula uses an average value for switch loss, so it

may be several percent in error.)

With the conditions above:

−+ +

50 5

47 03 5 0 5

This duty cycle is close enough to 50% that IP can be

assumed to be 1.5A.

OUTPUT DIVIDER

If the adjustable part is used, the resistor connected to

VOUT (R2) should be set to approximately 5k. R1 is

calculated from:

RVOUT

22 42

242

−

()

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