LTM8022V datasheet(11/20 Pages) LINER

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LTM8022

8022fb

APPLICATIONS INFORMATION

Load Sharing

Two or more LTM8022’s may be paralleled to produce higher

currents. To do this, tie the VIN, VOuT and SHARE pins of

all the paralleled LTM8022’s together. Synchronize the

LTM8022s to avoid beat frequencies if required. To ensure

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

may be tied together, as well. 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

LTM8022 modules conﬁgured for load sharing is given in

the Typical Applications section.

Soft-Start

The RUN/SS pin can be used to soft-start the LTM8022,

reducing the maximum input current during start-up. The

RUN/SS pin is driven through an external RC ﬁlter to create

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

shutdown waveforms with the soft-start circuit. By choos-

ing 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.

Synchronization

The internal oscillator of the LTM8022 can be synchronized

by applying an external 250kHz to 2MHz clock signal to

the SYNC pin. The resistor tied from the RT pin to ground

should be chosen such that the LTM8022 would free run

20% lower than the intended synchronization frequency

(see Frequency Selection section).

When the LTM8022 is synchronized to an external clock

source, Burst Mode operation is disabled. The part will

skip power switching cycles as necessary to maintain

regulation. Ensure that the SYNC pin is not left ﬂoating.

Tie it to GND if not used.

Shorted Input Protection

Care needs to be taken in systems where the output will be

held high when the input to the LTM8022 is absent. This

may occur in battery charging applications, or in battery

backup systems where a battery or some other supply is

diode ORed with the LTM8022’s output. If the VIN pin is

allowed to ﬂoat, and the RUN/SS pin is held high (either

by a logic signal or because it is tied to VIN), then the

LTM8022’s internal circuitry will pull its quiescent current

through its internal power switch. This is ﬁne if your system

can tolerate a few milliamps in this state. If you ground the

RUN/SS pin, the SW pin current will drop to essentially

zero. However, if the VIN pin is grounded while the output

is held high, then parasitic diodes inside the LTM8022 can

pull large currents from the output through the VIN pin.

Figure 3 shows a circuit that will run only when the input

voltage is present and that protects against a shorted or

reversed input.

Figure 2. To Soft-Start the LTM8022, Add a

Resistor and Capacitor to the RUN/SS Pin

Figure 3. The Input Diode Prevents a Shorted Input from

Discharging a Back-Up Battery Tied to the Output. It Also

Protects the Circuit From a Reversed Input. The LTM8022

Runs Only When the Input is Present

8022 F02

0.5A/DIV

VRUN/SS

2V/DIV

VOUT

2V/DIV

RUN/SS

GND

0.22μF

RUN

15k

2ms/DIV

LTM8022

8022 F03

VIN

VOUT

RUN/SS

VIN

SYNC

BIAS

AUX

VOUT

RT GND

ADJ

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