Datasheet

C6 =
t
2
x 10.5 PA
2.5V
= 0.021 PF
C1 =
I
OUT (max)
x t
ON
'V
= 0.5 PF
LM34919B
LM34919B-Q1
www.ti.com SNVS623A –MAY 2010–REVISED FEBRUARY 2013
A standard value 0.27Ω 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 current 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 more than 0.5V. The minimum value
for C1 is calculated from:
(14)
where t
ON
is the maximum on-time, and ΔV is the allowable ripple voltage (0.5V). 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 capacitor. C3’s value, and the V
CC
current
limit, determine a portion of the turn-on-time (t
1
in Figure 25).
C4: The recommended value for C4 is 0.022 µF. A high quality ceramic capacitor with low ESR is recommended
as C4 supplies 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
2
in Figure 25). The capacitor value is determined from the following:
(15)
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 maximum ripple current are reached simultaneously. The diode’s average power
dissipation is calculated from:
P
D1
= V
F
x I
OUT
x (1-D) (16)
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 29, and its performance is shown in Figure 30 and Figure 31. Current limit
measured approximately 780 mA at 6V, and 812 mA at 24V.
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