LM34923 Datasheet, PDF (1/12 Pages) National Semiconductor (TI) – EVAL evaluation board provides the design engineer with a fully

LM34923 Evaluation Board
National Semiconductor
Application Note 2147
Vijay Choudhary
May 16, 2011
Introduction
The LM34923 EVAL evaluation board provides the design
engineer with a fully functional buck regulator, employing the
constant on-time (COT) operating principle. This evaluation
board provides a 5V output over an input range of 6V to 75V.
The circuit delivers load currents to 500 mA, with current limit
set at a nominal 1 Amp.
The board’s specification are:
• Input Voltage: 6V to 75V
• Output Voltage: 5V
• Maximum load current: 500 mA
• Minimum load current: 0A
• Current Limit: 1 Amp (nominal)
• Measured Efficiency: 94.75% (V_IN = 6V, I_OUT = 100 mA)
• Nominal Switching Frequency: 200 kHz
• Size: 2.6 in. x 1.6 in.
FIGURE 1. Evaluation Board - Top Side
30161530
Theory of Operation
Refer to the evaluation board schematic in Figure 6. When
the circuit is in regulation, the buck switch is on each cycle for
a time determined by R1 and VIN according to the equation:
The on-time of this evaluation board ranges from ≊4.38 µs at
VIN = 6V, to ≊351 ns at VIN = 75V. The on-time varies in-
versely with VIN to maintain a nearly constant switching fre-
quency. At the end of each on-time the Minimum Off-Timer
ensures the buck switch is off for at least 260 ns. In normal
operation, the off-time is much longer. During the off-time, the
load current is supplied by the output capacitor (C2). When
the output voltage falls sufficiently that the voltage at FB is
below 2.5V, the regulation comparator initiates a new on-time
period. For stable, fixed frequency operation, a minimum of
25 mV of ripple is required at FB to switch the regulation com-
parator. Refer to the LM34923 data sheet for a more detailed
block diagram, and a complete description of the various
functional blocks.
© 2011 National Semiconductor Corporation 301615
www.national.com