LM34923MM/NOPB National Semiconductor, LM34923MM/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM34923MM/NOPB
• Description: "75V, 650mA Non-synch Buck "
• Manufacturer: National Semiconductor
• Series: -r
• Type: Step-Down (Buck)r

Specifications of LM34923MM/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 2.5 V ~ 75 V
• Current - Output: 600mA
• Frequency - Switching: 50kHz ~ 600kHz
• Voltage - Input: 6 V ~ 75 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-TFSOP, 10-MSOP (0.118", 3.00mm Width)
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

b) Minimum ripple configuration: If the application requires
a lower value of ripple (<10 mVp-p), the circuit of
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-
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. If a Schottky diode is used for the flyback
function, the off-time voltage is in the range of 0.5V to 1V,
depending on the specific diode used, and the maximum load
current. V
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 10 kΩ to 300
kΩ. CB is then chosen large compared to CA, typically 0.1 µF.
FIGURE 8. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in
Figure 9
output voltage is taken from the junction of R3 and C2. The
ripple at V
Calculate V
- Calculate RA x CA = (V
ON
FIGURE 7. Reduced Ripple Configuration
SW
is the same as that in the Block Diagram, except the
A
OUT
is the maximum on-time (at minimum input volt-
is the absolute value of the voltage at the SW pin
is the DC voltage at the RA/CA junction, and is
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)
Figure 8
/ΔV
)))
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can
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and C2’s characteristics. However, R3 slightly degrades the
load regulation. This circuit may be suitable if the load current
is fairly constant.
PC Board Layout
The LM34923 regulation, over-voltage, and current limit com-
parators are very fast, and respond to short duration noise
pulses. Layout considerations are therefore critical for opti-
mum performance. The layout must be as neat and compact
as possible, and all of the components must be as close as
possible to the associated pins. The two major current loops
have currents which switch very fast, and so the loops should
be as small as possible to minimize conducted and radiated
EMI. The first loop is formed by C1, through VIN to the SW
pin, L1, C2, and back to C1. The second loop is formed by L1,
C2, D1, and back to L1. Since a current equal to the load
current switches between these two loops with each transition
from on-time to off-time and back to on-time, it is imperative
that the ground end of C1 have a short and direct connection
to D1’s anode, without going through vias or a lengthy route.
The power dissipation in the LM34923 can be approximated
by determining the total conversion loss (P
subtracting the power losses in D1, and in the inductor. The
power loss in the diode is approximately:
where V
time duty cycle.
where R
is an approximation for the AC losses. If it is expected that the
internal dissipation of the LM34923 will produce excessive
junction temperatures during normal operation, good use of
the PC board’s ground plane can help to dissipate heat. Ad-
ditionally 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 temperature.
FIGURE 9. Alternate Minimum Output Ripple
F
L
is the diode’s forward voltage drop, and D is the on-
is the inductor’s DC resistance, and the 1.1 factor
P
P
D1
L1
= I
= I
OUT
OUT
x V
2
x R
F
x (1–D)
L
x 1.1
IN
– P
OUT
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), and then
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