Datasheet

t
ON
=
V
IN
- 1.5V
0.565 x 10
-10
x (R1 + 1.4 k:)
+ 55 ns
National
Semiconductor
J 2 OUT
J 3 GND
D 1
J1
G N D I N
J4
L 1
C 1
C 2 R 1
U 1
R 3 C 6
C 10
R 5
C 5
R 4
C 7
C 8
R 6
MADE IN U.S.
P/N 551600404-001
Rev A (c)2009 NSC
LM34919B EVALUATION BOARD
980600404
J5
Note: R2, C3, C4, and C9 are located on board's back side.
Theory of Operation www.ti.com
Figure 1. Evaluation Board - Top Side
2 Theory of Operation
Figure 1 shows the evaluation board schematic, 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 V
IN
according to Equation 1:
(1)
The on-time of this evaluation board ranges from ≊424 ns at V
IN
=6V, to ≊129 ns at V
IN
= 24V. The on-time
varies inversely with V
IN
to maintain a nearly constant switching frequency. 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 minimum of 25 mV of ripple is required at FB
to switch the regulation comparator. The current limit threshold is ≊780 mA at V
IN
= 6V, and ≊812 mA at
V
IN
= 24V. The variation is due to the change in ripple current amplitude as V
IN
varies. for a more detailed
block diagram, and a complete description of the various functional blocks, see the LM34919B,
LM34919B-Q1 Ultra-Small 40-V 600-mA Constant On-Time Buck Switching Regulator Data Sheet
(SNVS623).
2
AN-2016 LM34919B Evaluation Board SNVA418A–May 2010–Revised April 2013
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