LTM4643 datasheet(29/43 Pages) AD

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Data Sheet

LTM8060F

analog.com

Rev. 0

29 of 43

Table 10. 5V Output

DERATING CURVE

VIN

(V)

POWER LOSS

CURVES

AIRFLOW

(LFM)

HEAT SINK

θJA (°C/W)

Figure 63, Figure 64, Figure 65

12, 24, 36

Figure 56

None

Figure 63, Figure 64, Figure 65

12, 24, 36

Figure 56

200

None

7.5

Figure 63, Figure 64, Figure 65

12, 24, 36

Figure 56

400

None

6.5

Load Sharing

The four LTM8060F channels may be paralleled to produce higher currents. To do this on two or more LTM8060F

modules, connect the VINn, VOUTn, FBn, and SHAREn pins of all the paralleled channels/modules together. To ensure

that paralleled channels startup together, the TRSSn pins may be all connected together, as well. If it is

inconvenient to connect the TRSSn pins together, ensure that the same value soft start capacitors are used for each

µModule regulator. When load sharing among n units and using a single RFB resistor, the value of the resistor is

given by Equation 2.

�� =

199.2

��(�� − 0.8)

, ��ℎ�� Ω

(2)

Examples of load-sharing applications are given in Figure 70 through Figure 73 in the Typical Applications section.

Burst Mode Operation

To enhance efficiency at light loads, the LTM8060F automatically 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 LTM8060F delivers single-cycle bursts of current to the output capacitor followed by sleep

periods where most of the internal circuitry is powered off, and energy is delivered to the load by the output

capacitor. During the sleep time, VINn and BIASn quiescent currents are greatly reduced, so, as the load current

decreases towards a no-load condition, the percentage of time that the LTM8060F operates in sleep mode

increases and the average input current is greatly reduced, resulting in higher light load efficiency.

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

Minimum Input Voltage

The LTM8060F is a step-down converter, therefore, a minimum amount of headroom is required to keep the output

in regulation. Keep the input above 3V to ensure proper operation. Voltage transients or ripple valleys that cause

the input to fall below 3V may turn off the LTM8060F.

The VIN1 must be above 3V for Channel 1 and Channel 2 to operate. If VIN1 is above 3V, Channel 2 will operate if

VIN2 is above 2V.

The VIN34 must be above 3V for Channel 3 and Channel 4 to operate.

Output Voltage Tracking and Soft Start

The LTM8060F allows the user to adjust its output voltage ramp rate using the TRSSn pin. An internal 2μA pulls up

the TRSSn pin to about 2.4V. Putting an external capacitor on TRSSn enables the soft starting the output to reduce

current surges on the input supply. During the soft start ramp the output voltage will proportionally track the

TRSSn pin voltage. For output tracking applications, TRSSn can be externally driven by another voltage source.

From 0V to 0.8V, the TRSSn voltage will override the internal 0.8V reference input to the error amplifier, thus

regulating the FBn pin voltage to that of the TRSSn pin. When TRSSn is above 0.8V, tracking is disabled, and the

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