LM5010AMH datasheet(12/17 Pages) NSC

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Applications Information (Continued)

a higher efficiency, but with larger components. Generally, if

PC board space is tight, a higher frequency is better. The

resulting on-time and frequency have a ±25% tolerance.

Using equation 7 at a nominal V

IN of 8V,

A value of 200 k

Ω will be used for R

ON, yielding a nominal

frequency of 161 kHz at V

IN = 6V, and 205 kHz at VIN = 60V.

L1: The guideline for choosing the inductor value in this

example is that it must keep the circuit’s operation in con-

tinuous conduction mode at minimum load current. This is

not a strict requirement since the LM5010A regulates cor-

rectly when in discontinuous conduction mode, although at a

lower frequency. However, to provide an initial value for L1

the above guideline will be used.

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

mum allowed ripple current is twice the minimum load cur-

rent, or 400 mAp-p. Using this value of ripple current, the

inductor (L1) is calculated using the following:

(11)

where F

S(min) is the minimum frequency of 154 kHz (205 kHz

- 25%) at V

IN(max).

This provides a minimum value for L1 - the next higher

standard value (100 µH) will be used. To prevent saturation,

and possible destructive current levels, L1 must be rated for

the peak current which occurs if the current limit and maxi-

mum ripple current are reached simultaneously (I

PK in Figure

4). The maximum ripple amplitude is calculated by re-

arranging Equation 11 using V

IN(max),FS(min), and the mini-

mum inductor value, based on the manufacturer’s tolerance.

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

(12)

PK =ILIM +IOR(max) = 1.5A + 0.372A = 1.872A

where I

LIM is the maximum guaranteed current limit thresh-

old. At the nominal maximum load current of 1.0A, the peak

inductor current is 1.186A.

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 5), 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 entitled Increasing The Current Limit

Threshold.

C1: This capacitor limits the ripple voltage at VIN resulting

from the source impedance of the supply feeding this circuit,

and the on/off nature of the switch current into VIN. At

maximum load current, when the buck switch turns on, the

current into VIN steps up from zero to the lower peak of the

inductor current waveform (I

PK- in Figure 5), ramps up to the

peak value (I

PK+), then drops to zero at turn-off. The average

current into VIN during this on-time is the load current. For a

worst case calculation, C1 must supply this average current

during the maximum on-time. The maximum on-time is cal-

culated at V

IN = 6V using Equation 5, with a 25% tolerance

added:

The voltage at VIN should not be allowed to drop below 5.5V

in order to maintain V

CC above its UVLO.

Normally a lower value can be used for C1 since the above

calculation is a worst case calculation which assumes the

power source has a high source impedance. A quality ce-

ramic capacitor with a low ESR should be used for C1.

C2 and R3: Since the LM5010A requires a minimum of 25

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

required ripple at V

OUT is increased by R1 and R2, and is

equal to:

RIPPLE = 25 mVp-p x (R1 + R2)/R2 = 50 mVp-p

20153822

FIGURE 5. Inductor Current

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