LM34919EVAL National Semiconductor, LM34919EVAL Datasheet - Page 12

• Manufacturer Part Number: LM34919EVAL
• Description: BOARD EVAL FOR LM34919 SW REG
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM34919EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 600mA
• Voltage - Input: 8 ~ 40V
• Regulator Topology: Buck
• Frequency - Switching: 800kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34919
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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L1: The main parameter affected by the inductor is the in-
ductor current ripple amplitude (I
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 LM34919, but serves as a guideline for selecting L1. For
this case the maximum ripple current is:
If the minimum load current is zero, use 20% of I
I
then used in the following equation:
A standard value 15 µH inductor is selected. The maximum
ripple amplitude, which occurs at maximum V
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
mVp-p ripple at the FB pin for proper operation, the required
ripple at V
ple 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 7, rear-
ranged to solve for I
The minimum value for R3 is equal to:
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 cur-
rent during the on-time, and limit the voltage ripple at V
the assumption that the voltage source feeding V
output impedance greater than zero.
At maximum load current, when the buck switch turns on, the
current into V
OUT(min)
in equation 6. The ripple calculated in Equation 6 is
OUT
I
OR(MAX)
IN
is increased by R1 and R2. This necessary rip-
suddenly increases to the lower peak of the
= 2 x I
OR
at minimum V
OUT(min)
OR
= 400 mA
). The minimum load cur-
IN
.
IN
, calculates to
OUT(max)
IN
has an
IN
, on
(6)
(7)
for
12
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
imum value for C1 is calculated from:
where t
ripple voltage (0.5V at V
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 ca-
pacitor. C3’s value, and the V
portion of the turn-on-time (t
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 softstart time,
i.e. the time for the output voltage, to reach its final value (t
in Figure 1). The capacitor value is determined from the fol-
lowing:
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 IC’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 when the current limit and maxi-
mum ripple current are reached simultaneously. The diode’s
average power dissipation is calculated from:
where V
time duty cycle.
FINAL CIRCUIT
The final circuit is shown in Figure 5, and its performance is
shown in Figure 6 and Figure 7. Current limit measured ap-
proximately 650 mA at 8V, and 740 mA at 40V.
ON
F
is the diode’s forward voltage drop, and D is the on-
is the maximum on-time, and ΔV is the allowable
P
D1
= V
IN
= 8V). C5’s purpose is to minimize
F
1
x I
in Figure 1).
IN
OUT
CC
drop below
current limit, determine a
x (1-D)
≊
7.5V. The min-
2