LM34919_14 Datasheet, PDF (12/21 Pages) Texas Instruments – Ultra-Small 40-V 600-mA Constant On-Time Buck Switching Regulator

LM34919
SNOSAY2D – MAY 2007 – REVISED JUNE 2008
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L1: The main parameter affected by the inductor is the inductor current ripple amplitude (I_OR). The minimum load
current 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:
I_OR(MAX) = 2 x I_OUT(min) = 400 mA
(10)
If the minimum load current is zero, use 20% of I_OUT(max) for I_OUT(min) in Equation 10. The ripple calculated in
Equation 10 is then used in the following equation:
L1 = ^{VOUT x (VIN(max) - VOUT)} = 13.6 PH
I_OR(max) x f_SW x V_IN(max)
(11)
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 mVpp ripple at the FB pin for proper operation, the
required ripple at V_OUT is increased by R1 and R2. This necessary ripple 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 11, rearranged to solve for I_OR at minimum V_IN.
V_OUT x (V_IN(min) ± V_OUT)
I_OR(min) =
= 155 mAp-p
L1 x f_SW x V_IN(min)
(12)
The minimum value for R3 is equal to:
25 mV x (R1 + R2)
R3_(min) =
R2 x I_{OR (min)}
= 0.32:
(13)
A standard value 0.39Ω 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 below ≊7.5V. The minimum value for
C1 is calculated from:
I_{OUT (max)} x t_ON
C1 =
= 1 PF
'V
(14)
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 capacitor. C3's value, and the V_CC current
limit, determine a portion of the turn-on-time (t₁ in Figure 3).
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 (t₂ in Figure 3). The capacitor value is determined from the following:
12
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