LTM4613 datasheet(13/20 Pages) LINER

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LTM8048

8048fa

APPLICATIONS INFORMATION

Isolation and Working Voltage

The LTM8048 isolation is tested by tying all of the primary

pins together, all of the secondary pins together and

subjecting the two resultant circuits to a differential of

±725VDC for one second. This establishes the isolation

voltage rating, but it does not determine the working volt-

age rating, which is subject to the application board layout

and possibly other factors. The metal to metal separation

of the primary and secondary throughout the LTM8048

substrate is 0.44mm.

VOUT2 Post Regulator

VOUT2 is produced by a high performance low dropout

300mA regulator. At full load, its dropout is less than

430mV over temperature. Its output is set by applying a

resistor from the RADJ2 pin to GND; the value of RADJ2

can be calculated by the equation:

RADJ2 =

608.78

VOUT2 –1.22

VOUT1 to VOUT– Reverse Voltage

The LTM8048 cannot tolerate a reverse voltage from VOUT1

to VOUT– during operation. If VOUT– raises above VOUT1

during operation, the LTM8048 may be damaged. To protect

against this condition, a low forward drop power Schottky

diode has been integrated into the LTM8048, anti-parallel

to VOUT1/VOUT–. This can protect the output against many

reverse voltage faults. Reverse voltage faults can be both

steady state and transient. An example of a steady state

voltage reversal is accidentally misconnecting a powered

LTM8048 to a negative voltage source. An example of

transient voltage reversals is a momentary connection to

a negative voltage. It is also possible to achieve a VOUT1

reversal if the load is short-circuited through a long cable.

The inductance of the long cable forms an LC tank circuit

with the VOUT1 capacitance, which drive VOUT1 negative.

Avoid these conditions.

VOUT2 Post Regulator Bypass Capacitance and Low

Noise Performance

The VOUT2 linear regulator may be used with the addition

of a 0.01μF bypass capacitor from VOUT to the BYP pin

to lower output voltage noise. A good quality low leakage

capacitor, such as a X5R or X75 ceramic, is recommended.

This capacitor will bypass the reference of the regulator,

lowering the output voltage noise to as low as 20μVRMS.

Using a bypass capacitor has the added benefit of improv-

ing transient response.

PCB Layout

Most of the headaches associated with PCB layout have

been alleviated or even eliminated by the high level of

integration of the LTM8048. The LTM8048 is neverthe-

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

minimize electrical noise to 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 1 for a suggested layout. Ensure that the grounding

and heat sinking are acceptable.

Figure 1. Layout Showing Suggested External Components,

Planes and Thermal Vias

8048 F01

BIAS

RUN

GND

ADJ2

BYP

ADJ1

LTM8048

COUT2

COUT1

VOUT

–

VOUT2

VIN

VOUT1

CIN

THERMAL/INTERCONNECT VIAS

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