LTM8032MPVPBF datasheet(11/20 Pages) LINER

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LTM8032

8032fa

APPLICATIONS INFORMATION

than 56V. If BIAS power is applied from a remote or noisy

voltage source, it may be necessary to apply a decoupling

capacitor locally to the LTM8032.

Load Sharing

Two or more LTM8032s may be paralleled to produce higher

currents. This may, however, alter the EMI performance of

the LTM8032s. To do this, tie the VIN, ADJ, VOUT and SHARE

pins of all the paralleled LTM8032s together. To ensure that

paralleled modules start up together, the RUN/SS pins may

be tied together, as well. Synchronize the LTM8032s to an

external clock to eliminate beat frequencies, if required. If

the RUN/SS pins are not tied together, make sure that the

same valued soft-start capacitors are used for each module.

An example of two LTM8032 modules conﬁgured for load

sharing is given in the Typical Applications section.

Burst Mode Operation

To enhance efﬁciency at light loads, the LTM8032 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 LTM8032 delivers single cycle bursts of

current to the output capacitor followed by sleep periods

where the output power is delivered to the load by the output

capacitor. In addition, VIN and BIAS quiescent currents are

reduced to typically 20μA and 50μA respectively during

the sleep time. As the load current decreases towards a

no-load condition, the percentage of time that the LTM8032

operates in sleep mode increases and the average input

current is greatly reduced, resulting in higher efﬁciency.

Burst Mode operation is enabled by tying SYNC to GND. To

disable Burst Mode operation, tie SYNC to a stable voltage

above 0.7V. Do not leave the SYNC pin ﬂoating.

Minimum Input Voltage

The LTM8032 is a step-down converter, so a minimum

amount of headroom is required to keep the output in

regulation. In addition, the input voltage required to turn

on is higher than that required to run, and depends upon

whether the RUN/SS is used. As shown in Figure 1, it

takes only about 3.6VIN for the LTM8032 to run a 3.3V

output at light load. If RUN/SS is pulled up to VIN, it takes

LOAD CURRENT (mA)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

500

1000

1500

2000

8032 F01a

TO RUN

TO START

RUN/SS ENABLED

VOUT = 3.3V

LOAD CURRENT (mA)

6.5

7.0

7.5

8032 F01b

6.0

5.5

5.0

500

1000

1500

2000

TO RUN

TO START

RUN/SS ENABLED

VOUT = 5V

Figure 1. The LTM8032 Needs More Voltage to Start Than Run

5.5VIN to start. If the LTM8032 is enabled via the RUN/SS

pin, the minimum voltage to start at light loads is lower,

about 4.5V. A similar curve for 5VOUT operation is also

provided in Figure 1.

Soft-Start

The RUN/SS pin can be used to soft-start the LTM8032,

reducing the maximum input current during start-up. The

RUN/SS pin is driven through an external RC network to

create a voltage ramp at this pin. Figure 2 shows the start-

up and shutdown waveforms with the soft-start circuit. By

choosing an appropriate RC time constant, the peak start-up

current can be reduced to the current that is required to

regulate the output, with no overshoot. Choose the value

of the resistor so that it can supply at least 20μA when

the RUN/SS pin reaches 2.5V.

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