LT3957 datasheet(13/24 Pages) LINER

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LT3959

3959f

Boost Converter: Switch Duty Cycle and Frequency

The LT3959 can be configured as a boost converter for

the applications where the converter output voltage is

higher than the input voltage. Remember that boost con-

verters are not short-circuit protected. Under a shorted

output condition, the inductor current is limited only by

the input supply capability. For applications requiring a

step-up converter that is short-circuit protected, please

refer to the Applications Information section covering

SEPIC converters.

The conversion ratio as a function of duty cycle is:

VOUT

VIN

−D

in continuous conduction mode (CCM).

For a boost converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT) and the input voltage (VIN). The maximum

duty cycle (DMAX) occurs when the converter has the

minimum input voltage:

DMAX =

VOUT − VIN(MIN)

VOUT

The alternative to CCM, discontinuous conduction mode

(DCM) is not limited by duty cycle to provide high con-

version ratios at a given frequency. The price one pays

is reduced efficiency and substantially higher switching

current.

Boost Converter: Maximum Output Current Capability

and Inductor Selection

For the boost topology, the maximum average inductor

current is:

IL(MAX) = IO(MAX) •

−DMAX

APPLICATIONS INFORMATION

Due to the current limit of its internal power switch, the

LT3959 should be used in a boost converter whose maxi-

mum output current (IO(MAX)) is less than the maximum

output current capability by a sufficient margin (10% or

higher is recommended):

IO(MAX) <

VIN(MIN)

VOUT

• (6A – 0.5 •

ΔISW )

The inductor ripple current ΔISW has a direct effect on the

choice of the inductor value and the converter’s maximum

output current capability. Choosing smaller values of

ΔISW increases output current capability, but requires

large inductances and reduces the current loop gain (the

converter will approach voltage mode). Accepting larger

values of ΔISW provides fast transient response and

allows the use of low inductances, but results in higher

input current ripple and greater core losses, and reduces

output current capability.

Given an operating input voltage range, and having chosen

the operating frequency and ripple current in the inductor,

the inductor value of the boost converter can be determined

using the following equation:

VIN(MIN)

ΔISW •fOSC

•DMAX

The peak inductor current is the switch current limit (7A

typical), and the RMS inductor current is approximately

equal to IL(MAX). The user should choose the inductors

having sufficient saturation and RMS current ratings.

Boost Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with low

forward drop and low reverse leakage is desirable. The

peak reverse voltage that the diode must withstand is

equal to the regulator output voltage plus any additional

ringing across its anode-to-cathode during the on-time.

The average forward current in normal operation is equal

to the output current.

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