Datasheet

I
LR
=
(V
IN
- V
OUT
) x t
on
L
LM3102
www.ti.com SNVS515G –SEPTEMBER 2007–REVISED JANUARY 2012
Figure 24 illustrates an inductor current waveform. On average, the output current I
OUT
is the same as the
inductor current I
L
, which is the average of the rippled inductor current. In case of current limit (the current limit
portion of Figure 24), the next on-time will not initiate until that the current drops below 2.7A (assume the voltage
at the FB pin is lower than 0.8V). During each on-time the current ramps up an amount equal to:
(6)
During current limit, the LM3102 operates in a constant current mode with an average output current I
OUT(CL)
equal to 2.7A + I
LR
/ 2.
Figure 24. Inductor Current - Current Limit Operation
N-CHANNEL MOSFET AND DRIVER
The LM3102 integrates an N-Channel main MOSFET and an associated floating high voltage main MOSFET
gate driver. The gate drive circuit works in conjunction with an external bootstrap capacitor C
BST
and an internal
high voltage diode. C
BST
connecting between the BST and SW pins powers the main MOSFET gate driver during
the main MOSFET on-time. During each off-time, the voltage of the SW pin falls to approximately -1V, and C
BST
charges from V
CC
through the internal diode. The minimum off-time of 260 ns provides enough time for charging
C
BST
in each cycle.
SOFT-START
The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing
startup stresses and current surges. Upon turn-on, after V
CC
reaches the under-voltage threshold, an 8 µA
internal current source charges up an external capacitor C
SS
connecting to the SS pin. The ramping voltage at
the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the output voltage V
OUT
in
a controlled manner.
An internal switch grounds the SS pin if any of the following three cases happens: (i) V
CC
is below the under-
voltage lock-out threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the output
voltage can be shut off by connecting the SS pin to ground using an external switch. Releasing the switch allows
the SS pin to ramp up and the output voltage to return to normal. The shutdown configuration is shown in
Figure 25.
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