LM34919C-Q1 Datasheet, PDF (17/29 Pages) Texas Instruments – LM34919C-Q1 Ultra Small 50V, 600 mA Constant On-Time Buck Switching Regulator

LM34919C-Q1
www.ti.com
SNVS831A – SEPTEMBER 2013 – REVISED DECEMBER 2013
APPLICATION INFORMATION
External Components
The procedure for calculating the external components is illustrated with the following design example. Referring
to Figure 5, the circuit is to be configured for the following specifications:
- V_OUT = 3.3 V
- V_IN = 4.5 V to 24 V
- Minimum load current = 200 mA
- Maximum load current = 600 mA
- Switching Frequency = 1.5 MHz
- Soft-start time = 5 ms
R1 and R2: These resistors set the output voltage. The ratio of the feedback resistors is calculated from:
R1/R2 = (V_OUT/2.52 V) - 1
(7)
For this example, R1/R2 = 0.32. R1 and R2 should be chosen from standard value resistors in the range of 1.0
kΩ - 10 kΩ which satisfy the above ratio. For this example, 2.49 kΩ is chosen for R2 and 787 Ω for R1.
R_ON: This resistor sets the on-time and the switching frequency. The switching frequency must be less than 1.53
MHz to ensure the minimum forced on-time does not cause cycle skipping when operating at the maximum input
voltage. The R_ON resistor is calculated from Equation 8:
R_ON
V_OUT
F_SW x 35.5 x10^12
61.9k:
(8)
Check that this value resistor does not set an on-time less than 90 ns at maximum V_IN.
A standard value 61.9 kΩ resistor is used, resulting in a nominal frequency of 1.50 MHz. The minimum on-time is
calculated ≊92 ns at Vin = 24 V, and the maximum on-time is ≊488 ns at Vin = 4.5 V. Alternately, R_ON can be
determined using Equation 4 if a specific on-time is required.
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 LM34919C, 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
(9)
If the minimum load current is zero, use 20% of I_OUT(max) for I_OUT(min) in Equation 9. The ripple calculated in
Equation 9 is then used in Equation 10:
L1 = ^{( V IN (m ax) - V OUT) x t on (min )} = 4.76 µH
IOR (MAX )
(10)
A standard value 8.2 µH inductor is selected. The maximum ripple amplitude, which occurs at maximum V_IN,
calculates to 232 mA p-p, and the peak current is 716 mA at maximum load current. Ensure the selected inductor
is rated for this peak current.
C2 and R3: Since the LM34919C 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 ripple is created by the inductor ripple current
flowing through R3, and to a lesser extent by the ESR of C2. The minimum inductor ripple current is calculated
using Equation 6, rearranged to solve for I_OR at minimum V_IN.
I_{OR (MIN )}
=
kV_{IN:min ;}
F
V_OUT
L1
o
x
t_{on (max}
)
=
71.4
mA
(11)
The minimum value for R3 is equal to:
R3_{(min ?)}
=
25mV
R2 x
x (R1+ R2)
I_{OR (MI N ?)}
=
0.47 !
(12)
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