LTM4613 datasheet(18/28 Pages) AD

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LTM8060

Rev. 0

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APPLICATIONS INFORMATION

Frequency Foldback

The LTM8060 is equipped with frequency foldback which

acts to reduce the thermal and energy stress on the inter-

nal power elements during a short circuit or output over-

load condition. If the LTM8060 detects that the output

has fallen out of regulation, the switching frequency is

reduced as a function of how far the output is below the

target voltage. This in turn limits the amount of energy

that can 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 capaci-

tance of the load. When a clock is applied to the SYNCn

pin, the SYNCn 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 SYNCn

pin below about 0.8V (this can be ground or a logic low

output). To synchronize the LTM8060 oscillator to an

external frequency, connect a square wave (with about

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

wave amplitude should have valleys that are below 0.8V

and peaks above 1.5V.

The LTM8060 may be synchronized over a 200kHz to

3MHz range. The LTM8060 will not enter Burst Mode

operation at light output loads while synchronized to an

external clock. The RT resistor should be chosen to set

the switching frequency equal to or below the lowest

synchronization input. For example, if the synchroniza-

tion signal will be 500kHz and higher, the RT should be

selected for 500kHz or lower.

The LTM8060 features spread spectrum operation to fur-

ther reduce EMI/EMC emissions. To enable spread spec-

trum operation, apply between 2.8V and 4V to the SYNCn

pin. In this mode, triangular frequency modulation is used

to vary the switching frequency between the value pro-

grammed by RT to about 20% higher than that value. The

modulation frequency is about 7kHz. For example, when

the LTM8060 is programmed to 2MHz, the frequency will

vary from 2MHz to 2.4MHz at a 7kHz rate. When spread

spectrum operation is selected, Burst Mode operation is

disabled, and the part may run in discontinuous mode.

Shorted Input Protection

Care needs to be taken in systems where the output is

held high when the input to the LTM8060 is absent. This

may occur in battery charging applications or in battery

backup systems where a battery or some other supply

is diode OR’ed with the LTM8060’s output. If the VINn

pin is allowed to float and the RUN

n pin is held high

(either by a logic signal or because it is tied to VINn),

then the LTM8060’s internal circuitry pulls its quiescent

current through its internal power switch. This is fine if

your system can tolerate a few milliamps in this state.

If you ground the RUN

n pin, the internal current drops

to essentially zero. However, if the VINn pin is grounded

while the output is held high, parasitic diodes inside the

LTM8060 can pull large currents from the output through

the VINn pin. Figure 2 shows a circuit that runs only when

the input voltage is present and that protects against a

shorted or reversed input.

Figure 2. The Input Diode Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output. It Also Protects

the Circuit from a Reversed Input. The LTM8060 Runs Only

When the Input Is Present.

VIN

RUN

VIN

LTM8060

8060 F02

PCB Layout

Most of the headaches associated with PCB layout have

been alleviated or even eliminated by the high level of

integration of the LTM8060. The LTM8060 is neverthe-

less a switching power supply, and care must be taken to

minimize EMI and ensure proper operation. Even with the

high level of integration, you may fail to achieve specified

operation with a haphazard or poor layout. See Figure 3

for a suggested layout. Ensure that the grounding and

heat sinking are acceptable.

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