LTM8002 datasheet(18/26 Pages) LINER

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LTM8065

8065fa

For more information www.linear.com/LTM8065

APPLICATIONS INFORMATION

the LTM8065 GND, as shown in Figure 2, so fast load

transients can disrupt the LTM8065’s operation or even

cause damage.

occur in battery charging applications or in battery backup

systems where a battery or some other supply is diode

ORed with the LTM8065’s output. If the VIN pin is allowed

tofloatandtheRUNpinisheldhigh(eitherbyalogicsignal

or because it is tied to VIN), then the LTM8065’s internal

circuitry pulls 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 pin, the

internal current drops to essentially zero. However, if the

VIN pin is grounded while the output is held high, parasitic

diodes inside the LTM8065 can pull large currents from

the output through the VIN pin. Figure 4 shows a circuit

that runs only when the input voltage is present and that

protects against a shorted or reversed input.

FAST LOAD

TRANSIENT

OUTPUT TRANSIENT

RESPONSE

LTM8065

VOUT

VIN

8065 F02

GND

VIN

Figure 2. Any Output Voltage Transient Appears on LTM8065 GND

Figure 3. A Schottky Diode Can Limit the Transient

Caused by a Fast Rising VIN to Safe Levels

Figure 4. The Input Diode Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output. It Also

Protects the Circuit from a Reversed Input. The LTM8065

Runs Only When the Input Is Present

FAST VIN

TRANSIENT

OUTPUT EXPERIENCES

A POSITIVE TRANSIENT

LTM8065

VOUT

VIN

8065 F03

GND

COUT

CIN

OPTIONAL

SCHOTTKY

DIODE

AC DIVIDER

VIN

The CIN and COUT capacitors in Figure 3 form an AC divider

at the negative output voltage node. If VIN is hot-plugged

or rises quickly, the resultant VOUT will be a positive tran-

sient, which may be unhealthy for the application load.

An anti-parallel Schottky diode may be able to prevent

this positive transient from damaging the load. The loca-

tion of this Schottky diode is important. For example, in

a system where the LTM8065 is far away from the load,

placing the Schottky diode closest to the most sensitive

load component may be the best design choice. Carefully

evaluatewhetherthenegativebuckconfigurationissuitable

for the application. For negative outputs, connect BIAS to

LTM8065 GND.

Shorted Input Protection

Care needs to be taken in systems where the output is held

high when the input to the LTM8065 is absent. This may

LTM8065

VIN

8065 F04

RUN

PCB Layout

Most of the headaches associated with PCB layout have

been alleviated or even eliminated by the high level of

integration of the LTM8065. The LTM8065 is neverthe-

less a switching power supply, and care must be taken to

minimize EMI and ensure proper operation. Even with the

high level of integration, you may fail to achieve specified

operation with a haphazard or poor layout. See Figure 5

for a suggested layout. Ensure that the grounding and

heat sinking are acceptable.

A few rules to keep in mind are:

1. Place CFF, RFB and RT as close as possible to their

respective pins.

2. Place the CIN capacitor as close as possible to the VIN

and GND connection of the LTM8065.

3. Place the COUT capacitor as close as possible to the

VOUT and GND connection of the LTM8065.

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