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LTM8003

8003fc

For more information www.linear.com/LTM8003

APPLICATIONS INFORMATION

BIAS Pin Considerations

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

ternal power switching stage and operate other internal

circuitry. 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 VOUT. 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 LTM8003 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 unused

orgeneratinganegativeoutput,tieBIAStoLTM8003GND.

Maximum Load

ThemaximumpracticalcontinuousloadthattheLTM8003

can drive, while rated at 3.5A, actually depends upon both

the internal current limit and the internal temperature.

The internal current limit is designed to prevent damage

to the LTM8003 in the case of overload or short-circuit.

The internal temperature of the LTM8003 depends upon

operating conditions such as the ambient temperature,

the power delivered, and the heat sinking capability of the

system. For example, if the LTM8003H is configured to

regulateat1.2V,itmaycontinuouslydeliver6Afrom12VIN

if the ambient temperature is controlled to less than 50°C.

This is quite a bit higher than the 3.5A continuous rating.

Please see the “Derating, H-Grade, VOUT = 1.2V” curve in

theTypicalPerformanceCharacteristicssection.Similarly,

if the output voltage is 18V and the ambient temperature

is 100°C, the LTM8003H will deliver at most 2.7A from

24VIN, which is less than the 3.5A continuous rating.

Load Sharing

Neither the LTM8003 nor LTM8003-3.3 are designed to

load share.

Burst Mode Operation

To enhance efficiency at light loads, the LTM8003 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 LTM8003 delivers single cycle bursts

ofcurrenttotheoutputcapacitorfollowedbysleepperiods

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 LTM8003 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 LTM8003 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 LTM8003.

Output Voltage Tracking and Soft-Start

The LTM8003 allows the user to adjust its output voltage

ramp rate by means of the TR/SS pin. An internal 2A pulls

up the TR/SS pin to about 2.4V. Putting an external capaci-

tor on TR/SS enables soft starting the output to reduce

current surges on the input supply. During the soft-start

ramp the output voltage will proportionally track the TR/

SS pin voltage. For output tracking applications, TR/SS

can be externally driven by another voltage source. From

0V to 0.97V, the TR/SS voltage will override the internal

0.97V reference input to the error amplifier, thus regulat-

ing the FB pin voltage to that of the TR/SS pin. When TR/

SS is above 0.97V, tracking is disabled and the feedback

voltage will regulate to the internal reference voltage. The

TR/SS pin may be left floating if the function is not needed.

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