LM34923MM/NOPB National Semiconductor, LM34923MM/NOPB Datasheet - Page 14

• 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

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The maximum allowed frequency is the lesser of the two
above calculations. See the graph “Maximum Switching Fre-
quency”. For this exercise, F
F
lowed frequency for this example is 667 kHz, which is greater
than the 300 kHz specified for this design. Using Equation 1,
RT calculates to 258 kΩ. A standard value 261 kΩ resistor is
used. The minimum on-time calculates to 469 ns, and the
maximum on-time calculates to 2.28 µs.
L1: The main parameter affected by the inductor is the output
current ripple amplitude. The choice of inductor value there-
fore depends on both the minimum and maximum load cur-
rents, keeping in mind that the maximum ripple current occurs
at maximum Vin.
a) Minimum load current: To maintain continuous conduc-
tion at minimum Io (100 mA) if a flyback diode is used, the
ripple amplitude (I
lower peak of the waveform does not reach zero. L1 is cal-
culated using the following equation:
At Vin = 75V, L1(min) calculates to 146µH. The next larger
standard value (150 µH) is chosen and with this value I
calculates to 195 mA p-p at Vin = 75V, and 75 mA p-p at Vin
= 15V.
b) Maximum load current: At a load current of 400 mA, the
peak of the ripple waveform must not reach the minimum
guaranteed value of the LM34923’s current limit threshold
(700 mA). Therefore the ripple amplitude must be less than
600 mA p-p, which is already satisfied in the above calcula-
tion. With L1 = 150 µH, at maximum Vin and Io, the peak of
the ripple is 498 mA. While L1 must carry this peak current
without saturating or exceeding its temperature rating, it also
must be capable of carrying the maximum guaranteed value
of the LM34923’s current limit threshold without saturating,
since the current limit is reached during startup.
The DC resistance of the inductor should be as low as pos-
sible. For example, if the inductor’s DCR is 0.5 ohm, the power
dissipated at maximum load current is 0.08W. While small, it
is not insignificant compared to the load power of 4W.
C3: The capacitor on the V
filtering and stability, but its primary purpose is to prevent false
triggering of the V
tions. C3 should be no smaller than 1 µF.
C2 and R3: When selecting the output filter capacitor C2, the
items to consider are ripple voltage due to its ESR, ripple
voltage due to its capacitance, and the nature of the load.
A low ESR for C2 is generally desirable so as to minimize
power losses and heating within the capacitor. However, the
regulator requires a minimum amount of ripple voltage at the
feedback input for proper loop operation. For the LM34923
the minimum ripple required at pin 7 is 25 mV p-p, requiring
a minimum ripple at V
s(max)2
calculates to 1.28 MHz. Therefore the maximum al-
OR
CC
) must be less than 200 mA p-p so the
UVLO at the buck switch on/off transi-
OUT
of 100 mV for this example. Since
CC
s(max)1
output provides not only noise
calculates to 667 kHz, and
OR
14
the minimum ripple current (at minimum Vin) is 75 mA p-p,
the minimum ESR required at V
1.33Ω. Since quality capacitors for SMPS applications have
an ESR considerably less than this, R3 is inserted as shown
in the Block Diagram. R3’s value, along with C2’s ESR, must
result in at least 25 mV p-p ripple at pin 7. See the Low Output
Ripple Configuration section for techniques to reduce the out-
put ripple voltage.
D1: A power Schottky diode is recommended. Ultra-fast re-
covery diodes are not recommended as the high speed tran-
sitions at the SW pin may inadvertently affect the IC’s
operation through external or internal EMI. The important pa-
rameters are reverse recovery time and forward voltage. The
reverse recovery time determines how long the reverse cur-
rent surge lasts with each turn-on of the internal buck switch.
The forward voltage drop affects efficiency. The diode’s re-
verse voltage rating must be at least as great as the maximum
input voltage, plus ripple and transients, and its current rating
must be at least as great as the maximum current limit spec-
ification. The diode’s average power dissipation is calculated
from:
Where V
time duty cycle.
C1: This capacitor’s purpose is to supply most of the switch
current during the on-time, and limit the voltage ripple at Vin,
on the assumption that the voltage source feeding Vin has an
output impedance greater than zero. At maximum load cur-
rent, when the buck switch turns on, the current into the VIN
pin suddenly increases to the lower peak of the output current
waveform, ramp up to the peak value, then drop to zero at
turn-off. The average input current during this on-time is the
load current (400 mA). For a worst case calculation, C1 must
supply this average load current during the maximum on-time.
To keep the input voltage ripple to less than 1V (for this ex-
ercise), C1 calculates to:
Quality ceramic capacitors in this value have a low ESR which
adds only a few millivolts to the ripple. It is the capacitance
which is dominant in this case. To allow for the capacitor’s
tolerance, temperature effects, and voltage effects, a 1.0 µF,
100V, X7R capacitor is used.
C4: The recommended value is 0.01µF for C4, as this is ap-
propriate in the majority of applications. A high quality ceramic
capacitor, with low ESR is recommended as C4 supplies the
surge current to charge the buck switch gate at turn-on. A low
ESR also ensures a quick recharge during each off-time.
C5: This capacitor helps avoid supply voltage transients and
ringing due to long lead inductance at V
ceramic chip capacitor is recommended, located close to the
LM34923.
UV and UVO pins: The Under Voltage Detector function is
used to monitor a system voltage, such as the input voltage
at VIN, by connecting the UV pin to two resistors (R
R
the UV pin increases above its threshold the UVO pin switch-
es low. The UVO pin is high when the voltage at the UV input
pin is below its threshold. Hysteresis is provided by the inter-
nal 5µA current source which is enabled when the voltage at
the UV pin is below its threshold. The resistor values are cal-
culated using the following procedure:
UV2
) as shown in the Block Diagram. When the voltage at
F
is the diode’s forward voltage drop, and D is the on-
P
D1
= V
F
x I
OUT
x (1–D)
OUT
is 100 mV/75 mA =
IN
. A low ESR, 0.1µF
UV1
(4)
,