LM34923MM/NOPB National Semiconductor, LM34923MM/NOPB Datasheet - Page 13

• 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

Current Limit
The LM34923 contains an intelligent current limit OFF timer.
If the current in the Buck switch reaches the current limit
threshold, the present cycle is immediately terminated, and a
non-resetable OFF timer is triggered. The length of off-time is
controlled by the FB voltage and V
Limit Off-Time vs. V
is required. This condition occurs when the output is shorted,
and during the initial part of start-up. This amount of time en-
sures safe short circuit operation up to the maximum input
voltage of 75V. In cases of overload where the FB voltage is
above zero volts (not a short circuit) the required current limit
off-time is less. Reducing the off-time during less severe over-
loads reduces the amount of foldback, recovery time, and the
start-up time. The off-time in microseconds is calculated from
the following equation:
The current limit sensing circuit is blanked for the first 50-70
ns of each on-time so it is not falsely tripped by the current
surge which occurs at turn-on. The current surge is required
by the re-circulating diode (D1) for its turn-off recovery.
N - Channel Buck Switch and Driver
The LM34923 integrates an N-Channel Buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capac-
itor and an internal high voltage diode. A 0.01 µF ceramic
capacitor (C4) connected between the BST pin and SW pin
provides the voltage to the driver during the on-time.
During each off-time, the SW pin is at approximately 0V, and
the bootstrap capacitor charges from Vcc through the internal
diode. The minimum OFF timer, set to 260 ns, ensures a min-
imum time each cycle to recharge the bootstrap capacitor.
The internal pre-charge switch at the SW pin is turned on for
≊
cient voltage exists across the bootstrap capacitor for the on-
time. This feature helps prevent operating problems which
can occur during very light load conditions, involving a long
off-time, during which the voltage across the bootstrap ca-
pacitor could otherwise reduce below the Gate Drive UVLO
threshold. The pre-charge switch also helps prevent startup
problems which can occur if the output voltage is pre-charged
prior to turn-on. After current limit detection, the pre-charge
switch is turned on for the entire duration of the forced off-
time .
150 ns during the minimum off-time period, ensuring suffi-
FIGURE 3. Shutdown Implementation
FB
). When FB = 0V, a maximum off-time
IN
(see the graph Current
30141715
13
Under Voltage Detector
The Under Voltage Detector can be used to monitor the input
voltage, or any other system voltage as long as the voltage at
the UV pin does not exceed its maximum rating.
The Under Voltage Output indicator pin (UVO) is connected
to the drain of an internal N-channel MOSFET capable of
sustaining 10V in the off-state. An external pull-up resistor is
required at UVO to an appropriate voltage to indicate the sta-
tus to downstream circuitry. The off-state voltage at the UVO
pin can be higher or lower than the voltage at VIN, but must
not exceed 10V.
The UVO pin switches low when the voltage at the UV input
pin is above its threshold. Typically the monitored voltage
threshold is set with a resistor divider (R
in the Block Diagram. When the voltage at the UV pin is below
its threshold, the internal 5 µA current source at UV is en-
abled. As the input voltage increases, taking UV above its
threshold, the current source is disabled, raising the voltage
at UV to provide threshold hysteresis.
The UVO output is high when the VCC voltage is below its
UVLO threshold, or when the LM34923 is shutdown using the
RT/SD pin (see Figure 3), regardless of the voltage at the UV
pin.
Thermal Protection
The LM34923 should be operated so the junction temperature
does not exceed 125°C during normal operation. An internal
Thermal Shutdown circuit is provided to shutdown the
LM34923 in the event of a higher than normal junction tem-
perature. When activated, typically at 165°C, the controller is
forced into a low power reset state by disabling the buck
switch. This feature prevents catastrophic failures from acci-
dental device overheating. When the junction temperature
reduces below 145°C (typical hysteresis = 20°C) normal op-
eration is resumed.
Applications Information
SELECTION OF EXTERNAL COMPONENTS
A guide for determining the component values is illustrated
with a design example. Refer to the Block Diagram. The fol-
lowing steps will configure the LM34923 for:
•
•
•
•
R
V
dard values of 3.01 kΩ and 1.00 kΩ are chosen. Other values
could be used as long as the 3:1 ratio is maintained.
F
cy, the choice of frequency is generally a compromise. A
higher frequency allows for a smaller inductor, input capaci-
tor, and output capacitor (both in value and physical size),
while providing a lower conversion efficiency. A lower fre-
quency provides higher efficiency, but generally requires
higher values for the inductor, input capacitor and output ca-
pacitor. The maximum allowed switching frequency for the
LM34923 is limited by the minimum on-time (200 ns) at the
maximum input voltage, and by the minimum off-time (260 ns)
at the minimum input voltage. The maximum frequency limit
for each application is defined by the following two calcula-
tions:
s
FB
FB1
and R
Input voltage range (Vin): 15V to 75V
Output voltage (V
Load current (for continuous conduction mode): 100 mA
to 400 mA
Switching Frequency: 300 kHz
= 2.5V, the ratio of R
, R
FB2
T
: Unless the application requires a specific frequen-
: V
OUT
= V
OUT
FB
): 10V
FB2
x (R
to R
FB1
FB1
+ R
calculates as 3:1. Stan-
FB2
UV1
) / R
, R
FB1
UV2
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, and since
) as shown