LM5010AMH datasheet(13/17 Pages) NSC

13 / 17 page

Applications Information (Continued)

This necessary ripple voltage is created by the inductor

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

ripple current, which occurs at minimum VIN, maximum in-

ductor value, and maximum frequency, is determined.

(13)

The minimum ESR for C2 is then equal to:

If the capacitor used for C2 does not have sufficient ESR, R3

is added in series as shown in the Block Diagram. The value

chosen for C2 is application dependent, and it is recom-

mended that it be no smaller than 3.3 µF. C2 affects the

ripple at V

OUT, and transient response. Experimentation is

usually necessary to determine the optimum value for C2.

C3: The capacitor at the VCC pin provides noise filtering and

stability, prevents false triggering of the V

CC UVLO at the

buck switch on/off transitions, and limits the peak voltage at

CC when a high voltage with a short rise time is initially

applied at V

IN. C3 should be no smaller than 0.47 µF, and

should be a good quality, low ESR, ceramic capacitor, physi-

cally close to the IC pins.

C4: The recommended value for C4 is 0.022 µF. A high

quality ceramic capacitor with low ESR is recommended as

C4 supplies the surge current to charge the buck switch gate

at each turn-on. A low ESR also ensures a complete re-

charge during each off-time.

C5: This capacitor suppresses transients and ringing due to

lead inductance at VIN. A low ESR, 0.1 µF ceramic chip

capacitor is recommended, located physically close to the

LM5010A.

C6: The capacitor at the SS pin determines the soft-start

time, i.e. the time for the reference voltage at the regulation

comparator, and the output voltage, to reach their final value.

The capacitor value is determined from the following:

Fora5ms softstart time, C6 calculates to 0.022 µF.

D1: A Schottky diode is recommended. Ultra-fast recovery

diodes are not recommended as the high speed transitions

at the SW pin may inadvertently affect the IC’s operation

through external or internal EMI. The diode should be rated

for the maximum V

IN (60V), the maximum load current (1A),

and the peak current which occurs when current limit and

maximum ripple current are reached simultaneously (I

PK in

Figure 4), previously calculated to be 1.87A. The diode’s

forward voltage drop affects efficiency due to the power

dissipated during the off-time. The average power dissipa-

tion in D1 is calculated from:

D1 =VF xIO x(1-D)

where I

O is the load current, and D is the duty cycle.

FINAL CIRCUIT

The final circuit is shown in Figure 6, and its performance is

shown in Figures7&8. Current limit measured approxi-

mately 1.3A.

20153833

FIGURE 6. Example Circuit

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