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LM3100-2006 View Datasheet(PDF) - National ->Texas Instruments

Part NameDescriptionManufacturer
LM3100(2006) SIMPLE SWITCHER® Synchronous 1MHz 1.5A Step-Down Voltage Regulator National-Semiconductor
National ->Texas Instruments National-Semiconductor
LM3100 Datasheet PDF : 15 Pages
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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 syn-
chronous switches and is available in a thermally enhanced
eTSSOP-20 package. The Constant ON-Time (COT) regu-
lation scheme requires no loop compensation, results in fast
load transient response, and simplifies circuit implementa-
tion. 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 cur-
rent 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, VCC 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 in-
versely with the line voltage (VIN). Control is based on a
comparator and the one-shot on-timer, with the output volt-
age 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 (RON). 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 refer-
ence. 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 conduc-
tion mode, and varies with load current. Conversion effi-
ciency is maintained since the switching losses are reduced
with the reduction in load and switching frequency. The
discontinuous operating frequency can be calculated ap-
proximately as follows:
(2)
The output voltage is set by two external resistors (RFB1,
RFB2). The regulated output voltage is calculated as follows:
VOUT = 0.8V x (RFB1 + RFB2)/RFB2
(3)
Start-up Regulator (VCC)
The start-up regulator is integrated within LM3100. The input
pin (VIN) can be connected directly to line voltage up to 36V,
with transient capability to 40V. The VCC output regulates at
6V, and is current limited to 65 mA. Upon power up, the
regulator sources current into the external capacitor at VCC
(CVCC). CVCC must be at least 680nF for stability. When the
voltage on the VCC pin reaches the under-voltage lockout
threshold of 3.75V, the buck switch is enabled and the
Soft-start pin is released to allow the soft-start capacitor
(CSS) to charge.
The minimum input voltage is determined by the dropout
voltage of VCC, and the VCC UVLO falling threshold ()3.7
V). If VIN is less than )4.0V, the VCC UVLO activates to shut
off the output.
Regulation Comparator
The feedback voltage at FB is compared to the internal
reference voltage of 0.8V. In normal operation (the output
voltage is regulated), an on-time period is initiated when the
voltage at FB falls below 0.8V. The buck switch stays on for
the on-time, causing the FB voltage to rise above 0.8V. After
the on-time period, the buck switch stays off until the FB
voltage falls below 0.8V. Bias current at the FB pin is nomi-
nally 100 nA.
Over-Voltage Comparator
The voltage at FB is compared to an internal 0.92V refer-
ence. If the voltage at FB rises above 0.92V the on-time
pulse is immediately terminated. This condition can occur if
the input voltage, or the output load, changes suddenly.
Once the OVP is activated, the buck switch remains off until
the voltage at FB pin falls below 0.92V. The low side switch
will stay on to discharge the inductor energy and until the
inductor current decays to zero. The low side switch will be
turned off.
ON-Time Timer, Shutdown
The LM3100 main switch is determined by the RON resistor
and the input voltage (VIN), and is calculated from:
(1)
where RL = the load resistance
In continuous conduction mode (CCM), current always flows
through the inductor and never reaches zero during the
off-time. In this mode, the operating frequency remains rela-
tively constant with load and line variations. The CCM oper-
ating frequency can be calculated approximately as follows:
(4)
The inverse relationship with VIN results in a nearly constant
frequency as VIN is varied. RON should be selected for a
minimum on-time (at maximum VIN) greater than 200 ns for
proper current limit operation. This requirement limits the
maximum frequency for each application, depending on VIN
and VOUT, calculated from the following:
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