Datasheet

1.3 x 10
-10
200 ns x V
IN(MAX)
R
ON
t
R
FB2
R
FB1
0.8V
V
OUT
=
-1
LM3100
STOP
RUN
V
IN
VIN
SS
+
LM3100
www.ti.com SNVS421G –JANUARY 2006–REVISED APRIL 2013
Soft-Start
The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing
start-up stresses and current surges. Upon turn-on, after V
CC
reaches the under-voltage threshold, an internal
8µA current source charges up the external capacitor at the SS pin. The ramping voltage at SS (and the non-
inverting input of the regulation comparator) ramps up the output voltage in a controlled manner.
An internal switch grounds the SS pin if any of the following cases happen: (i) VCC falls below the under-voltage
lock-out threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the converter can
be disabled by connecting the SS pin to ground using an external switch. Releasing the switch allows the SS pin
return to pull high and the output voltage returns to normal. The shut-down configuration is shown in Figure 22 .
Figure 22. Alternate Shutdown Implementation
Thermal Protection
The LM3100 should be operated so the junction temperature does not exceed the maximum limit. An internal
Thermal Shutdown circuit, which activates (typically) at 165°C, takes the controller to a low power reset state by
disabling the buck switch and the on-timer, and grounding the SS pin. This feature 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 the external components.
R
FB1
and R
FB2
: The ratio of these resistors is calculated from:
(7)
R
FB1
and R
FB2
should be chosen from standard value resistors in the range of 1.0 kΩ - 10 kΩ which satisfy the
above ratio.
For V
OUT
= 0.8V, the FB pin can be connected to the output directly. However, the converter operation needs a
minimum inductor current ripple to maintain good regulation when no load is connected. This minimum load is
about 10 µA and can be implemented by adding a pre-load resistor to the output.
R
ON
: The minimum value for R
ON
is calculated from:
(8)
Equation 2 in Hysteretic Control Circuit Overview section can be used to select R
ON
if a specific frequency is
desired as long as the above limitation is met.
L: The main parameter affected by the inductor is the output current ripple amplitude (I
OR
). The maximum
allowable (I
OR
must be determined at both the minimum and maximum nominal load currents. At minimum load
current, the lower peak must not reach 0A. At maximum load current, the upper peak must not exceed the
current limit threshold (1.9A). The allowable ripple current is calculated from the following equations:
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