8

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

sn1767 1767fas

Figure 3. Output Ripple Voltage Waveform

and reduces the current at which discontinuous operation

occurs. The following formula gives maximum output

current for continuous mode operation, implying that the

peak to peak ripple (2x the term on the right) is less than

the maximum switch current.

Continuous Mode

−

()

()( )( )

VV

V

Lf V

OUT

IN

OUT

IN

2

Discontinuous operation occurs when

I

V

Lf

OUT DIS

OUT

()

()

()( )

=

2

I

A

OUT MAX

()

−

=−

()

()(

=−

=

15

58 5

2 3 3 10

1 25 10

8

15

0 23

1 27

66

.

.•

.

•

..

.

Note that the worst case (minimum output current avail-

able) condition is at the maximum input voltage. For the

same circuit at 15V, maximum output current would be

only 1.1A.

When choosing an inductor, consider maximum load

current, core and copper losses, allowable component

height, output voltage ripple, EMI, fault current in the

inductor, saturation, and of course, cost. The following

procedure is suggested as a way of handling these some-

what complicated and conflicting requirements.

1. Choose a value in microhenries from the graphs of

maximum load current. Choosing a small inductor with

lighter loads may result in discontinuous mode of

operation, but the LT1767 is designed to work well in

either mode.

Assume that the average inductor current is equal to

load current and decide whether or not the inductor

must withstand continuous fault conditions. If maxi-

mum load current is 0.5A, for instance, a 0.5A inductor

may not survive a continuous 2A overload condition.

Also, the instantaneous application of input or release

from shutdown, at high input voltages, may cause

INDUCTOR CHOICE AND MAXIMUM OUTPUT

CURRENT

Maximum output current for a buck converter is equal to

peak inductor current. In past designs, the maximum

switch current has been reduced by the introduction of

slope compensation. Slope compensation is required at

duty cycles above 50% to prevent an affect called

subharmonic oscillation (see Application Note 19 for

details). The LT1767 has a new circuit technique that

maintains a constant switch current rating at all duty

cycles. (Patent Pending)

For most applications, the output inductor will be in the

1

µH to 10µH range. Lower values are chosen to reduce the

physical size of the inductor, higher values allow higher

output currents due to reduced peak to peak ripple current,

Table 2. Surface Mount Solid Tantalum Capacitor ESR

and Ripple Current

E Case Size

ESR (Max,

Ripple Current (A)

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

AVX TAJ

0.7 to 0.9

0.4

D Case Size

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

C Case Size

AVX TPS

0.2 (typ)

0.5 (typ)

Figure 3 shows a comparison of output ripple for a ceramic

and tantalum capacitor at 200mA ripple current.

TANTALUM CAPACITOR

(10mV/DIV)

0.2

µs/DIV

1767 F03

CERAMIC CAPACITOR

(10mV/DIV)

(5V/DIV)