LM34919BTLX National Semiconductor Corporation, LM34919BTLX Datasheet - Page 11

• Manufacturer Part Number: LM34919BTLX
• Description: Lm34919b Ultra Small 40v, 600 Ma Constant On-time Buck Switching Regulator
• Manufacturer: National Semiconductor Corporation

Functional Description
The LM34919B Step Down Switching Regulator features all
the functions needed to implement a low cost, efficient buck
bias power converter capable of supplying at least 0.6A to the
load. This high voltage regulator contains an N-Channel buck
switch, is easy to implement, and is available in micro SMD
package. The regulator’s operation is based on a constant on-
time control scheme, where the on-time is determined by
V
relatively constant with load and input voltage variations. The
feedback control requires no loop compensation resulting in
very fast load transient response. The valley current limit de-
tection circuit, internally set at 0.64A, holds the buck switch
off until the high current level subsides. This scheme protects
against excessively high current if the output is short-circuited
when V
The LM34919B can be applied in numerous applications to
efficiently regulate down higher voltages. Additional features
include: Thermal shutdown, V
drive under-voltage lockout, and maximum duty cycle limiter.
IN
. This feature allows the operating frequency to remain
IN
is high.
CC
under-voltage lockout, gate
FIGURE 1. Start Up Sequence
11
Control Circuit Overview
The LM34919B buck DC-DC regulator employs a control
scheme based on a comparator and a one-shot on-timer, with
the output voltage feedback (FB) compared to an internal ref-
erence (2.5V). If the FB voltage is below the reference the
buck switch is turned on for a time period determined by the
input voltage and a programming resistor (R
on-time the switch remains off until the FB voltage falls below
the reference but not less than the minimum off-time. The
buck switch then turns on for another on-time period. Typi-
cally, during start-up, or when the load current increases
suddenly, the off-times are at the minimum. Once regulation
is established, the off-times are longer.
When in regulation, the LM34919B operates in continuous
conduction mode at heavy load currents and discontinuous
conduction mode at light load currents. In continuous con-
duction mode current always flows through the inductor, nev-
er reaching zero during the off-time. In this mode the
operating frequency remains relatively constant with load and
line variations. The minimum load current for continuous con-
ON
30102734
). Following the
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