Datasheet

V
OUT
1.3 x 10
-10
x R
ON
f
SW
=
(V
IN
± V
OUT
) x R
ON
2
V
OUT
(V
IN
- 1) x L x 1.18 x 10
20
x I
OUT
f
SW
=
LMR24210
SNVS738G –OCTOBER 2011–REVISED APRIL 2013 www.ti.com
FUNCTIONAL DESCRIPTION
The LMR24210 Step Down Switching Regulator features all required functions to implement a cost effective,
efficient buck power converter capable of supplying 1A to a load. It contains Dual N-Channel main and
synchronous MOSFETs. The Constant ON-Time (COT) regulation scheme requires no loop compensation,
results in fast load transient response and simple circuit implementation. The regulator can function properly
even with an all ceramic output capacitor network, and does not rely on the output capacitor’s ESR for stability.
The operating frequency remains constant with line variations due to the inverse relationship between the input
voltage and the on-time. The valley current limit detection circuit, with the limit set internally at 1.8A, inhibits the
main MOSFET until the inductor current level subsides.
The LMR24210 can be applied in numerous applications and can operate efficiently for inputs as high as 42V.
Protection features include output over-voltage protection, thermal shutdown, V
CC
under-voltage lock-out and
gate drive under-voltage lock-out. The LMR24210 is available in a small DSBGA chip scale package.
COT Control Circuit Overview
COT control is based on a comparator and a one-shot on-timer, with the output voltage feedback (feeding to the
FB pin) compared with an internal reference of 0.8V. If the voltage of the FB pin is below the reference, the main
MOSFET is turned on for a fixed on-time determined by a programming resistor R
ON
and the input voltage V
IN
,
upon which the on-time varies inversely. Following the on-time, the main MOSFET remains off for a minimum of
260 ns. Then, if the voltage of the FB pin is below the reference, the main MOSFET is turned on again for
another on-time period. The switching will continue to achieve regulation.
The regulator will operate in the discontinuous conduction mode (DCM) at a light load, and the continuous
conduction mode (CCM) with a heavy load. In the DCM, the current through the inductor starts at zero and
ramps up to a peak during the on-time, and then ramps back to zero before the end of the off-time. It remains
zero and the load current is supplied entirely by the output capacitor. The next on-time period starts when the
voltage at the FB pin falls below the internal reference. The operating frequency in the DCM is lower and varies
larger with the load current as compared with the CCM. Conversion efficiency is maintained since conduction
loss and switching loss are reduced with the reduction in the load and the switching frequency respectively. The
operating frequency in the DCM can be calculated approximately as follows:
(1)
In the continuous conduction mode (CCM), the current flows through the inductor in the entire switching cycle,
and never reaches zero during the off-time. The operating frequency remains relatively constant with load and
line variations. The CCM operating frequency can be calculated approximately as follows:
(2)
Please consider Equation 4 and Equation 5 when choosing the switching frequency.
The output voltage is set by two external resistors R
FB1
and R
FB2
. The regulated output voltage is
V
OUT
= 0.8V x (R
FB1
+ R
FB2
)/R
FB2
(3)
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