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LTM8063

8063fa

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APPLICATIONS INFORMATION

be delivered to the load under fault. During the start-up

time, frequency foldback is also active to limit the energy

delivered to the potentially large output capacitance of

the load. When a clock is applied to the SYNC pin, the

SYNC pin is floated or held high, the frequency foldback

is disabled, and the switching frequency will slow down

only during overcurrent conditions.

Synchronization

To select low ripple Burst Mode operation, tie the SYNC

pin below about 0.4V (this can be ground or a logic low

output). To synchronize the LTM8063 oscillator to an

external frequency, connect a square wave (with about

20% to 80% duty cycle) to the SYNC pin. The square

wave amplitude should have valleys that are below 0.4V

and peaks above 1.5V.

The LTM8063 will not enter Burst Mode operation at low

output loads while synchronized to an external clock, but

insteadwillpulseskiptomaintainregulation.TheLTM8063

may be synchronized over a 200kHz to 2.2MHz range.

The RT resistor should be chosen to set the switching

frequency equal to or below the lowest synchronization

input. For example, if the synchronization signal will be

500kHz and higher, the RT should be selected for 500kHz.

For some applications it is desirable for the LTM8063 to

operate in pulse-skipping mode, offering two major dif-

ferences from Burst Mode operation. The first is that the

clock stays awake at all times and all switching cycles

are aligned to the clock. The second is that full switching

frequency is reached at lower output load than in Burst

Modeoperation.Thesetwodifferencescomeattheexpense

of increased quiescent current. To enable pulse-skipping

mode, the SYNC pin is floated.

The LTM8063 features spread spectrum operation to

further reduce EMI/EMC emissions. To enable spread

spectrum operation, apply between 2.9V and 4.2V to the

SYNC pin. In this mode, triangular frequency modulation

is used to vary the switching frequency between the value

programmed by RT to about 20% higher than that value.

Themodulationfrequencyisabout3kHz.Forexample,when

the LTM8063 is programmed to 2MHz, the frequency will

vary from 2MHz to 2.4MHz at a 3kHz rate. When spread

spectrum operation is selected, Burst Mode operation is

disabled, and the part will run in pulse-skipping mode.

TheLTM8063doesnotoperateinforcedcontinuousmode

regardless of SYNC signal.

Negative Output

The LTM8063 is capable of generating a negative output

voltage by connecting its VOUT to system GND and the

LTM8063 GND to the negative voltage rail. An example

of this is shown in the Typical Applications section. The

most versatile way to generate a negative output is to

use a dedicated regulator that was designed to generate

a negative voltage, but using a buck regulator like the

LTM8063 to generate a negative voltage is a simple and

cost effective solution, as long as certain restrictions are

kept in mind.

Figure 1showsatypicalnegativeoutputvoltageapplication.

Note that LTM8063 VOUT is tied to system GND and input

power is applied from VIN to LTM8063 VOUT. As a result,

the LTM8063 is not behaving as a true buck regulator,

and the maximum output current depends upon the input

voltage. In the example shown in the Typical Applications

section, there is an attending graph that shows how much

current the LTM8063 can deliver for given input voltages.

Figure 1.

NEGATIVE

OUTPUT VOLTAGE

LTM8063

VOUT

VIN

8063 F01

GND

VIN

The LTM8063 Can Be Used to Generate a Negative Voltage

Note that this configuration requires that any load current

transient will directly impress the transient voltage onto

the LTM8063 GND, as shown in Figure 2, so fast load

transients can disrupt the LTM8063’s operation or even

cause damage.

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