LM34914EVAL National Semiconductor, LM34914EVAL Datasheet - Page 2

• Manufacturer Part Number: LM34914EVAL
• Description: BOARD EVALUATION LM34914
• Manufacturer: National Semiconductor

Specifications of LM34914EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 1A
• Voltage - Input: 8 ~ 40V
• Regulator Topology: Buck
• Frequency - Switching: 275kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34914
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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Board Connection/Start-up
The input connections are made to the J1 connector. The load
is connected to the J2 (OUT) and J3 (GND) terminals. Ensure
the wires are adequately sized for the intended load current.
Before start-up a voltmeter should be connected to the input
terminals, and to the output terminals. The load current should
be monitored with an ammeter or a current probe. It is rec-
ommended that the input voltage be increased gradually to
8V, at which time the output voltage should be 5V. If the output
voltage is correct with 8V at VIN, then increase the input volt-
age as desired and proceed with evaluating the circuit. DO
NOT EXCEED 40V AT VIN.
Output Ripple Control
The LM34914 requires a minimum of 25 mVp-p ripple at the
FB pin, in phase with the switching waveform at the SW pin,
for proper operation. The required ripple can be supplied from
ripple at V
in options B and C below, or the ripple can be generated sep-
arately (using R6, C9, C10) keeping the ripple at V
minimum as described in option A below.
A) Minimum Output Ripple: This evaluation board is sup-
plied configured for minimum ripple at V
nents R6, C9 and C10. The output ripple, which ranges from
≊
primarily by the ESR of output capacitor (C7), and the
inductor’s ripple current, which ranges from 85 mAp-p to 190
mAp-p over the input voltage range. The ripple voltage re-
quired by the FB pin is generated by R6, C9 and C10 since
the SW pin switches from -1V to V
is a virtual ground. The values for R6 and C9 are chosen to
B) Intermediate Ripple Level Configuration: This configu-
ration generates more ripple at V
uration, but uses two less capacitors. If some ripple can be
3mVp-p at V
OUT
, through the feedback resistors, as described
IN
= 8V to
≊
8 mVp-p at V
FIGURE 2. Minimum Output Ripple Configuration Using R6,C9,C10
OUT
IN
, and the right end of C9
than the above config-
IN
OUT
= 40V, is determined
by using compo-
OUT
to a
2
generate a 30-40 mVp-p triangle waveform at their junction.
That triangle wave is then coupled to the FB pin through C10.
The following procedure is used to calculate values for R6,
C9 and C10:
1) Calculate the voltage V
where V
during the off-time (typically 1V), and V
voltage. For this circuit V
proximate DC voltage at the R6/C9 junction, and is used in
the next equation.
2) Calculate the R6 x C9 product:
where t
imum input voltage, and ΔV is the desired ripple amplitude at
the R6/C9 junction, 30 mVp-p for this example.
R6 and C9 are then chosen from standard value components
to satisfy the above product. For example, C9 can be 3300
pF requiring R6 to be
large compared to C9. The circuit as supplied on this EVB is
shown in Figure 2.
tolerated in the application, this configuration is slightly more
economical, and simpler. R3, C2 and C12 are used instead
of R6 and C7-C10, as shown in Figure 3.
ON
SW
is the maximum on-time (
is the absolute value of the voltage at the SW pin
V
A
= V
OUT
≤
91.8 kΩ. C10 is chosen to be 0.01 µF,
- (V
A
A
calculates to 4.63V. This is the ap-
:
SW
x (1 - (V
≊
2700 ns), V
IN
OUT
is the minimum input
/V
IN
)))
IN
30000305
is the min-