L1 =

t

1.3 x 10

-10

LM34910C

SNVS517B – MAY 2007 – REVISED MARCH 2013

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APPLICATIONS INFORMATION

EXTERNAL COMPONENTS

The following guidelines can be used to select the external components.

R1 and R2: The ratio of these resistors is calculated from:

(8)

R1 and R2 should be chosen from standard value resistors in the range of 1.0 k

Ω - 10 kΩ which satisfy the

above ratio.

(9)

must be determined at both the minimum and maximum nominal load currents.

a) If the maximum load current is less than the current limit threshold (1.25A), the minimum load current is used

to determine the maximum allowable ripple. To maintain continuous conduction mode the lower peak should not

reach 0 mA. For this case, the maximum ripple current is:

(10)

The ripple calculated in Equation 6 is then used in the following equation:

where

•

•

Fs is determined from Equation 1

(11)

This provides a minimum value for L1. The next larger standard value should be used, and L1 should be rated

b) If the maximum load current is greater than the current limit threshold (1.25A), the LM34910C ensures the

however, must not exceed 3.5A. For this case, the ripple limits are:

(12)

and

(13)

used in Equation 11. The next smaller value should then be used for L1. L1 must be rated for the peak value of

0.1 µF, and should be a good quality, low ESR, ceramic capacitor.

10

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