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LTM8073

8073fa

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applicaTions inForMaTion

BIAS Pin Considerations

The BIAS pin is used to provide drive power for the inter-

nal power switching stage and operate other internal cir-

cuitry. For proper operation, it must be powered by at

least 3.2V. If the output voltage is programmed to 3.2V

or higher, BIAS may be simply tied to AUX. If VOUT is less

than 3.2V, BIAS can be tied to VIN or some other voltage

source. If the BIAS pin voltage is too high, the efficiency

of the LTM8073 may suffer. The optimum BIAS voltage is

dependent upon many factors, such as load current, input

voltage, output voltage and switching frequency. In all

cases, ensure that the maximum voltage at the BIAS pin

is less than 19V. If BIAS power is applied from a remote

or noisy voltage source, it may be necessary to apply a

decoupling capacitor locally to the pin. A 1µF ceramic

capacitor works well. The BIAS pin may also be left open

at the cost of a small degradation in efficiency.

If the LTM8073 is configured to provide a negative output,

do not connect BIAS to VOUT or AUX. Instead, tie to BIAS

to the LTM8073 GND, which should be the negative output.

Maximum Load

The maximum practical continuous load that the LTM8073

can drive, while rated at 3A, actually depends upon both

the internal current limit and the internal temperature.

The internal current limit is designed to prevent damage

to the LTM8073 in the case of overload or short-circuit.

The internal temperature of the LTM8073 depends upon

operating conditions such as the ambient temperature,

the power delivered, and the heat sinking capability of the

system. For example, if the LTM8073 is configured to reg-

ulate at 1.2V, it may continuously deliver 4.5A from 12VIN

if the ambient temperature is controlled to less than 55°C;

this is more than the 3A continuous rating. Please see the

derating curve for VOUT = 1.2V in the Typical Performance

Characteristics section. Similarly, if the output voltage is

15V and the ambient temperature is 95°C, the LTM8073

will deliver at most 2A from 24VIN, which is less than the

3A continuous rating.

Load Sharing

LTM8073s may be paralleled to produce higher currents.

To do this, tie the VIN, VOUT and SHARE pins of all the

paralleled LTM8073s together. To ensure that paralleled

modules start up together, the TR/SS pins may be tied

together, as well. If it is inconvenient to tie the TR/SS pins

together, make sure that the same valued soft-start capac-

itors are used for each µModule regulator. An example of

two LTM8073s configured for load sharing is given in the

Typical Applications section.

For closer load sharing, synchronize the LTM8073s to an

external clock source. When load sharing among n units

and using a single RFB resistor, the value of the resistor is:

RFB =

199.7

n VOUT –0.8

(

)

where RFB is in kΩ.

Burst Mode Operation

To enhance efficiency at light loads, the LTM8073 auto-

matically switches to Burst Mode operation which keeps

the output capacitor charged to the proper voltage while

minimizing the input quiescent current. During Burst

Mode operation, the LTM8073 delivers single cycle bursts

of current to the output capacitor followed by sleep peri-

ods where most of the internal circuitry is powered off

and energy is delivered to the load by the output capacitor.

During the sleep time, VIN and BIAS quiescent currents

are greatly reduced, so, as the load current decreases

towards a no load condition, the percentage of time that

the LTM8073 operates in sleep mode increases and the

average input current is greatly reduced, resulting in

higher light load efficiency.

Burst Mode operation is enabled by tying SYNC to GND.

Minimum Input Voltage

The LTM8073 is a step-down converter, so a minimum

amount of headroom is required to keep the output in

regulation. Keep the input above 3.4V to ensure proper

operation. Voltage transients or ripple valleys that cause

the input to fall below 3.4V may turn off the LTM8073.

Output Voltage Tracking and Soft-Start

The LTM8073 allows the user to program its output volt-

age ramp rate by means of the TR/SS pin. An internal 2μA

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