LINEAR LTM8047 User Guide

design ideas

µModule Converters Take the Hassle Out of Designing Isolated Power Supplies David Ng

Sometimes a system needs a little bit of isolated power, but designing an isolated power supply is rarely easy. The nature of isolated supplies makes them complicated and touchy,

resulting in late nights and long weekends spent on design and debug. The LTM8047 AX IM UM OU T1 OA (m A)

and LTM8048 µModule converters take the hassle out of designing isolated power supplies, placing a flyback regulator in a compact 9mm × 11.25mm × 4.92mm BGA Ro HS compliant package. The controller, power switching and rectification elements, as well as transformer and isolated feedback circuitry are all integrated. Both parts operate from 3.1V to 32V inputs and produce over 1W of isolated power. The LTM8048 is identical to the LTM8047, but adds an integrated 300m A linear post regulator.

Designing with the LTM8047 and LTM8048 LTM8047

Figure 1. The LTM8047 only requires four VIN VOUT

VOUTVIN

is easy. Figure 1 shows a complete additional components to implement an 3.1V TO 29V 5V

RUN 280m A

isolated 5V power supply that accepts an

LTM8047-based isolated power supply, (15VIN)

3.1V–29V input.

requiring only the addition of capaci-

LTspice IV ADJ

tors for input, output, and biasing, and circuits.linear.com/553

a resistor to set the output voltage. The –IS GND VOUT

LTM8048 requires only one more com-

725VDC ISOLATION

ponent: a resistor to set the voltage of the LDO output, as shown in Figure 2.

OL AT IO BA RR IE LTM8048

Figure 2. The LTM8048 is the VIN 5.7V

VIN VOUT1

As is the case with most flyback convert- LTM8047 with the addition of 3.1V TO 30V

2.2µF VOUT2

RUN VOUT2

an LDO post regulator. 5V

ers, the output voltage can be above

BYPBIAS

OL AT IO BA RR IE 4.7µF

or below the input, accommodating LTspice IV

circuits.linear.com/550

ADJ1 ADJ2

a wide range of operating conditions.

SS 162k 10µF And, as is nature of flyback converters, –IS

GND VOUT

the amount of current that the LTM8047 725VDC ISOLATION and LTM8048 can deliver depends on the input voltage. Figure 3 shows the

BIAS = VIN IF VIN ≤ 5V load capability of a typical LTM8047 at BIAS = 5V IF VIN > 5V

2.5V, 3.3V and 5V outputs. The LTM8048 features the same load capability.

VOUT 500µV/DIV

The linear post regulator integrated

(AC-COUPLED)

into the LTM8048 is a high perfor- mance 300m A device, boasting a low

2.5VOUT1

dropout of less than 450m V at room 3.3VOUT1

5VOUT1

temp, full load. As shown in Figure 4,

1µs/DIV

the output noise and ripple of the post VIN (V)

Figure 3. Maximum load capability of the LTM8047 Figure 4. The output noise of the LTM8048 post and LTM8048 depends on the input voltage. regulator is less than 1m V.

April 2012 : LT Journal of Analog Innovation | 37