LM2640MTC-ADJ National Semiconductor, LM2640MTC-ADJ Datasheet - Page 18

• Manufacturer Part Number: LM2640MTC-ADJ
• Description: Dual Adjustable Step-Down Switching Power Supply Controller
• Manufacturer: National Semiconductor

Design Procedure
(Continued)
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 LM2640, 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
50 mA, with voltage rating
30V.
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 LM2640 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 LM2640 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.
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