LTC1700EMS 查看數據表（PDF） - Linear Technology

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LTC1700EMS

No RSENSE Synchronous Step-Up DC/DC Controller

Linear Technology 

LTC1700

APPLICATIONS INFORMATION

To eliminate this subharmonic oscillation, a compensat-

ing ramp is added internally to the LTC1700 on the

inductor current waveform when the duty cycle exceeds

5%. This scheme, known as slope compensation, makes

the loop perceive that there is more inductor current than

it actually has. As a result, the maximum current capability

of the regulator is reduced. This reduction is proportional

to the duty cycle and is shown in Figure 5. Hence for

applications that operate at high duty cycles, the

N-channel MOSFET chosen should have a lower RDS(ON)

to make up for this reduction (See Design Example).

1.2

1.0

0.8

0.6

0.4

0.2

0

0 10 20 30 40 50 60 70

DUTY CYCLE (%)

1700 F05

Figure 5. Maximum Output Current vs Duty Cycle

Inductor Value Selection

Given the input voltage, inductor value and operating

frequency, the ripple current can be calculated:

∆IL=

VIN⎛⎝⎜

DC

fL

⎞⎠⎟

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Thus, highest efficiency operation is obtained at

low frequency with small ripple current. To achieve this,

however, requires a larger inductor.

A reasonable starting point is to choose a ripple current

that is about 40% of IO(MAX). Note that the largest ripple

current occurs at the highest VIN. To guarantee that ripple

current does not exceed a specified maximum, the induc-

tor should be chosen according to:

10

LMIN≥

VIN(MAX) ⎛⎝⎜

DC

f∆IL

⎞

⎠⎟

With Burst Mode operation enabled on the LTC1700, the

ripple current is normally set such that the inductor

current is continuous during burst periods. Remember

that during bursting, the peak current is clamped at

approximately:

IBURST(PEAK) ≅ 36mV/RDS(ON)

Hence the peak-to-peak ripple selected for optimal burst

mode operation should not exceed IBURST(PEAK). This

implies a minimum inductance of:

( ) L MINBURST

=

VIN(MAX) DC

(f)(0.66)⎛⎝⎜

IOMAX

1– DC

⎞

⎠⎟

In applications that invoke Burst Mode operation, the

inductor should be chosen so it has low ripple (0.4IOMAX)

current during heavy load and continuous operation dur-

ing bursting. The criteria for selecting which equation to

use is:

Use LMIN for Duty Cycle > 36%

Use LMINBURST for Duty Cycle ≤ 36%

A smaller value than LMIN could be used in the circuit;

however, the inductor current will not be continuous

during burst periods. The advantage of using a smaller

inductance than LMIN is primarily size. The disadvantage is

higher output ripple.

Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. High efficiency converters generally cannot af-

ford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite, molypermalloy,

or Kool Mµ® cores. Actual core loss is independent of core

size for a fixed inductor value, but it is very dependent on

inductance selected. As inductance increases, core losses

go down. Unfortunately, increased inductance requires

more turns of wire and therefore copper losses will

increase. Ferrite designs have very low core losses and are

Kool Mµ is a registered trademark of Magnetics, Inc.

1700fa

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