LT1738 datasheet(13/20 Pages) LINER

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LT1738

1738fa

Positive Output Voltage Setting

Sensing of a positive output voltage is usually done using

a resistor divider from the output to the FB pin. The

positive input to the error amp is connected internally to a

1.25V bandgap reference. The FB pin will regulate to this

voltage.

Referring to Figure 3, R1 is determined by:

VOUT

125

=−









The FB bias current represents a small error and can

usually be ignored for values of R1||R2 up to 10k.

One word of caution, sometimes a feedback zero is added

to the control loop by placing a capacitor across R1. If the

feedback capacitively pulls the FB pin above the internal

regulator voltage (2.4V), output regulation may be dis-

rupted. A series resistance with the feedback pin can

eliminate this potential problem. There is an internal clamp

on FB that clamps at 0.7V above the regulation voltage that

should also help prevent this problem.

APPLICATIO S I FOR ATIO

Smaller value capacitors can be made in two ways. The

first is simply combining two capacitors in series. The

equivalent capacitance is then (C1 • C2)/(C1 + C2).

The second method makes use of a capacitor divider. Care

should be taken that the voltage rating of the capacitor

satisfies the full voltage swing thus essentially the same

rating as the MOSFET.

The equivalent slew capacitance for Figure 2 is

(C1 • C2)/(C1 + C2 + C3).

Negative Output Voltage Setting

Negative output voltage can be sensed using the NFB pin.

In this case regulation will occur when the NFB pin is at

–2.5V. The nominal input bias current for the NFB is –25

µA

(INFB), which needs to be accounted for in setting up the

divider.

Referring to Figure 4, R1 is chosen such that:

OUT

2 25

−

+µ









.•

A suggested value for R2 is 2.5k. The NFB pin is normally

left open if the FB pin is being used.

MOSFET DRAIN

CAP

1738 F02

Table 2

INPUT VOLTAGE

CAPACITOR VALUE

< 25V

5pF

50V

2.5pF

100V

1pF

Figure 2

Figure 3

Figure 4

FB PIN

1738 F03

VOUT

NFB PIN

INFB

1738 F04

–VOUT

Dual Polarity Output Voltage Sensing

Certain applications may benefit from sensing both posi-

tive and negative output voltages. When doing this each

output voltage resistor divider is individually set as previ-

ously described. When both FB and NFB pins are used, the

LT1738 will act to prevent either output from going

beyond its set output voltage. The highest output (lightest

load) will dominate control of the regulator. This technique

would prevent either output from going unregulated high

at no load. However, this technique will also compromise

output load regulation.

Shutdown

If SHDN is pulled low, the regulator will turn off. As the

SHDN pin voltage is increased from ground the internal

bandgap regulator will be powered on. This will set a 1.39V

threshold for turn on of the internal regulator that runs

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