LM34923 Datasheet, PDF (13/26 Pages) National Semiconductor (TI) – EVAL evaluation board provides the design engineer with a fully

LM34923
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SNVS695A – MARCH 2011 – REVISED FEBRUARY 2013
Over-Voltage Comparator
The feedback voltage at FB is compared to an internal 2.85V reference. If the voltage at FB rises above 2.85V
the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load,
change suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5V.
On-Time Generator and Shutdown
The on-time for the LM34923 is determined by the R_T resistor, and is inversely proportional to the input voltage
(Vin), resulting in a nearly constant frequency as Vin is varied over its range. The on-time equation for the
LM34923 is:
T_ON
=
1.25
x
10^-10 x (R_T +
(V_IN - 0.5V)
500:)
+
30
ns
(5)
R_T should be selected for a minimum on-time (at maximum V_IN) greater than 200 ns, for proper current limit
operation. This requirement limits the maximum frequency for each application, depending on V_IN and V_OUT.
The LM34923 can be remotely disabled by taking the RT/SD pin to ground. See Figure 32. The voltage at the
RT/SD pin is between 1.5 and 5.0 volts, depending on Vin and the value of the R_T resistor.
Input
Voltage
VIN
R_T
STOP
RUN
R_T/SD
LM34923
Figure 32. Shutdown Implementation
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_IN (see the graph Current Limit Off-Time vs. V_FB). When
FB = 0V, a maximum off-time is required. This condition occurs when the output is shorted, and during the initial
part of start-up. This amount of time ensures 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 overloads reduces the amount of foldback, recovery time,
and the start-up time. The off-time in microseconds is calculated from the following equation:
(V_IN + 1.83V) x 0.28
T_OFF = _{(VFB x 1.05) + 0.58}
(6)
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 associated floating high voltage gate driver. The gate
driver circuit works in conjunction with an external bootstrap capacitor 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.
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