LM34919B_1 Datasheet, PDF (1/12 Pages) National Semiconductor (TI) – Evaluation Board provides the design engineer with a fully functional

LM34919B Evaluation Board
National Semiconductor
Application Note 2016
Dennis Morgan
May 27, 2010
Introduction
The LM34919BEVAL 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 3.3V output over an input range of 6V to 24V.
The circuit delivers load currents to 600 mA, with current limit
set at a nominal 800 mA. The board is populated with all
components except R5, C9 and C10. These components pro-
vide options for managing the output ripple as described later
in this document.
The board’s specification are:
• Input Voltage: 6V to 24V
• Output Voltage: 3.3V
• Maximum load current: 600 mA
• Minimum load current: 0A
• Current Limit: 780 mA to 815 mA
• Measured Efficiency: 88.7% (V_IN = 6V, I_OUT = 300 mA)
• Nominal Switching Frequency: 1.5 MHz
• Size: 2.6 in. x 1.6 in. x 0.5 in
FIGURE 1. Evaluation Board - Top Side
30110020
Theory of Operation
Refer to the evaluation board schematic in Figure 1, which
contains a simplified block diagram of the LM34919B. 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 ≊424 ns at
VIN =6V, to ≊129 ns at VIN = 24V. 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 88 ns. In normal
operation, the off-time is much longer. During the off-time, the
load current is supplied by the output capacitor (C7, C8).
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 min-
imum of 25 mV of ripple is required at FB to switch the
regulation comparator. The current limit threshold is ≊780 mA
at Vin = 6V, and ≊812 mA at Vin = 24V. The variation is due
to the change in ripple current amplitude as Vin varies. Refer
to the LM34919B data sheet for a more detailed block dia-
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