LTM4644 datasheet(20/28 Pages) AD

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LTM8051

Rev. 0

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APPLICATIONS INFORMATION

Pre-Biased Output

As discussed in the Output Voltage Tracking and Soft-Start

section, the LTM8051 regulates the output to the FB

n volt-

age determined by the TRSS

n pin whenever TRSSn is less

than 0.8V. If the LTM8051 output is higher than the target

output voltage, and SYNC

n is not held below 0.8V, the

LTM8051 will attempt to regulate the output to the target

voltage by returning a small amount of energy back to the

input supply. If there is nothing loading the input supply,

its voltage may rise. Take care that it does not rise so high

that the input voltage exceeds the absolute maximum rat-

ing of the LTM8051. If SYNC is grounded, the LTM8051

will not return current to the input.

Synchronization

To select low ripple Burst Mode operation, tie the SYNC

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

output). To synchronize the LTM8051 oscillator to an

external frequency, connect a square wave (with about

20% to 80% duty cycle) to the SYNC

n pin. The square

wave amplitude should have valleys that are below 0.8V

and peaks above 1.5V.

The LTM8051 may be synchronized over a 300kHz to

3MHz range. The LTM8051 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 LTM8051 features spread spectrum operation to fur-

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

trum operation, apply between 2.8V and 4.2V to the SYNC

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 LTM8051 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 LTM8051 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 LTM8051’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 LTM8051’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

LTM8051 can pull large currents from the output through

the VINn pin. Figure 5 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 LTM8051 Runs Only

When the Input Is Present

VIN

RUN

VIN

LTM8051

8051 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 LTM8051. The LTM8051 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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