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LT1766/LT1766-5

1766fc

APPLICATIONS INFORMATION

IP = Maximum rated switch current

VIN = Minimum input voltage

VOUT = Output voltage

VF = Catch diode forward voltage

0.3 = Switch voltage drop at 1.5A

Example: with VIN(MIN) = 5.5V, VOUT = 12V, L = 18μH,

VF = 0.63V, IP = 1.5A: IMAX = 0.280A.

OUTPUT DIVIDER

Refer to Applications Information Feedback Pin Functions

to calculate R1 and R2 for the (negative) output voltage

(from Table 1).

mode formula to calculate minimum inductor needed. If

load current is higher, use the continuous mode formula.

Output current where continuous mode is needed:

CONT

OUT

() ( )

()(

)

Minimum inductor discontinuous mode:

MIN

OUT

()(

)

()( )

Minimum inductor continuous mode:

MIN

OUT

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢

⎤

⎦

⎥

()(

)

()(

)

–

()

For a 40V to –12V converter using the LT1766 with peak

switch current of 1.5A and a catch diode of 0.63V:

CONT

() ( . )

()(

)

1 5

4 4012 40120 63

0 573

For a load current of 0.25A, this says that discontinuous

mode can be used and the minimum inductor needed is

found from:

MIN

212 0 25

200 10

1 5

13 3

()( .

)

(•

)( . )

In practice, the inductor should be increased by about

30% over the calculated minimum to handle losses and

variations in value. This suggests a minimum inductor of

18μH for this application.

Ripple Current in the Input and Output Capacitors

Positive-to-negative converters have high ripple current

in the input capacitor. For long capacitor lifetime, the RMS

value of this current must be less than the high frequency

ripple current rating of the capacitor. The following formula

will give an

approximate value for RMS ripple current. This

formula assumes continuous mode and large inductor

value. Small inductors will give somewhat higher ripple

current, especially in discontinuous mode. The exact

formulas are very complex and appear in Application

OUTPUT**

–12V, 0.25A

INPUT†

5.5V TO

48V

1766 F15

0.33μF

10MQO60N

44.2k

100μF

25V

TANT

2.2μF

100V

CER

D2†

1N4148W

L1*

18μH

BOOST

LT1766

VIN

VSW

GND

4.99k

* INCREASE L1 TO 30μH OR 60μH FOR HIGHER CURRENT APPLICATIONS.

SEE APPLICATIONS INFORMATION

** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE

AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION

† FOR V

IN > 44V AND VOUT = –12V, ADDITIONAL VOLTAGE DROP IN THE

PATH OF D2 IS REQUIRED TO ENSURE BOOST PIN MAXIMUM RATING IS

NOT EXCEEDED. SEE APPLICATIONS INFORMATION (BOOST PIN VOLTAGE)

Figure 15. Positive-to-Negative Converter

Inductor Value

The criteria for choosing the inductor is typically based on

ensuring that peak switch current rating is not exceeded.

This gives the lowest value of inductance that can be

used, but in some cases (lower output load currents) it

may give a value that creates unnecessarily high output

ripple voltage.

The difﬁculty in calculating the minimum inductor size

needed is that you must ﬁrst decide whether the switcher

will be in continuous or discontinuous mode at the critical

point where switch current reaches 1.5A. The ﬁrst step is

to use the following formula to calculate the load current

above which the switcher must use continuous mode. If

your load current is less than this, use the discontinuous

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