Datasheet

V
OUT
1.3 x 10
-10
x R
ON
f
SW
=
(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
=
LM3102
www.ti.com SNVS515G –SEPTEMBER 2007–REVISED JANUARY 2012
FUNCTIONAL DESCRIPTION
The LM3102 Step Down Switching Regulator features all required functions to implement a cost effective,
efficient buck power converter capable of supplying 2.5A to a load. It contains 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 shutdown, V
CC
under-voltage lock-out, gate
drive under-voltage lock-out. The LM3102 is available in the thermally enhanced TSSOP-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
ON
and the input voltage V
IN
,
upon which the on-time varies 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 conduction
loss and switching loss are reduced with the reduction in the load and the switching frequency respectively. The
operating 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 remains 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
= 0.8V x (R
FB1
+ R
FB2
)/R
FB2
(3)
STARTUP REGULATOR (V
CC
)
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
CC
output regulates at 6V, and is current limited to 65 mA. Upon power up, the regulator sources
current into an external capacitor C
VCC
, which is connected to the VCC pin. For stability, C
VCC
must be at least
680 nF. When the voltage on 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
SS
to charge.
The minimum input voltage is determined by the dropout voltage of the regulator and the V
CC
UVLO falling
threshold (≊3.7V). If V
IN
is less than ≊4.0V, the regulator shuts off and V
CC
goes to zero.
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