be less than 200 mA p-p so the lower peak of the waveform

does not reach zero. L1 is calculated using the following

equation:

At Vin = 90V, L1(min) calculates to 132 µH. The next larger

standard value (150 µH) is chosen and with this value I

OR

calculates to 176 mA p-p at Vin = 90V, and 33 mA p-p at Vin

= 12V.

b) Maximum load current: At a load current of 150 mA, the

peak of the ripple waveform must not reach the minimum

guaranteed value of the LM5009’s current limit threshold

(250 mA). Therefore the ripple amplitude must be less than

200 mA p-p, which is already satisfied in the above calcula-

tion. With L1 = 150 µH, at maximum Vin and Io, the peak of

the ripple will be 238 mA. While L1 must carry this peak

current without saturating or exceeding its temperature rat-

ing, it also must be capable of carrying the maximum guar-

anteed value of the LM5009’s current limit threshold (370

mA) without saturating, since the current limit is reached

during startup.

C3: The capacitor on the V

filtering and stability, but also prevents false triggering of the

V

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 LM5009 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 33 mA p-p, the minimum ESR required at V

Ω.

Since quality capacitors for SMPS applications 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

Ω to

5.0

Ω.

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

OUT1

or V

to 13 mV) but lower regulation due to R3.

C2 should generally be no smaller than 3.3 µF. Typically, its

value is 10 µF to 20 µF, with the optimum value determined

by the load. If the load current is fairly constant, a small value

suffices for C2. If the load current includes significant tran-

sients, a larger value is necessary. For each application,

experimentation is needed to determine the optimum values

for R3 and C2.

C) Ripple Reduction: The ripple amplitude at V

reduced by reducing R3, and adding a capacitor across R1

so as to tranfer the ripple at V

without attenuation. The new value of R3 is calculated from:

R3 = 25 mV/I

OR(min)

where I

mAp-p in this example. The added capacitor’s value is cal-

culated from:

C= T

where T

The selected capacitor should be larger than the value cal-

culated above.

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 0.329 µs, yielding a

maximum off-time of 2.63 µs. This is increased by 82 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 169 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 or Schottky diode with a reverse recovery time of )30

ns, and a forward voltage drop of )0.7V. Other types of

diodes may have a lower forward voltage drop, but may have

longer recovery times, or greater reverse leakage. D1’s re-

verse voltage rating must be at least as great as the maxi-

mum Vin, and its current rating be greater than the maximum

current limit threshold (370 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 (150 mA). For a worst case calculation, C1 must

supply this average load current during the maximum on-

time. To keep the input voltage ripple to less than 2V (for this

exercise), C1 calculates to:

Quality ceramic capacitors in this value have a low ESR

which adds only a few millivolts to the ripple. It is the capaci-

tance which is dominant in this case. To allow for the capaci-

tor’s tolerance, temperature effects, and voltage effects, a

1.0 µF, 100V, X7R capacitor will be used.

C4: The recommended value is 0.01µF for C4, as this is

appropriate in the majority of applications. A high quality

ceramic capacitor, with low ESR is recommended as C4

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