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LT1767

Monolithic 1.5A, 1.25MHz Step-Down Switching Regulators

Linear Technology 

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

APPLICATIONS INFORMATION

input capacitor is reduced so values in the range of 1µF

to 4.7µF are suitable for most applications. Y5V or similar

type ceramics can be used since the absolute value of ca-

pacitance is less important and has no significant effect on

loop stability. If operation is required close to the minimum

input required by the output of the LT1767, a larger value

may be required. This is to prevent excessive ripple causing

dips below the minimum operating voltage, resulting in

erratic operation.

If tantalum capacitors are used, values in the 22µF to 470µF

range are generally needed to minimize ESR and meet ripple

current and surge ratings. Care should be taken to ensure

the ripple and surge ratings are not exceeded. The AVX TPS

and Kemet T495 series are surge rated. AVX recommends

derating capacitor operating voltage by 2:1 for high surge

applications.

OUTPUT CAPACITOR

Unlike the input capacitor, RMS ripple current in the output

capacitor is normally low enough that ripple current rating

is not an issue. The current waveform is triangular, with

an RMS value given by:

IRIPPLE(RMS)

=

0.29

(VOUT

(L)(

) (VIN −

f) (VIN )

VOUT

)

The LT1767 will operate with both ceramic and tantalum

output capacitors. Ceramic capacitors are generally chosen

for their small size, very low ESR (effective series resistance),

and good high frequency operation, reducing output ripple

voltage. Typical ceramic output capacitors are in the 4.7µF

to 47µF range. Since the absolute value of capacitance

defines the pole frequency of the output stage, an X7R or

X5R type ceramic, which have good temperature stability,

is recommended.

Tantalum capacitors are usually chosen for their bulk capac-

itance properties, useful in high transient load applications.

ESR rather than capacitive value defines output ripple at

1.25MHz. Typical LT1767 applications require a tantalum

capacitor with less than 0.3Ω ESR at 22µF to 500µF.

Figure 3 shows a comparison of output ripple for a ceramic

and tantalum capacitor at 200mA ripple current.

VOUT USING 47µF, 0.1Ω

TANTALUM CAPACITOR

(10mV/DIV)

VOUT USING 2.2µF

CERAMIC CAPACITOR

(10mV/DIV)

VSW

(5V/DIV)

0.2µs/DIV

1767 F03

Figure 3. Output Ripple Voltage Waveform

INDUCTOR CHOICE AND MAXIMUM OUTPUT

CURRENT

Maximum output current for a buck converter is equal

to the maximum switch rating (IP) minus one half peak

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 sub-

harmonic 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.

For most applications, the output inductor will be in the

1µH 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, 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.

For more information www.linear.com/LT1767

1767fb

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