Datasheet

FB
VIN
VCC
SW
RTN
BST
2.5V
Vth
V
OUT
Input
RT/SD
CURRENT
LIMIT
+
-
VCC UVLO
ON/OFF
TIMERS
DRIVER
LOGIC
0.9V
2.85
V
OVER-VOLTAGE
START-UP
REGULATOR
FEEDBACK
THERMAL
SHUTDOWN
Current
Limit
One-Shot
UVO
UV
5 PA
PRE
-
CHARGE
R
T
R
UV2
C1
C3
R
UV1
R
FB2
R
FB1
L1
C4
D1
C2
R3
LM34923
Delay
C5
V
IN
2.5V
LM34923
SNVS695A –MARCH 2011–REVISED FEBRUARY 2013 www.ti.com
BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
The LM34923 Step Down Switching Regulator features all the functions needed to implement a low cost,
efficient, Buck bias power converter. This high voltage regulator contains an 80 V N-Channel Buck Switch, is
easy to implement and is provided in the VSSOP-10 package. The regulator is based on a control scheme using
an on-time inversely proportional to V
IN
. The control scheme requires no loop compensation. Current limit is
implemented with forced off-time, which is inversely proportional to V
OUT
. This scheme ensures short circuit
control while providing minimum foldback.
The LM34923 can be applied in numerous applications to efficiently regulate down higher voltages. This
regulator is well suited for 48 Volt Telecom and the new 42V Automotive power bus ranges. Features include:
Thermal Shutdown, V
CC
under-voltage lockout, Gate drive under-voltage lockout, Max Duty Cycle limit timer,
intelligent current limit off timer, a pre-charge switch, and a programmable under voltage detector with status flag.
Control Circuit Overview
The LM34923 is a Buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with
line voltage (V
IN
). Control is based on a comparator and the on-time one-shot, with the output voltage feedback
(FB) compared to an internal reference (2.5V). If the FB level is below the reference the buck switch is turned on
for a fixed time determined by the line voltage and a programming resistor (R
T
). Following the ON period the
switch remains off for at least the minimum off-timer period of 260 ns. If FB is still below the reference at that
time the switch turns on again for another on-time period. This continues until regulation is achieved.
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