LM34917A_14 Datasheet, PDF (11/21 Pages) Texas Instruments – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

LM34917A
www.ti.com
SNOSAX7C – DECEMBER 2007 – REVISED MAY 2008
- Minimum load current = 200 mA
- Maximum load current = 1000 mA
- Switching Frequency = 1.5 MHz
- Soft-start time = 5 ms
- Output voltage ripple level: Minimum
R1 and R2: These resistors set the output voltage. The ratio of the feedback resistors is calculated from:
R1/R2 = (V_OUT/2.5V) - 1
(7)
For this example, R1/R2 = 1. 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 R1 and R2.
R_ON: This resistor sets the on-time, and (by default) the switching frequency. Since the maximum frequency is
limited by the minimum off-time forced by the LM34917A, first check that the desired frequency is less than:
V_IN - V_OUT
f_SW < _{VIN x 105 ns} = 3.57 MHz at VIN = 8V
(8)
The R_ON resistor is calculated from equation 5 using the minimum input voltage:
V_OUT x (V_IN(min) - 1.35V)
R_ON =
- 1.4 k: = 22.49 k:
V_IN(min) x 1.16 x 10^-10 x f_SW
(9)
Equation 4 is used to verify that this value resistor does not set an on-time less than 120 ns at maximum input
voltage. A standard value 22.1 kΩ resistor is used, resulting in a nominal frequency of 1.49 MHz. The minimum
on-time is 188 ns at Vin = 33V, and the maximum on-time is 510 ns at Vin = 8V.
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 LM34917A, but serves as a
guideline for selecting L1. For this example, the maximum ripple current should be less than:
I_OR(MAX) = 2 x I_OUT(min) = 400 mAp-p
(10)
For other applications, if the minimum load current is zero, use 20% of I_OUT(max) for I_OUT(min) in equation 6. The
ripple amplitude calculated in Equation 6 is then used in the following equation:
t_on(min) x (V_IN(max) ± V_OUT)
L1_(min) =
I_OR(max)
= 13.2 PH
(11)
A standard value 15 µH inductor is selected. The maximum ripple amplitude, which occurs at maximum V_IN,
calculates to 351 mA p-p, and the peak current is 1175 mA at maximum load current. Ensure the selected
inductor is rated for this peak current.
C2: C2 should typically 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, since it is assumed the voltage source feeding VIN has some amount of source impedance.
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 =
= 1.02 PF
'V
(12)
where t_ON is the maximum on-time, and ΔV is the allowable ripple voltage at V_IN (0.5V at V_IN = 8V). C5’s purpose
is to minimize transients and ringing due to long lead inductance leading the VIN pin. A low ESR 0.1 µF ceramic
chip capacitor must be located close to the VIN and RTN pins.
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