LM34923 Datasheet, PDF (11/26 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
The LM34923 operates in discontinuous conduction mode at light load currents, and continuous conduction
mode at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero
and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-
time period starts when the voltage at FB falls below the internal reference - until then the inductor current
remains zero. In this mode the operating frequency is lower than in continuous conduction mode, and varies with
load current. Therefore at light loads the conversion efficiency is maintained, since the switching losses reduce
with the reduction in load and frequency. The discontinuous operating frequency can be calculated as follows:
F=
V_OUT² x L x 1.28 x 10²⁰
R_L x (R_T)²
(1)
where R_L = the load resistance
In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero.
In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively
constant with load and line variations. The approximate continuous mode operating frequency can be calculated
as follows:
V_OUT x (V_in - 0.5V)
F = _{1.25 x 10-10 x VIN x (RT + 500:)}
(2)
The buck switch duty cycle is approximately equal to:
DC =
t_ON
t_ON + t_OFF
=
V_OUT
V_IN
(3)
The output voltage (V_OUT) is programmed by two external resistors as shown in the Block Diagram. The
regulation point can be calculated as follows:
V_OUT = 2.5 x (R_FB1 + R_FB2) / R_FB1
(4)
The LM34923 regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor C2. A minimum of 25mV to 50mV of ripple voltage at the feedback pin
(FB) is required for the LM34923. In cases where the capacitor ESR is too small, additional series resistance
may be required (R3 in the Block Diagram).
For applications where lower output voltage ripple is required the output can be taken directly from a low ESR
output capacitor, as shown in Figure 30. However, R3 slightly degrades the load regulation.
L1
SW
LM34923
FB
R_FB2
R_FB1
R3
V_OUT
C2
Figure 30. Low Ripple Output Configuration
Start-Up Regulator (V_CC)
The high voltage bias regulator is integrated within the LM34923. The input pin (VIN) can be connected directly
to line voltages between 6V and 75V, with transient capability to 80V. The V_CC output is regulated at 7.5V. The
V_CC regulator output current is limited at approximately 30 mA.
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