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LT1959

4.5A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1959

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 VC to switch current transcon-

ductance prevents current hogging.

A common VC voltage forces each LT1959 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.

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 5V 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.

1.8MHz

3-BIT RING

COUNTER

C1, C3: MARCON THCS50E1E106Z

D1: ROHM RB051L-40

D2: 1N914

L1: DO3316P-682

LT1959

VC SYNC SW GND VIN BOOST FB

INPUT

4.3V TO 15V

+ C3A

10µF

25V

D1A

L1A

6.8µH

D2A

C2A

330nF

10V

22

LT1959

VC SYNC SW GND VIN BOOST FB

LT1959

VC SYNC SW GND VIN BOOST FB

+ C3B

+ C4 + C3C

10µF

68nF

10µF

25V

D2B

25V

25V

D2C

D1B

D1C

R1

2.67k

1% +

R2

2.49k

1%

2.5V

12A

C1

10µF

25V

L1B

6.8µH

C2B

330nF

10V

L1C

6.8µH

C2C

330nF

10V

1959 F15

Figure 15. Current Sharing 12A Supply

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