23

LT1506

Redundant Operation

The circuit shown in Figure 15 is fault tolerant when

operating at less than 8A of output current. If one device

fails, the output will remain in regulation. The feedback

increasing switch current of the two remaining devices.

BUCK CONVERTER WITH ADJUSTABLE SOFT START

Large capacitive loads can cause high input currents at

start-up. Figure 16 shows a circuit that limits the dv/dt of

the output at start-up, controlling the capacitor charge

rate. The buck converter is a typical configuration with the

maintain a constant dv/dt at the output. Output rise time is

the circuit operates normally. R3 is transient protection for

the base of Q1.

RiseTime

RC

V

V

SS

OUT

BE

=

()(

)(

)

()

4

Using the values shown in Figure 16,

RiseTime

ms

==

(•

)(

•

)( )

.

–

47 10

15 10

5

07

5

39

The ramp is linear and rise times in the order of 100ms are

possible. Since the circuit is voltage controlled, the ramp

rate is unaffected by load characteristics and maximum

output current is unchanged. Variants of this circuit can be

used for sequencing multiple regulator outputs.

Dual Output SEPIC Converter

The circuit in Figure 17 generates both positive and

negative 5V outputs with a single piece of magnetics. The

two inductors shown are actually just two windings on a

standard B H Electronics inductor. The topology for the 5V

output is a standard buck converter. The – 5V topology

would be a simple flyback winding coupled to the buck

converter if C4 were not present. C4 creates a SEPIC

(Single-Ended Primary Inductance Converter) topology

whicn improves regulation and reduces ripple current in

L1. Without C4, the voltage swing on L1B compared to

L1A would vary due to relative loading and coupling

losses. C4 provides a low impedance path to maintain an

equal voltage swing in L1B, improving regulation. In a

flyback converter, during switch on time, all the converter’s

energy is stroed in L1A only, since no current flows in L1B.

At switch off, energy is transferred by magnetic coupling

into L1B, powering the – 5V rail. C4 pulls L1B positive

during switch on time, causing current to flow, and energy

to build in L1B and C4. At switch off, the energy stored in

both L1B and C4 supply the –5V rail. This reduces the

current in L1A and changes L1B current waveform from

square to triangular. For details on this circuit see Design

Note 100.

Figure 16. Buck Converter with Adjustable Soft Start

OUTPUT

5V

OUTPUT

* L1 IS A SINGLE CORE WITH TWO WINDINGS

BH ELECTRONICS #501-0726

** TOKIN IE475ZY5U-C304

PRELOAD OF 1k TO 5k MAY BE USED TO

IMPROVE LOAD REGULATION

D1, D3: MBRD340

INPUT

6V TO 15V

GND

1506 F17

C2

0.27

µF

1.5nF

D1

C1**

100

µF

10V TANT

C5**

100

µF

10V TANT

C3

10

µF

25V

CERAMIC

C4**

4.7

µF

D2

1N914

D3

L1*

6.8

µH

L1*

R1

5.36k

R2

4.99k

+

+

+

+

BOOST

LT1506

FB

SHDN

GND

BOOST

LT1506

OUTPUT

5V

4A

INPUT

12V

1506 F16

C2

0.33

µF

C1

100

µF

15nF

1.5nF

D1

C3

10

µF

D2

1N914

L1

5

µH

R1

5.36k

R3

2k

FB

SHDN

GND

+

R2

4.99k

R4

47k

Q1

Figure 17. Dual Output SEPIC Converter

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.