Datasheet

V
OUT
V
IN(MAX)
x 200 ns
F =
SW(MAX)
V
IN
1.3 x 10
-10
x R
ON
t
ON
=
V
OUT
1.3 x 10
-10
x R
ON
F
SW
=
LM3100
www.ti.com SNVS421G –JANUARY 2006–REVISED APRIL 2013
In continuous conduction mode (CCM), current always flows through the inductor and never reaches zero during
the off-time. In this mode, 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
, R
FB2
). The regulated output voltage is calculated as
follows:
V
OUT
= 0.8V x (R
FB1
+ R
FB2
)/R
FB2
(3)
Start-up Regulator (V
CC
)
The start-up regulator is integrated within LM3100. The input pin (VIN) can be connected directly to line voltage
up to 36V, with transient capability of 40V. The V
CC
output regulates at 6V, and is current limited to 65 mA. Upon
power up, the regulator sources current into the external capacitor at V
CC
(C
VCC
). C
VCC
must be at least 680nF
for stability. When the voltage on the VCC pin reaches the under-voltage lockout threshold of 3.75V, the buck
switch is enabled and the Soft-start pin is released to allow the soft-start capacitor (C
SS
) to charge.
The minimum input voltage is determined by the dropout voltage of V
CC
regulator, and the V
CC
UVLO falling
threshold (≊3.7 V). If VIN is less than ≊4.0V, the V
CC
UVLO activates to shut off the output.
Regulation Comparator
The feedback voltage at FB pin is compared to the internal reference voltage of 0.8V. In normal operation (the
output voltage is regulated), an on-time period is initiated when the voltage at FB falls below 0.8V. The buck
switch stays on for the on-time, causing the FB voltage to rise above 0.8V. After the on-time period, the buck
switch stays off until the FB voltage falls below 0.8V again. Bias current at the FB pin is nominally 100 nA.
Over-Voltage Comparator
The voltage at FB pin is compared to an internal 0.92V reference. If the feedback voltage rises above 0.92V the
on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load, changes
suddenly. Once the OVP is activated, the buck switch remains off until the voltage at FB pin falls below 0.92V.
The low side switch will stay on to discharge the inductor energy until the inductor current decays to zero. The
low side switch will be turned off.
ON-Time Timer, Shutdown
The ON-Time of LM3100 main switch is determined by the R
ON
resistor and the input voltage (V
IN
), and is
calculated from:
(4)
The inverse relationship of t
ON
and V
IN
results in a nearly constant switching frequency as V
IN
is varied. R
ON
should be selected for a minimum on-time (at maximum V
IN
) greater than 200 ns for proper current limit
operation. This requirement limits the maximum frequency for each application, depending on V
IN
and V
OUT
,
calculated from Equation 5:
(5)
The LM3100 can be remotely shut down by taking the EN pin below 1.1V. Refer to Figure 20. In this 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. For normal operation, the voltage at the EN pin is set between 1.5V and
3.0V, depending on V
IN
and the external pull-up resistor. For all cases, this voltage must be limited not to exceed
7V.
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