LM34919 datasheet(12/18 Pages) NSC

12 / 18 page

L1: The main parameter affected by the inductor is the in-

ductor current ripple amplitude (I

OR). The minimum load cur-

rent is used to determine the maximum allowable ripple in

order to maintain continuous conduction mode, where the

lower peak does not reach 0 mA. This is not a requirement of

the LM34919, but serves as a guideline for selecting L1. For

this case the maximum ripple current is:

OR(MAX) = 2 x IOUT(min) = 400 mA

(6)

If the minimum load current is zero, use 20% of I

OUT(max) for

OUT(min) in equation 6. The ripple calculated in Equation 6 is

then used in the following equation:

(7)

A standard value 15 µH inductor is selected. The maximum

ripple amplitude, which occurs at maximum V

IN, calculates to

362 mA p-p, and the peak current is 781 mA at maximum load

current. Ensure the selected inductor is rated for this peak

current.

C2 and R3: Since the LM34919 requires a minimum of 25

mVp-p ripple at the FB pin for proper operation, the required

ripple at V

OUT is increased by R1 and R2. This necessary rip-

ple is created by the inductor ripple current flowing through

R3, and to a lesser extent by C2 and its ESR. The minimum

inductor ripple current is calculated using equation 7, rear-

ranged to solve for I

OR at minimum VIN.

The minimum value for R3 is equal to:

A standard value 0.39

Ω resistor is used for R3 to allow for

tolerances. C2 should generally be no smaller than 3.3 µF,

although that is dependent on the frequency and the desired

output characteristics. C2 should be a low ESR good quality

ceramic capacitor. Experimentation is usually necessary to

determine the minimum value for C2, as the nature of the load

may require a larger value. A load which creates significant

transients requires a larger value for C2 than a non-varying

load.

C1 and C5: C1’s purpose is to supply most of the switch cur-

rent during the on-time, and limit the voltage ripple at V

IN, on

the assumption that the voltage source feeding V

IN has an

output impedance greater than zero.

At maximum load current, when the buck switch turns on, the

current into V

IN suddenly increases to the lower peak of the

inductor’s ripple current, ramps up to the upper peak, then

drops to zero at turn-off. The average current during the on-

time is the load current. For a worst case calculation, C1 must

supply this average load current during the maximum on-time,

without letting the voltage at V

IN drop below ≊7.5V. The min-

imum value for C1 is calculated from:

where t

ON is the maximum on-time, and ΔV is the allowable

ripple voltage (0.5V at V

IN = 8V). C5’s purpose is to minimize

transients and ringing due to long lead inductance leading to

the VIN pin. A low ESR, 0.1 µF ceramic chip capacitor must

be located close to the VIN and RTN pins.

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

stability for the Vcc regulator. C3 should be no smaller than

0.1 µF, and should be a good quality, low ESR, ceramic ca-

pacitor. C3’s value, and the V

CC current limit, determine a

portion of the turn-on-time (t

1 in Figure 1).

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

ceramic capacitor with low ESR is recommended as C4 sup-

plies a surge current to charge the buck switch gate at each

turn-on. A low ESR also helps ensure a complete recharge

during each off-time.

C6: The capacitor at the SS pin determines the softstart time,

i.e. the time for the output voltage, to reach its final value (t

in Figure 1). The capacitor value is determined from the fol-

lowing:

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 input voltage, the maximum load current, and the

peak current which occurs when the current limit and maxi-

mum ripple current are reached simultaneously. The diode’s

average power dissipation is calculated from:

D1 = VF x IOUT x (1-D)

where V

F is the diode’s forward voltage drop, and D is the on-

time duty cycle.

FINAL CIRCUIT

The final circuit is shown in Figure 5, and its performance is

shown in Figure 6 and Figure 7. Current limit measured ap-

proximately 650 mA at 8V, and 740 mA at 40V.

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