LM2694 datasheet(13/19 Pages) NSC

13 / 19 page

Applications Information (Continued)

•

S = 250 kHz

•

Minimum load current = 100 mA

•

Maximum load current = 600 mA

•

Softstart time=5ms.

R1 and R2: These resistors set the output voltage, and their

ratio is calculated from:

R1/R2 = (V

OUT/2.5V) - 1

(7)

R1/R2 calculates to 1.0. The resistors should be chosen

from standard value resistors in the range of 1.0 k

Ω -10kΩ.

A value of 2.5 k

Ω will be used for R1 and for R2.

ON,FS: RON can be chosen using Equation 5 to set the

nominal frequency, or from Equation 4 if the on-time at a

particular V

IN is important. A higher frequency 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 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 5 at a V

IN of 8V,

A value of 140 k

Ω will be used for R

ON, yielding a nominal

frequency of 252 kHz.

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 LM2694 regulates correctly

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 200 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 of 189 kHz (252 kHz

- 25%).

This provides a minimum value for L1 - the next higher

standard value (150 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. The maxi-

mum ripple amplitude 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 =ILIM +IOR(max) = 0.74A + 0.18A = 0.92A

where I

LIM is the maximum guaranteed current limit thresh-

old. At the nominal maximum load current of 0.6A, the peak

inductor current is 692 mA.

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 12), 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 calculated at V

IN = 8V using Equation 4, with a

25% tolerance added:

The voltage at VIN should not be allowed to drop below 7.5V

in order to maintain V

CC above its UVLO.

20187037

FIGURE 12. Inductor Current

LM2694 датащи(PDF) 13 Page - National Semiconductor (TI)

LM2694 датащи(HTML) 13 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

датащи скачать

ссылки URL