Datasheet

f
SW(MAX)
=
V
OUT
V
IN(MAX)
x 150 ns
V
IN
1.3 x 10
-10
x R
ON
t
on
=
LMR24210
www.ti.com SNVS738G –OCTOBER 2011–REVISED APRIL 2013
Startup Regulator (V
CC
)
A startup regulator is integrated within the LMR24210. The input pin VIN can be connected directly to a line
voltage up to 42V. The V
CC
output regulates at 6V, and is current limited to 65 mA. Upon power up, the regulator
sources current into an external capacitor C
VCC
, which is connected to the VCC pin. For stability, C
VCC
must be at
least 680 nF. When the voltage on the VCC pin is higher than the under-voltage lock-out (UVLO) threshold of
3.75V, the main MOSFET is enabled and the SS pin is released to allow the soft-start capacitor C
SS
to charge.
The minimum input voltage is determined by the dropout voltage of the regulator and the V
CC
UVLO falling
threshold (≊3.7V). If V
IN
is less than ≊4.0V, the regulator shuts off and V
CC
goes to zero.
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 switches off.
ON-Time Timer, Shutdown
The on-time of the LMR24210 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 LMR24210 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.
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