LM2641MTC-ADJ Datasheet, PDF (23/31 Pages) Texas Instruments – LM2641 Dual Adjustable Step-Down Switching Power Supply Controller

LM2641
www.ti.com
SNVS040B – JANUARY 2000 – REVISED APRIL 2013
F is the switching frequency, F_S.
T_S = 1/F_S.
D = Duty Cycle.
The ESR term predominates in determining output ripple voltage. Good quality Tantalum capacitors have
specified maximum specifications for ESR, but the typical values for ESR are usually considerably lower than the
maximum limit.
POWER MOSFETs
Two N-channel logic-level MOSFETs are required for each output. The voltage rating should be at least 1.2 times
the maximum input voltage.
Maximizing efficiency for a design requires selecting the right FET. The ON-resistance of the FET determines the
ON-state (conduction) losses, while gate charge defines the losses during switch transitions. These two
parameters require a trade-off, since reducing ON-resistance typically requires increasing gate capacitance
(which increases the charge required to switch the FET). Improved FETs are currently being released which are
designed specifically for optimized ON-resistance and gate charge characteristics.
The V_IN and V_OUT for a specific application determines the ON time of each switch. In some cases where one
FET is on most of the time, efficiency may be improved slightly by selecting a low ON-resistance FET for one of
the FET switches and a different type with lower gate charge requirement for the other FET switch. However, for
most applications this would give no measurable improvement.
CURRENT SENSE RESISTOR
A sense resistor is placed between the inductor and the output capacitor to measure the inductor current. The
value of this resistor is set by the current limit voltage of the LM2641 (see Electrical Characteristics) and the
maximum (peak) inductor current. The value of the sense resistor can be calculated from:
(9)
Where:
V_CL(MIN) is the minimum specified current limit voltage (see Electrical Characteristics).
I_MAX is the maximum output current for the application.
I_RIPPLE is the inductor ripple current for the application.
TOL is the tolerance (in %) of the sense resistor.
The physical placement of the sense resistors should be as close as possible to the LM2641 to minimize the lead
length of the connections to the CSH and CSL pins. Keeping short leads on these connections reduces the
amount of switching noise conducted into the current sense circuitry of the LM2641.
EXTERNAL DIODES
FET Diodes
Both of the low-side MOSFET switches have an external Schottky diode connected from drain to source. These
diodes are electrically in parallel with the intrinsic body diode present inside the FET. These diodes conduct
during the dead time when both FETs are off and the inductor current must be supplied by the catch diode
(which is either the body diode or the Schottky diode).
Converter efficiency is improved by using external Schottky diodes. Since they have much faster turn-off
recovery than the FET body diodes, switching losses are reduced.
The voltage rating of the Schottky must be at least 25% higher than the maximum input voltage. The average
current rating of the diode needs to be only about 30% of the output current, because the duty cycle is low.
The physical placement of the Schottky diode must be as close as possible to the FET, since any parasitic (lead)
inductance in series with the Schottky will slow its turn-ON and cause current to flow through the FET body
diode.
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