LT1506CS8 datasheet(22/24 Pages) LINER

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LT1506

APPLICATIONS INFORMATION

converters 120

° out of phase with each other reduces

input and output ripple currents. This reduces the ripple

rating, size and cost of filter capacitors.

Current Sharing/Split Input Supplies

Current sharing is accomplished by joining the VC pins to

a common compensation capacitor. The output of the

error amplifier is a gm stage, so any number of devices can

be connected together. The effective gm of the composite

error amplifier is the multiple of the individual devices. In

Figure 15, the compensation capacitor C4 has been

increased by

×3. Tolerances in the reference voltages

result in small offset currents to flow between the VC pins.

The overall effect is that the loop regulates the output at a

voltage between the minimum and maximum reference of

the devices used. Switch current matching between

devices will be typically better than 300mA. The negative

temperature coefficient of the VCtoswitchcurrenttranscon-

ductance prevents current hogging.

A common VC voltage forces each LT1506 to operate at the

same switch current, not duty cycle. Each device operates

at the duty cycle defined by its respective input voltage. In

Figure 15, the input could be split and each device oper-

ated at a different voltage. The common VC ensures

loading is shared between inputs.

Figure 15. Current Sharing 12A Supply

68nF

25V

C1, C3: MARCON THCS50E1E106Z

D1: ROHM RB051L-40

D2: 1N914

L1: DO3316P-682

C3C

µF

25V

C2C

330nF

10V

D1C

D2C

1506 F15

L1C

6.8

µH

µF

25V

12A

5.36k

4.99k

1.8MHz

3-BIT RING

COUNTER

C3B

µF

25V

C2B

330nF

10V

D1B

D2B

L1B

6.8

µH

C3A

µF

25V

INPUT

6V TO 15V

C2A

330nF

10V

D1A

D2A

L1A

6.8

µH

VC SYNC SW GND

LT1506-SYNC

VIN BOOST FB

VC SYNC SW GND

LT1506-SYNC

VIN BOOST FB

VC SYNC SW GND

LT1506-SYNC

VIN BOOST FB

Synchronized Ripple Currents

A ring counter generates three synchronization signals at

600kHz, 33% duty cycle phased 120

° apart. The sync

input will operate over a wide range of duty cycles, so no

further pulse conditioning is needed. Each device’s maxi-

mum input ripple current is a 4A square wave at 600kHz.

When synchronously added together, the ripple remains

at 4A but frequency increases to 1.8MHz. Likewise, the

output ripple current is a 1.8MHz triangular waveform,

with maximum amplitude of 350mA at 10V VIN. Interest-

ingly, at 7.6V and 15V VIN, the theoretical summed output

ripple current cancels completely. To reduce board space

and ripple voltage, C1 and C3 are ceramic capacitors. Loop

compensation C4 must be adjusted when using ceramic

output capacitors due to the lack of effective series resis-

tance. The typical tantalum compensation of 1.5nF is

increased to 22nF (

×3) for the ceramic output capacitor.

If synchronization is not used and the internal oscillators

free run, the circuit will operate correctly, but ripple

cancellation will not occur. Input and output capacitors

must be ripple rated for the total output current.

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