LM34919EVAL National Semiconductor, LM34919EVAL Datasheet - Page 8

• Manufacturer Part Number: LM34919EVAL
• Description: BOARD EVAL FOR LM34919 SW REG
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM34919EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 600mA
• Voltage - Input: 8 ~ 40V
• Regulator Topology: Buck
• Frequency - Switching: 800kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34919
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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Functional Description
The LM34919 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 LM34919 can be applied in numerous applications to ef-
ficiently 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
8
Control Circuit Overview
The LM34919 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 LM34919 operates in continuous con-
duction mode at heavy load currents and discontinuous con-
duction mode at light load currents. In continuous conduction
mode current always flows through the inductor, never reach-
ing zero during the off-time. In this mode the operating fre-
quency remains relatively constant with load and line
variations. The minimum load current for continuous conduc-
ON
30004434
). Following the