Datasheet

f
SW(MAX)
=
V
OUT
V
IN(MAX)
x 150 ns
V
IN
1.3 x 10
-10
x R
ON
t
on
=
LM3102
SNVS515G –SEPTEMBER 2007–REVISED JANUARY 2012 www.ti.com
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 nominally 5 nA.
ZERO COIL CURRENT DETECT
The current of the synchronous MOSFET is monitored by a zero coil current detection circuit which inhibits the
synchronous 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 protection (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
ON
and the input voltage V
IN
. It is
calculated as follows:
(4)
The inverse relationship of t
on
and V
IN
gives a nearly constant frequency as V
IN
is varied. R
ON
should be selected
such that the on-time at maximum V
IN
is greater than 150 ns. The on-timer has a limiter to ensure a minimum of
150 ns for t
on
. This limits the maximum operating frequency, which is governed by the following equation:
(5)
The LM3102 can be remotely shutdown by pulling the voltage of the EN pin below 1V. In this shutdown mode,
the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the EN pin
allows normal operation to resume because the EN pin is internally pulled up.
Figure 23. Shutdown Implementation
CURRENT LIMIT
Current limit detection is carried out during the off-time by monitoring the re-circulating current through the
synchronous MOSFET. Referring to the Functional Block Diagram, when the main MOSFET is turned off, the
inductor current flows through the load, the PGND pin and the internal synchronous MOSFET. If this current
exceeds 2.7A, the current limit comparator toggles, and as a result disabling the start of the next on-time period.
The next switching cycle starts when the re-circulating current falls back below 2.7A (and the voltage at the FB
pin is below 0.8V). The inductor current is monitored during the on-time of the synchronous MOSFET. As long as
the inductor current exceeds 2.7A, the main MOSFET will remain inhibited to achieve current limit. The operating
frequency is lower during current limit due to a longer off-time.
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