7

LT1373

Negative Output Voltage Setting

NFB pin to ground. Output voltage is set by connecting the

NFB pin to an output resistor divider (Figure 2). The – 7

µA

and should not be ignored. This has been accounted for in

the formula in Figure 2. The suggested value for R2 is

2.49k. The FB pin is normally left open for negative output

applications. See Dual Polarity Output Voltage Sensing for

limitations of FB pin loading when using the NFB pin.

A logic low on the S/S pin activates shutdown, reducing

the part’s supply current to 12

µA. Typical synchronization

range is from 1.05 and 1.8 times the part’s natural switch-

ing frequency, but is only guaranteed between 300kHz and

340kHz. A 12

µs resetable shutdown delay network guar-

antees the part will not go into shutdown while receiving

a synchronization signal.

Caution should be used when synchronizing above

330kHz because at higher sync frequencies the ampli-

tude of the internal slope compensation used to prevent

subharmonic switching is reduced. This type of

subharmonic switching only occurs when the duty cycle

of the switch is above 50%. Higher inductor values will

tend to eliminate problems.

Thermal Considerations

Care should be taken to ensure that the worst-case input

voltage and load current conditions do not cause exces-

sive die temperatures. The packages are rated at 120

°C/W

for SO (S8) and 130

°C/W for PDIP (N8).

Average supply current (including driver current) is:

DC = switch duty cycle

Switch power dissipation is given by:

Total power dissipation of the die is the sum of supply

current times supply voltage plus switch power:

Choosing the Inductor

For most applications the inductor will fall in the range of

10

µHto50µH.Lowervaluesarechosentoreducephysical

size of the inductor. Higher values allow more output

current because they reduce peak current seen by the

power switch which has a 1.5A limit. Higher values also

reduce input ripple voltage, and reduce core loss.

When choosing an inductor you might have to consider

maximum load current, core and copper losses, allowable

R1

1 +

R2

LT1373 • F02

NFB

PIN

R2

R1 =

()

2.45

R2

Figure 2. Negative Output Resistor Divider

Dual Polarity Output Voltage Sensing

Certain applications benefit from sensing both positive

and negative output voltages. One example is the Dual

Output Flyback Converter with Overvoltage Protection

circuit shown in the Typical Applications section. Each

output voltage resistor divider is individually set as de-

scribed above. When both the FB and NFB pins are used,

the LT1373 acts to prevent either output from going

beyond its set output voltage. For example in this applica-

tion, if the positive output were more heavily loaded than

the negative, the negative output would be greater and

would regulate at the desired set-point voltage. The posi-

tive output would sag slightly below its set-point voltage.

This technique prevents either output from going unregu-

lated high at no load. Please note that the load on the FB

pin should not exceed 100

µA when the NFB pin is used.

This situation occurs when the resistor dividers are used

at both FB

and NFB. True load on FB is not the full divider

current unless the positive output is shorted to ground.

See Dual Output Flyback Converter application.

Shutdown and Synchronization

The dual function S/S pin provides easy shutdown and

synchronization. It is logic level compatible and can be

APPLICATIO S I FOR ATIO