Datasheet

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LT1683

1683fd

APPLICATIONS INFORMATION

This part creates a second higher frequency rolloff of

harmonics that inversely depends on the slew time, the

time that voltage or current spends between the off state

and on state. This time is adjustable through the choice of

the slew resistors, the external resistors to ground on the

VSL

and R

CSL

pins and the external components used for

the external voltage feedback capacitors C

, C

(from

CAP A or CAP B to their respective MOSFET drains) and

the sense resistor. Lower slew rates (longer slew times,

lower frequency for harmonics rolloff) is created with

higher values of R

VSL

, R

CSL

, C

and the current

sense resistor.

Setting the voltage and current slew rates should be done

empirically. The most practical way of determining these

components is to set C

, C

and the sense resistor value.

Then, start by making R

VSL

, R

CSL

each a 50k resistor pot

in series with 3.3k. Starting from the lowest resistor set-

ting (fast slew) adjust the pots until the noise level meets

your guidelines. Note that slower slewing waveforms will

dissipate more power so that efﬁciency will drop. You

can monitor this as you make your slew adjustment by

measuring input and output voltage and their respective

currents. Monitor the MOSFET temperature as slew rates

are slowed. These components will heat up as efﬁciency

decreases.

Measuring noise should be done carefully. It is easy to

introduce noise by poor measurement techniques. Consult

AN70 for recommended measurement techniques. Keeping

probe ground leads very short is essential.

Usually it will be desirable to keep the voltage and cur-

rent slew resistors approximately the same. There are

circumstances where a better optimization can be found by

adjusting each separately, but as these values are separated

further, a loss of independence of control may occur.

It is possible to use a single slew setting resistor. In this

case the R

VSL

and R

CSL

pins are tied together. A resistor

with a value of 1.8k to 34k (one-half the individual resis-

tors) can then be tied from these pins to ground.

In general only the R

CSL

value will be available for adjust-

ment of current slew. The current slew time does also

depend on the current sense resistor but this resistor is

normally set with consideration of the maximum current

in the MOSFETs.

Setting the voltage slew also involves selection of the

capacitors C

, C

. The voltage slew time is proportional

to the output voltage swing (basically input voltage), the

external voltage feedback capacitor and the R

VSL

value.

Thus at higher input voltages smaller capacitors will be

used with lower R

VSL

values. For a starting point use

Table 2.

Table 2

INPUT VOLTAGE CAPACITOR VALUE

<25V 5pF

50V 2.5pF

100V 1pF

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 ratings of the capacitors

satisfy the full voltage swing (2x input voltage for push-

pull conﬁgurations) thus essentially the same rating as

the MOSFETs.

MOSFET DRAIN

CAP A OR B

1683 F02

Figure 2

The equivalent slew capacitance for Figure 2 is

(C1 • C2)/(C1 + C2 + C3).

Positive Output Voltage Setting

Sensing of a positive output voltage is usually done us-

ing 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:

R1= R2

OUT

1.25

− 1

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