Datasheet

8.3 x 10
-11
V
IN(MAX)
x 100 ns
R
ON
t
LM3103
www.ti.com SNVS523F –SEPTEMBER 2007–REVISED APRIL 2013
N-Channel MOSFET and Driver
The LM3103 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
connected 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 240 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 and a 180 µs
fixed delay, a 70 µA internal current source charges 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 happen: (i) V
CC
is below the under-voltage lockout 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 the ground using an
external switch. Releasing the switch allows the voltage of the SS pin to ramp up and the output voltage to return
to normal. The shutdown configuration is shown in Figure 22.
Figure 22. Alternate Shutdown Implementation
Thermal Protection
The junction temperature of the LM3103 should not exceed the maximum limit. Thermal protection is
implemented by an internal Thermal Shutdown circuit, which activates (typically) at 165°C to make the controller
enter a low power reset state by disabling the main MOSFET, disabling the on-timer, and grounding the SS pin.
Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction
temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation
resumes.
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select external components.
R
FB1
and R
FB2
: These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ,
satisfying the following ratio:
R
FB1
/R
FB2
= (V
OUT
/0.6V) - 1 (7)
For V
OUT
= 0.6V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than
20 µA. This is because the converter operation needs a minimum inductor current ripple to maintain good
regulation when no load is connected.
R
ON
: Equation 2 can be used to select R
ON
if a desired operating frequency is selected. But the minimum value
of R
ON
is determined by the minimum on-time. It can be calculated as follows:
(8)
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