Datasheet
(V
IN
± V
OUT
) x R
ON
2
V
OUT
(V
IN
- 1) x L x 1.18 x 10
20
x I
OUT
F
SW
=
EN
SS
BST
SW
6V LDO
ON TIMER
Ron
START
COMPLETE
START
COMPLETE
UVLO
THERMAL
SHUTDOWN
Gate Drive
UVLO
LEVEL
SHIFT
L
C
BST
LM 3100
AVDD
SD
DRIVER
REGULATION
COMPARATOR
VDD
LOGIC
DrvH
DrvL
OVER-VOLTAGE
COMPARATOR
0.92V
CURRENT LIMIT
COMPARATOR
32 mV
80
1200
1
DRIVER
GND
11
4,5
15
8
13
7
16
6
2,3
17,18
VREF
VDD
1.26V
0.92V
0.8V
1.26V
260 ns
OFF TIMER
0.8V
PMOS
input
FB
*optional
EN
V
CC
VIN
RON
Zero -
Coil
Current
Detect
V
out
VIN
C
VCC
VCC
R
ILIM
200
:
P
GND
V
IN
PGND
CIN
R
ON
8
P
A
V
CC
0.26
:
C
SS
C
FB
*
C
OUT
R
FB1
R
FB2
LM3100
SNVS421G –JANUARY 2006–REVISED APRIL 2013
www.ti.com
SIMPLIFIED FUNCTIONAL BLOCK DIAGRAM
Functional Description
The LM3100 Step Down Switching Regulator features all functions needed to implement a cost effective, efficient
buck power converter capable of supplying 1.5A to a load. This voltage regulator contains Dual 40V N-Channel
buck synchronous switches and is available in a thermally enhanced HTSSOP-20 package. The Constant ON-
Time (COT) regulation scheme requires no loop compensation, results in fast load transient response, and
simplifies circuit implementation. It will work correctly even with an all ceramic output capacitor network and does
not rely on the output capacitor’s ESR for stability. The operating frequency remains constant with line and load
variations due to the inverse relationship between the input voltage and the on-time. The valley current limit
detection circuit, internally set at 1.9A, inhibits the high-side switch until the inductor current level subsides.
Please refer to the functional block diagram with a typical application circuit.
The LM3100 can be applied in numerous applications and can operate efficiently from inputs as high as 36V.
Protection features include: Thermal shutdown, V
CC
under-voltage lockout, gate drive under-voltage lockout.
Hysteretic Control Circuit Overview
The LM3100 buck DC-DC regulator employs a control scheme in which the high-side switch on-time varies
inversely with the line voltage (V
IN
). Control is based on a comparator and the one-shot on-timer, with the output
voltage feedback (FB) compared with an internal reference of 0.8V. If the FB level is below the reference the
buck switch is turned on for a fixed time determined by the input voltage and a programming resistor (R
ON
).
Following the on-time, the switch remains off for a minimum of 260ns. If FB is below the reference at that time
the switch turns on again for another on-time period. The switching will continue until regulation is achieved.
The regulator will operate in discontinuous conduction mode at light load currents, and continuous conduction
mode with heavy load current. In discontinuous conduction mode (DCM), current through the output inductor
starts at zero and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time.
The next on-time period starts when the voltage at FB falls below the internal reference. Until then the inductor
current remains zero and the load is supplied entirely by the output capacitor. In this mode the operating
frequency is lower than in continuous conduction mode, and varies with load current. Conversion efficiency is
maintained since the switching losses are reduced with the reduction in load and switching frequency. The
discontinuous operating frequency can be calculated approximately as follows:
(1)
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