LM3102MH/NOPB National Semiconductor, LM3102MH/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM3102MH/NOPB
• Description: IC BUCK SYNC ADJ 2.5A 20TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM3102MH/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 7 V
• Current - Output: 2.5A
• Frequency - Switching: 1MHz
• Voltage - Input: 4.5 ~ 42 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Current, Output: 2.5 A
• Current, Supply: 0.7 mA
• Frequency, Oscillator: 1 MHz
• Package Type: eTSSOP
• Regulator Type: Buck (Step-Down)
• Resistance, Thermal, Junction To Case: 6.5 °C/W
• Temperature, Operating, Range: -40 to +125 °C
• Voltage, Dropout: 350 mV
• Voltage, Input: 4.5 to 42 V
• Voltage, Output: 0.8 to 38 V
• Dc To Dc Converter Type: Step Down
• Pin Count: 20
• Input Voltage: 42V
• Output Voltage: 0.8 to 7V
• Switching Freq: 1000KHz
• Output Current: 2.5A
• Efficiency: 92%
• Output Type: Adjustable
• Switching Regulator: Yes
• Mounting: Surface Mount
• Input Voltage (min): 4.5V
• Operating Temp Range: -40C to 125C
• Operating Temperature Classification: Automotive
• For Use With: LM3102EVAL - BOARD EVALUATION LM3102
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: RoHS Compliant part Electrostatic Device
• Other names: *LM3102MH; *LM3102MH/NOPB; LM3102MH

Functional Description
The LM3102 Step Down Switching Regulator features all re-
quired functions to implement a cost effective, efficient buck
power converter capable of supplying 2.5A to a load. It con-
tains Dual N-Channel main and synchronous MOSFETs. The
Constant ON-Time (COT) regulation scheme requires no loop
compensation, results in fast load transient response and
simple circuit implementation. The regulator can function
properly 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 variations
due to the inverse relationship between the input voltage and
the on-time. The valley current limit detection circuit, with the
limit set internally at 2.7A, inhibits the main MOSFET until the
inductor current level subsides.
The LM3102 can be applied in numerous applications and
can operate efficiently for inputs as high as 42V. Protection
features include output over-voltage protection, thermal shut-
down, V
lock-out. The LM3102 is available in the thermally enhanced
eTSSOP-20 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 an internal reference of 0.8V. 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
R
inversely. Following the on-time, the main MOSFET remains
off for a minimum of 260 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:
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:
The output voltage is set by two external resistors R
R
ON
FB2
and the input voltage V
. The regulated output voltage is
CC
under-voltage lock-out, gate drive under-voltage
IN
, upon which the on-time varies
FB1
and
(1)
(2)
11
Startup Regulator (V
A startup regulator is integrated within the LM3102. The input
pin VIN can be connected directly to a line voltage up to 42V.
The V
mA. Upon power up, the regulator sources current into an ex-
ternal capacitor C
stability, C
the VCC pin is higher than the under-voltage lock-out (UVLO)
threshold of 3.75V, the main MOSFET is enabled and the SS
pin is released to allow the soft-start capacitor C
The minimum input voltage is determined by the dropout volt-
age of the regulator and the V
(
V
Regulation Comparator
The feedback voltage at the FB pin is compared to a 0.8V
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 0.8V. The main MOSFET stays on for
the on-time, causing the output voltage and consequently the
voltage of the FB pin to rise above 0.8V. After the on-time
period, the main MOSFET stays off until the voltage of the FB
pin falls below 0.8V again. Bias current at the FB pin is nom-
inally 5 nA.
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 0.92V internal
reference. If it rises above 0.92V, 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
0.92V. 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 LM3102 main MOSFET is determined by
the resistor R
follows:
The inverse relationship of t
frequency as V
the on-time at maximum V
timer has a limiter to ensure a minimum of 150 ns for t
limits the maximum operating frequency, which is governed
by the following equation:
≊
CC
3.7V). If V
goes to zero.
CC
output regulates at 6V, and is current limited to 65
VCC
V
OUT
IN
ON
must be at least 680 nF. When the voltage on
is less than
IN
= 0.8V x (R
and the input voltage V
is varied. R
VCC
, which is connected to the VCC pin. For
≊
IN
on
FB1
4.0V, the regulator shuts off and
ON
is greater than 150 ns. The on-
and V
+ R
should be selected such that
CC
CC
FB2
IN
UVLO falling threshold
gives a nearly constant
)/R
)
IN
FB2
. It is calculated as
SS
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on
. This
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