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LT3430

sn3430 3430is

APPLICATIO S I FOR ATIO

DUAL OUTPUT SEPIC CONVERTER

The circuit in Figure 14 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 Coiltronics 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

which 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 stored 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, including

maximum output currents, see Design Note 100.

POSITIVE-TO-NEGATIVE CONVERTER

The circuit in Figure 15 is a positive-to-negative topology

using a grounded inductor. It differs from the standard

approach in the way the IC chip derives its feedback signal

because the LT3430 accepts only positive feedback sig-

nals. The ground pin must be tied to the regulated negative

output. A resistor divider to the FB pin then provides the

proper feedback voltage for the chip.

The following equation can be used to calculate maximum

load current for the positive-to-negative converter:

f L

MAX

OUT













–

()(

)

()( )( )

()(

– .

)

(– .

)(

)

015

IP = Maximum rated switch current

VIN = Minimum input voltage

VOUT = Output voltage

VF = Catch diode forward voltage

0.15 = Switch voltage drop at 3A

Example: with VIN(MIN) = 5.5V, VOUT = 12V, L = 10µH,

VF = 0.52V, IP = 3A: IMAX = 0.6A.

VOUT

–5V

†

* L1 IS A SINGLE CORE WITH TWO WINDINGS

COILTRONICS #CTX25-4A

† IF LOAD CAN GO TO ZERO, AN OPTIONAL

PRELOAD OF 1k TO 5k MAY BE USED TO

IMPROVE LOAD REGULATION

D1, D3: 30BQ060

VIN

7.5V TO 60V

GND

3430 F14

0.68

µF

220pF

100

µF

10V TANT

100

µF

10V TANT

4.7

µF

100V

CERAMIC

100

µF

10V

TANT

MMSD914TI

L1A*

µH

L1B*

15.4k

4.99k

3.3k

0.022

µF

BOOST

VIN

LT3430

SHDN

SYNC

GND

Figure 14. Dual Output SEPIC Converter

LT3430 Datasheet(PDF) 24 Page - Linear Technology

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