LM34919C-Q1 Datasheet, PDF (18/29 Pages) Texas Instruments – LM34919C-Q1 Ultra Small 50V, 600 mA Constant On-Time Buck Switching Regulator

LM34919C-Q1
SNVS831A – SEPTEMBER 2013 – REVISED DECEMBER 2013
www.ti.com
A standard value 0.47 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 current during the on-time and limit the voltage ripple at
V_IN.
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 0.5 V. The minimum value
for C1 is calculated from:
I_{OUT (max)} x t_ON
C1 =
V
= 0.5 2F
(13)
where t_ON is the maximum on-time, and ΔV is the allowable ripple voltage. Input ripple of 0.5 V is acceptable in
typical applications. 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 (t₁ in (Figure 6).
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 soft-start time, i.e. the time for the output voltage to reach its final
value (t₂ in Figure 6). The capacitor value is determined from the following:
C6
=
t₂
x 10.5
2.5V
2A
=
0.021
2F
(14)
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 device'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 in current limiting. The diode’s average power dissipation is calculated from:
P_D1 = V_F x I_OUT x (1-D)
(15)
where V_F is the diode forward voltage drop, and D is the duty cycle at the SW pin.
Final Circuit
The final circuit is shown in Figure 9, and its performance is shown in Figure 10 and Figure 11.
4.5V - 24V
Input
C1
2.2 µF
C5
0.1 µF
R4
VIN
100 k
VCC
VOUT
RON
61.9 k
R5 10 k
C6
0.022 µF
EN
BST
LM34919C
RON
SW
PGD
SS
RTN
ISEN
FB
SGND
C3
0.1 µF
C4
L1
0.022 µF ^{8.2 µH}
D1
R1
787
R2
2.49 k
VOUT
3.3V
R3
0.47
C2
22 µF
Figure 9. Example Circuit
18
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