LM2641MTC-ADJ National Semiconductor, LM2641MTC-ADJ Datasheet - Page 17

• Manufacturer Part Number: LM2641MTC-ADJ
• Description: Power Supply IC
• Manufacturer: National Semiconductor

Specifications of LM2641MTC-ADJ

• Power Dissipation Pd: 883mW
• No. Of Pins: 28
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Design Procedure
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 in-
creasing gate capacitance (which increases the charge re-
quired to switch the FET). Improved FETs are currently being
released which are designed specifically for optimized
ON-resistance and gate charge characteristics.
The V
ON time of each switch. In some cases where one FET is on
most of the time, efficiency may be improved slightly by se-
lecting 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 out-
put 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) in-
ductor current. The value of the sense resistor can be calcu-
lated from:
Where:
V
Electrical Characteristics).
I
I
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 di-
odes 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).
MAX
RIPPLE
CL
(MIN) is the minimum specified current limit voltage (see
is the maximum output current for the application.
IN
is the inductor ripple current for the application.
and V
OUT
for a specific application determines the
(Continued)
17
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) in-
ductance in series with the Schottky will slow its turn-ON and
cause current to flow through the FET body diode.
Bootstrap Diodes
As shown in the block diagram for the LM2641, the CBOOT
pin has an internal diode which is connected to the 5V inter-
nal rail (which is also connected to the LIN pin). This diode
charges up the bootstrap capacitor to about 5V when the
low-side FET switch turns ON and pulls its drain down to
ground. The internal diode works well until the pulse widths
get extremely narrow, and then the charge applied to the
bootstrap capacitor can become insufficient to fully turn ON
the gate of the FET.
For this reason, an external diode should be used which con-
nects directly between the bootstrap capacitor and the exter-
nal capacitor connected to the LIN pin (C17). A fast-recovery
silicon diode should be used which has an average current
rating
Output Diodes
It is recommended that diodes be placed between the regu-
lated outputs and ground to prevent the outputs from swing-
ing below ground. The diode used may be a Schottky or sili-
con type, and should have a current rating of 1A or more. If
the outputs are allowed to swing below ground more than a
Vbe, the substrate of the LM2641 will become forward bi-
ased which will cause the part to operate incorrectly. Another
potential problem which could be caused by negative output
transients is damage to the output capacitors, since tantalum
capacitors can be damaged if a reverse voltage is forced
across them
The operating conditions where this can occur are not typi-
cal: it can happen if one or both of the outputs are very lightly
loaded, and an undervoltage (or overvoltage) condition is
detected. When this happens, the LM2641 turns off the
switching oscillator and turns on both of the low-side FET’s
which abruptly grounds one end of the inductor. When this
happens, the other end of the inductor (which is connected
to the regulated output) will experience a transient ringing
voltage as the energy stored in the inductor is discharged.
The amplitude and duration of the ringing is a function of the
R-L-C tank circuit made up the output capacitance, inductor,
and resistance of the inductor windings.
Because of this, the choice of inductor influences how large
in amplitude the ringing will be. In tests performed on the
Typical Application Circuit, the Sumida inductor showed less
ringing than the Pulse inductor, but both showed a voltage
transient that would go slightly below ground. For this rea-
son, the output diodes are recommended.
50 mA, with voltage rating
>
30V.
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