LM5010 datasheet(13/19 Pages) NSC

13 / 19 page

Thermal Shutdown (Continued)

heating. When the junction temperature reduces below

155˚C (typical hysteresis = 20˚C), the Softstart pin is re-

leased and normal operation resumes.

Applications Information

EXTERNAL COMPONENTS

The procedure for calculating the external components is

illustrated with a design example. The circuit in Figure 1 is to

be configured for the following specifications:

•

OUT = 10V

•

IN = 15V to 75V

•

S = 625 kHz

•

Minimum load current = 150 mA

•

Maximum load current = 1.0A

•

Softstart time=5ms.

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

R1/R2 = (V

OUT/2.5V) - 1

(7)

R1/R2 calculates to 3.0. The resistors should be chosen

from standard value resistors in the range of 1.0 k

Ω -10kΩ.

Values of 3.0 k

Ω for R1, and 1.0 kΩ for R2 will be used.

ON,FS: RON sets the on-time, and can be chosen using

Equation 2 to set a nominal frequency, or from Equation 5 if

the on-time at a particular V

IN is important. A higher fre-

quency generally means a smaller inductor and capacitors

(value, size and cost), but higher switching losses. A lower

frequency means a higher efficiency, but with larger compo-

nents. If PC board space is tight, a higher frequency is better.

The resulting on-time and frequency have a ±25% toler-

ance. Re-arranging Equation 2 ,

The next larger standard value (137 k

Ω) is chosen for R

ON,

yielding a nominal frequency of 618 kHz.

L1: The inductor value is determined based on the load

current, ripple current, and the minimum and maximum input

voltage (V

IN(min),VIN(max)). Refer to Figure 13 .

To keep the circuit in continuous conduction mode, the maxi-

mum allowed ripple current is twice the minimum load cur-

rent, or 300 mAp-p. Using this value of ripple current, the

inductor (L1) is calculated using the following:

(8)

where F

S(min) is the minimum frequency (FS - 25%).

This provides a minimum value for L1 - the next higher

standard value (100 µH) will be used. L1 must be rated for

the peak current (I

PK+) to prevent saturation. The peak cur-

rent occurs at maximum load current with maximum ripple.

The maximum ripple is calculated by re-arranging Equation

8 using V

IN(max),FS(min), and the minimum inductor value,

based on the manufacturer’s tolerance. Assume, for this

exercise, the inductor’s tolerance is ±20%.

(9)

PK+ = 1.0A + 0.234A / 2 = 1.117A

CL: Since it is obvious that the lower peak of the inductor

current waveform does not exceed 1.0A at maximum load

current (see Figure 13), it is not necessary to increase the

current limit threshold. Therefore R

CL is not needed for this

exercise. For applications where the lower peak exceeds

1.0A, see the section below on increasing the current limit

threshold.

C2 and R3: Since the LM5010 requires a minimum of 25

mVp-p of ripple at the FB pin for proper operation, the

required ripple at V

OUT1 is increased by R1 and R2. This

necessary ripple is created by the inductor ripple current

acting on C2’s ESR + R3. First, the minimum ripple current is

determined.

20119922

FIGURE 13. Inductor Current

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