LT1374CS8 datasheet(25/32 Pages) LINER

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LT1374

1374fb

APPLICATIONS INFORMATION

likely to be changed in production is the output capacitor,

because that is the component most likely to have manu-

facturer variations (in ESR) large enough to cause prob-

lems. It would be a wise move to lock down the sources of

the output capacitor in production.

A possible exception to the “clean response” rule is at very

light loads, as evidenced in Figure 14 with ILOAD = 50mA.

Switching regulators tend to have dramatic shifts in loop

response at very light loads, mostly because the inductor

current becomes discontinuous. One common result is very

slow but stable characteristics. A second possibility is low

phase margin, as evidenced by ringing at the output with

transients. The good news is that the low phase margin at

light loads is not particularly sensitive to component varia-

tion, so if it looks reasonable under a transient test, it will

probably not be a problem in production. Note that

fre-

quency of the light load ringing may vary with component

tolerance but phase margin generally hangs in there.

POSITIVE-TO-NEGATIVE CONVERTER

The circuit in Figure 15 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 LT1374 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.

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 LT1374’s 4.5A

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 4.5A

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

−

()( )

()( )( )













()

−

()

+−

()

035

IP = Maximum rated switch current

VIN = Minimum input voltage

VOUT = Output voltage

VF = Catch diode forward voltage

0.35 = Switch voltage drop at 4.5A

Example: with VIN(MIN) = 5.5V, VOUT = 5V, L = 10µH,

VF = 0.5V, IP = 4.5A: IMAX = 2A. Note that this equation does

not take into account that maximum rated switch current

(IP) on the LT1374 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.)

OUTPUT**

–5V, 1.8A

INPUT

5.5V TO

20V

1374 F15

0.27

µF

MBRS330T3

100

µF

10V TANT

×2

µF TO

µF

CMDSH-3

L1*

µH

BOOST

LT1374-5

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 15. Positive-to-Negative Converter

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