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參數(shù)資料
| 型號： | LM3103 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | SIMPLE SWITCHER㈢ Synchronous 1MHz 0.75A Step-Down Voltage Regulator |
| 中文描述： | 的SIMPLE SWITCHER㈢1MHz的0.75A同步降壓穩(wěn)壓器 |
| 文件頁數(shù)： | 10/16頁 |
| 文件大?。?/td> | 304K |
| 代理商： | LM3103 |
Functional Description
The LM3103 Step Down Switching Regulator features all re-
quired functions to implement a cost effective, efficient buck
power converter which is capable of supplying 0.75A to loads.
It contains dual N-Channel main and synchronous MOS-
FETs. The Constant ON-Time (COT) regulation scheme re-
quires no loop compensation, results in a fast load transient
response and simple circuit implementation. The regulator
can function properly even with an all ceramic output capac-
itor network, and does not rely on the output capacitor’s ESR
for stability. The operating frequency remains constant with
line variations due to the inverse relationship between the in-
put voltage and the on-time. The valley current limit detection
circuit, with a limit set internally at 0.9A, inhibits the main
MOSFET until the inductor current level subsides.
The LM3103 can be applied in numerous applications and
can operate efficiently for inputs as high as 42V. Protection
features include V
under-voltage lockout, output over-volt-
age protection, thermal shutdown, gate drive under-voltage
lock-out. The LM3103 is available in the thermally enhanced
eTSSOP-16 package.
COT Control Circuit Overview
COT control is based on a comparator and a one-shot on-
timer, with the output voltage feedback (feeding to the FB pin)
compared with a 0.6V internal reference. If the voltage of the
FB pin is below the reference, the main MOSFET is turned on
for a fixed on-time determined by a programming resistor
RON and the input voltage V
, upon which the on-time varies
inversely. Following the on-time, the main MOSFET remains
off for a minimum of 240 ns. Then, if the voltage of the FB pin
is below the reference, the main MOSFET is turned on again
for another on-time period. The switching will continue to
achieve regulation.
The regulator will operate in the discontinuous conduction
mode (DCM) at a light load, and the continuous conduction
mode (CCM) with a heavy load. In the DCM, the current
through the inductor starts at zero and ramps up to a peak
during the on-time, and then ramps back to zero before the
end of the off-time. It remains zero and the load current is
supplied entirely by the output capacitor. The next on-time
period starts when the voltage at the FB pin falls below the
internal reference. The operating frequency in the DCM is
lower and varies larger with the load current as compared with
the CCM. Conversion efficiency is maintained since conduc-
tion loss and switching loss are reduced with the reduction in
the load and the switching frequency respectively. The oper-
ating frequency in the DCM can be calculated approximately
as follows:
(1)
In the continuous conduction mode (CCM), the current flows
through the inductor in the entire switching cycle, and never
reaches zero during the off-time. The operating frequency re-
mains relatively constant with load and line variations. The
CCM operating frequency can be calculated approximately as
follows:
(2)
The output voltage is set by two external resistors R
FB1
and
R
FB2
. The regulated output voltage is
V
OUT
= 0.6V x (R
FB1
+ R
FB2
)/R
FB2
Startup Regulator (V
CC
)
A startup regulator is integrated within the LM3103. The input
pin VIN can be connected directly to a line voltage up to 42V.
The V
output regulates at 6V, and is current limited to 30
mA. Upon power up, the regulator sources current into an ex-
ternal capacitor C
VCC
, which is connected to the VCC pin. For
stability, C
must be at least 1 μF. When the voltage on the
VCC pin is higher than the under-voltage lock-out (UVLO)
threshold of 3.7V, the main MOSFET is enabled and the SS
pin is released to allow the soft-start capacitor C
SS
to charge.
The minimum input voltage is determined by the dropout volt-
age of the regulator and the V
UVLO falling threshold
(
3.4V). If V
is less than
4.0V, the regulator shuts off and
V
CC
goes to zero.
Regulation Comparator
The feedback voltage at the FB pin is compared to a 0.6V
internal reference. In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at
the FB pin falls below 0.6V. The main MOSFET stays on for
the programmed on-time, causing the output voltage to rise
and consequently the voltage of the FB pin to rise above 0.6V.
After the on-time period, the main MOSFET stays off until the
voltage of the FB pin falls below 0.6V again. Bias current at
the FB pin is nominally 1 nA.
(3)
Zero Coil Current Detect
The current of the synchronous MOSFET is monitored by a
zero coil current detection circuit which inhibits the syn-
chronous MOSFET when its current reaches zero until the
next on-time. This circuit enables the DCM operation, which
improves the efficiency at a light load.
Over-Voltage Comparator
The voltage at the FB pin is compared to a 0.68V internal
reference. If it rises above 0.68V, the on-time is immediately
terminated. This condition is known as over-voltage protec-
tion (OVP). It can occur if the input voltage or the output load
changes suddenly. Once the OVP is activated, the main
MOSFET remains off until the voltage at the FB pin falls below
0.6V. The synchronous MOSFET will stay on to discharge the
inductor until the inductor current reduces to zero and then
switch off.
ON-Time Timer, Shutdown
The on-time of the LM3103 main MOSFET is determined by
the resistor R
ON
and the input voltage V
IN
. It is calculated as
follows:
(4)
The inverse relationship of t
on
and V
gives a nearly constant
frequency as V
is varied. R
should be selected such that
the on-time at maximum V
IN
is greater than 100 ns. The on-
timer has a limiter to ensure a minimum of 100 ns for t
. This
limits the maximum operating frequency, which is governed
by the following equation:
www.national.com
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| LM3103EVAL | 功能描述:電源管理IC開發(fā)工具 LM3103 EVAL BOARD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM3103EVAL/NOPB | 功能描述:電源管理IC開發(fā)工具 LM3103 EVAL BRD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM3103MH | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 4.5V to 42V 16-Pin TSSOP EP Tube 制造商:Texas Instruments 功能描述:IC, V REG, SYNCH, 0.75A, POWERWISE |
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| LM3103MHX | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 4.5V to 42V 16-Pin TSSOP EP T/R |
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