LM2694 datasheet(14/19 Pages) NSC

14 / 19 page

Applications Information (Continued)

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 LM2694 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

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.

(10)

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 Figure 7. The value chosen

for C2 is application dependent, and it is recommended that

it be no smaller than 3.3 µF. C2 affects the ripple at V

OUT,

and transient response. Experimentation is usually neces-

sary 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 0.1 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

LM2694.

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 0.024 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

(30V), the maximum load current

(0.6A), and the peak current which occurs when current limit

and maximum ripple current are reached simultaneously (I

in Figure 11), previously calculated to be 0.92A. 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 13, and its performance is

shown in Figure 14 and Figure 15. Current limit measured

approximately 0.64A.

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