LM2641MTC-ADJ Datasheet, PDF (24/31 Pages) Texas Instruments – LM2641 Dual Adjustable Step-Down Switching Power Supply Controller

LM2641
SNVS040B – JANUARY 2000 – REVISED APRIL 2013
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Bootstrap Diodes
As shown in the block diagram for the LM2641, the CBOOT pin has an internal diode which is connected to the
5V internal 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 connects directly between the bootstrap capacitor and
the external 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 regulated outputs and ground to prevent the outputs from
swinging below ground. The diode used may be a Schottky or silicon 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 biased 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 typical: 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 Circuits, the Sumida inductor showed less ringing than the Pulse inductor, but both
showed a voltage transient that would go slightly below ground. For this reason, the output diodes are
recommended.
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