false triggering of the V

transitions. For this reason, C3 should be no smaller than 0.1

µF.

C2, and R3: When selecting the output filter capacitor C2,

the items to consider are ripple voltage due to its ESR, ripple

voltage due to its capacitance, and the nature of the load.

a) ESR and R3: A low ESR for C2 is generally desirable so

as to minimize power losses and heating within the capaci-

tor. However, a hysteretic regulator requires a minimum

amount of ripple voltage at the feedback input for proper loop

operation. For the LM5008 the minimum ripple required at

pin 5 is 25 mV p-p, requiring a minimum ripple at V

100 mV. Since the minimum ripple current (at minimum Vin)

is 34 mA p-p, the minimum ESR required at V

34mA = 2.94

Ω. Since quality capacitors for SMPS applica-

tions have an ESR considerably less than this, R3 is inserted

as shown in Figure 1. R3’s value, along with C2’s ESR, must

result in at least 25 mV p-p ripple at pin 5. Generally, R3 will

be 0.5 to 3.0

Ω.

b) Nature of the Load: The load can be connected to V

OUT1

or V

lower regulation due to R3.

For a maximum allowed ripple voltage of 100 mVp-p at

V

Ω for C2. At

maximum Vin, the ripple current is 181 mAp-p, creating a

ripple voltage of 72 mVp-p. This leaves 28 mVp-p of ripple

due to the capacitance. The average current into C2 due to

the ripple current is calculated using the waveform in Figure

12.

Starting when the current reaches Io (300 mA in Figure 12)

half way through the on-time, the current continues to in-

crease to the peak (391 mA), and then decreases to 300 mA

half way through the off-time. The average value of this

portion of the waveform is 45.5mA, and will cause half of the

voltage ripple, or 14 mV. The interval is one half of the

frequency cycle time, or 2.23 µs. Using the capacitor’s basic

equation:

C=Ix

∆t/ ∆V

the minimum value for C2 is 7.2 µF. The ripple due to C2’s

capacitance is 90˚ out of phase from the ESR ripple, and the

two numbers do not add directly. However, this calculation

provides a practical minimum value for C2 based on its ESR,

and the target spec. To allow for the capacitor’s tolerance,

temperature effects, and voltage effects, a 15 µF, X7R ca-

pacitor will be used.

c) In summary: The above calculations provide a minimum

value for C2, and a calculation for R3. The ESR is just as

important as the capacitance. The calculated values are

guidelines, and should be treated as starting points. For

each application, experimentation is needed to determine

the optimum values for R3 and C2.

R

off-time set by this resistor must be greater than the maxi-

mum normal off-time which occurs at maximum Vin. Using

equation 2, the minimum on-time is 0.470 µs, yielding a

maximum off-time of 3.99 µs. This is increased by 117 ns (to

is then increased to allow for:

The response time of the current limit detection loop

(400ns),

ance,

t

Using equation 3, R

Ω (at V

The closest standard value is 267 k

Ω.

D1: The important parameters are reverse recovery time and

forward voltage. The reverse recovery time determines how

long the reverse current surge lasts each time the buck

switch is turned on. The forward voltage drop is significant in

the event the output is short-circuited as it is only this diode’s

voltage which forces the inductor current to reduce during

the forced off-time. For this reason, a higher voltage is better,

although that affects efficiency. A good choice is an ultrafast

power diode, such as the MURA110T3 from ON Semicon-

ductor. Its reverse recovery time is 30ns, and its forward

voltage drop is approximately 0.72V at 300 mA at 25˚C.

Other types of diodes may have a lower forward voltage

drop, but may have longer recovery times, or greater reverse

leakage. D1’s reverse voltage rating must be at least as

great as the maximum Vin, and its current rating be greater

than the maximum current limit threshold (610 mA).

C1: This capacitor’s purpose is to supply most of the switch

current during the on-time, and limit the voltage ripple at Vin,

on the assumption that the voltage source feeding Vin has

an output impedance greater than zero. At maximum load

current, when the buck switch turns on, the current into pin 8

will suddenly increase to the lower peak of the output current

waveform, ramp up to the peak value, then drop to zero at

turn-off. The average input current during this on-time is the

load current (300 mA). For a worst case calculation, C1 must

20097926

FIGURE 12. Inductor Current Waveform

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