LM34917A Datasheet, PDF (11/16 Pages) National Semiconductor (TI) – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

helps prevent catastrophic failures from accidental device
overheating. When the junction temperature reduces below
155°C (typical hysteresis = 20°C), normal operation resumes.
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is il-
lustrated with the following design example. Referring to the
Block Diagram, the circuit is to be configured for the following
specifications:
- V_OUT = 5V
- V_IN = 8V to 33V
- 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
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:
The R_ON resistor is calculated from equation 5 using the min-
imum input voltage:
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 in-
ductor current ripple amplitude (I_OR). The minimum load cur-
rent 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
(6)
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 am-
plitude calculated in Equation 6 is then used in the following
equation:
(7)
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 re-
quire a larger value. A load which creates significant tran-
sients requires a larger value for C2 than a non-varying load.
C1 and C5: C1’s purpose is to supply most of the switch cur-
rent 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 min-
imum value for C1 is calculated from:
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.
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 ca-
pacitor. C3’s value, and the VCC current limit, determine a
portion of the turn-on-time (t1 in Figure 1).
C4: The recommended value for C4 is 0.022 µF. A high quality
ceramic capacitor with low ESR is recommended as C4 sup-
plies 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 1). The capacitor value is determined from:
R3, C7, C8: The ripple amplitude at V_OUT is determined by
C2’s characteristics and the inductor’s ripple current ampli-
tude, and typically ranges from 5 mV to 30 mV over the Vin
range. Since the LM34917A’s regulation comparator requires
a minimum of 25 mVp-p ripple at the FB pin, these three com-
ponents are added to generate and provide the necessary
ripple to FB in phase with the waveform at SW. R3 and C8
are chosen to generate a sawtooth waveform at their junction,
and that voltage is AC coupled to the FB pin via C7. To de-
termine the values for R3, C7 and C8, the following procedure
is used:
Calculate V_A = V_OUT – (V_SW x (1 – (V_OUT/V_IN(min)))
11
www.national.com