LM34914EVAL National Semiconductor, LM34914EVAL Datasheet - Page 12

• 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: -

www.national.com
If it is expected that the internal dissipation of the LM34914
will produce excessive junction temperatures during normal
operation, good use of the PC board’s ground plane can help
to dissipate heat. The exposed pad on the bottom of the IC
package can be soldered to a ground plane, and that plane
should extend out from beneath the IC, and be connected to
ground plane on the board’s other side with several vias, to
help dissipate the heat. The exposed pad is internally con-
nected to the IC substrate. Additionally the use of wide PC
board traces, where possible, can help conduct heat away
from the IC. Judicious positioning of the PC board within the
end product, along with the use of any available air flow
(forced or natural convection) can help reduce the junction
temperatures.
LOW OUTPUT RIPPLE CONFIGURATIONS
For applications where low output ripple is required, the fol-
lowing options can be used to reduce or nearly eliminate the
ripple.
a) Reduced ripple configuration: In Figure 4, Cff is added
across R1 to AC-couple the ripple at V
pin. This allows the ripple at V
of 25 mVp-p by reducing R3, since the ripple at V
attenuated by the feedback resistors. The minimum value for
Cff is determined from:
where t
(min)
for Cff. R1 and R2 should each be towards the upper end of
the 1kΩ to 10kΩ range.
b) Minimum ripple configuration: If the application requires
a lower value of ripple (<10 mVp-p), the circuit of Figure 5 can
be used. R3 is removed, and the resulting output ripple volt-
age is determined by the inductor’s ripple current and C2’s
characteristics. RA and CA are chosen to generate a saw-
. The next larger standard value capacitor should be used
ON(max)
FIGURE 4. Reduced Ripple Configuration
is the maximum on-time, which occurs at V
OUT
to be reduced to a minimum
OUT
directly to the FB
20197326
OUT
is not
IN
12
tooth waveform at their junction, and that voltage is AC-
coupled to the FB pin via CB. To determine the values for RA,
CA and CB, use the following procedure:
where V
during the off-time (typically 1V). VA is the DC voltage at the
RA/CA junction, and is used in the next equation.
where t
age), and ΔV is the desired ripple amplitude at the RA/CA
junction (typically 40-50 mV). RA and CA are then chosen
from standard value components to satisfy the above product.
Typically CA is 1000 pF to 5000 pF, and RA is 100kΩ to 300
kΩ. CB is then chosen large compared to CA, typically 0.1 µF.
R1 and R2 should each be towards the upper end of the
1kΩ to 10kΩ range.
FIGURE 5. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in
Figure 6 is the same as that in the Block Diagram, except the
output voltage is taken from the junction of R3 and C2. The
ripple at V
and C2’s characteristics. However, R3 slightly degrades the
load regulation. This circuit may be suitable if the load current
is fairly constant.
Calculate V
FIGURE 6. Alternate Minimum Output Ripple
- Calculate RA x CA = (V
ON
SW
OUT
is the maximum on-time (at minimum input volt-
is the absolute value of the voltage at the SW pin
is determined by the inductor’s ripple current
A
= V
OUT
- (V
SW
IN(min)
x (1 - (V
- V
OUT
A
) x t
/V
ON
IN(min)
20197327
/ΔV
20197328
)))