Data Sheet

LTM8060F

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Rev. 0

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feedback voltage will be regulated to the internal reference voltage. The TRSSn pin may be left floating if the

function is not needed.

An active pull-down circuit is connected to the TRSSn pin, which will discharge the external soft start capacitor in

the case of fault conditions and restart the ramp when the faults are cleared. Fault conditions that clear the soft

start capacitor are the RUNn pin transitioning low, VINn voltage falling too low, or thermal shutdown.

Prebiased Output

As discussed in the Output Voltage Tracking and Soft Start section, the LTM8060F regulates the output to the FBn

voltage determined by the TRSSn pin whenever TRSSn is less than 0.8V. If the LTM8060F output is higher than the

target output voltage, and SYNCn is not held below 0.8V, the LTM8060F will attempt to regulate the output to the

target voltage by returning a small amount of energy back to the input supply. If nothing is 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 rating of the LTM8060F. If SYNC is grounded, the LTM8060F will not return current to the input.

Frequency Foldback

The LTM8060F is equipped with frequency foldback, which acts to reduce the thermal and energy stress on the

internal power elements during a short circuit or output overload condition. If the LTM8060F 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

startup 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 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, connect the SYNCn pin below about 0.8V (this can be ground or a logic

low output). To synchronize the LTM8060F 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 LTM8060F may be synchronized over a 200kHz to 3MHz range. The LTM8060F 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 synchronization signal

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

The LTM8060F features spread-spectrum operation to further reduce electromagnetic

interference/electromagnetic interference compatibility (EMI/EMC) emissions. To enable spread-spectrum

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

modulation frequency is about 7kHz. For example, when the LTM8060F is programmed to 2MHz, the frequency will

vary from 2MHz to 2.4MHz at a 7kHz rate. When spread-spectrum operation is selected, the Burst Mode operation is

disabled, and the part may run in discontinuous-conduction mode.

Shorted Input Protection

Care needs to be taken in systems where the output is held high when the input to the LTM8060F 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 LTM8060F’s output. If the VINn pin is allowed to float and the RUNn pin is held high (either by a

logic signal or because it is connected to VINn), then the LTM8060F’s internal circuitry pulls its quiescent current