4

2 × 4

1

(

59

× %

(.;

+ 4 × '54

Reg

2×

V '

2×V '

Copyright © 2016, Texas Instruments Incorporated

8

176

11

LM2750, LM2750-ADJ

www.ti.com

SNVS180N – APRIL 2002 – REVISED APRIL 2016

Product Folder Links: LM2750 LM2750-ADJ

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Feature Description (continued)

The schematic of Figure 11 is a simplified model of the LM2750 that is useful for evaluating output current

capability. The model shows a linear pre-regulation block (Reg), a voltage doubler (2×), and an output resistance

output resistance of the LM2750 is 5

Ω (typical), and is approximately equal to twice the resistance of the four

LM2750 switches. When the output voltage is in regulation, the regulator in the model controls the voltage V' to

increases. To prevent droop in output voltage, the voltage drop across the regulator is reduced, V' increases, and

regulator, V' equals the input voltage, and the output voltage is "on the edge" of regulation. Additional output

current causes the output voltage to fall out of regulation, and the LM2750 operation is similar to a basic open-

loop doubler. As in a voltage doubler, increase in output current results in output voltage drop proportional to the

output resistance of the doubler. The out-of-regulation LM2750 output voltage can be approximated by:

(1)

Again, Equation 1 only applies at low input voltage and high output current where the LM2750 is not regulating.

See Figure 1 and Figure 2 in for more details.

Figure 11. LM2750 Output Resistance Model

A more complete calculation of output resistance takes into account the effects of switching frequency, flying

capacitance, and capacitor equivalent series resistance (ESR). See Equation 2:

(2)

Switch resistance (5

Ω typical) dominates the output resistance equation of the LM2750. With a 1.7-MHz typical

switching frequency, the 1/(F×C) component of the output resistance contributes only 0.6

Ω to the total output

resistance. Increasing the flying capacitance only provides minimal improvement to the total output current

capability of the LM2750. In some applications it may be desirable to reduce the value of the flying capacitor

below 1 µF to reduce solution size and/or cost, but this must be done with care so that output resistance does

not increase to the point that undesired output voltage droop results. If ceramic capacitors are used, equivalent

series resistance (ESR) is a negligible factor in the total output resistance, as the ESR of quality ceramic

capacitors is typically much less than 100 m

Ω.

7.3.6 Thermal Shutdown

The LM2750 implements a thermal shutdown mechanism to protect the device from damage due to overheating.

When the junction temperature rises to 150°C (typical), the part switches into shutdown mode. The LM2750

releases thermal shutdown when the junction temperature of the part is reduced to 130°C (typical).

Thermal shutdown is most-often triggered by self-heating, which occurs when there is excessive power

dissipation in the device and/or insufficient thermal dissipation. LM2750 power dissipation increases with

increased output current and input voltage (see Power Efficiency And Power Dissipation). When self-heating

brings on thermal shutdown, thermal cycling is the typical result. Thermal cycling is the repeating process where

the part self-heats, enters thermal shutdown (where internal power dissipation is practically zero), cools, turns on,

and then heats up again to the thermal shutdown threshold. Thermal cycling is recognized by a pulsing output

voltage and can be stopped be reducing the internal power dissipation (reduce input voltage and/or output

current) or the ambient temperature. If thermal cycling occurs under desired operating conditions, thermal

dissipation performance must be improved to accommodate the power dissipation of the LM2750. The WSON

package has excellent thermal properties that, when soldered to a PCB designed to aid thermal dissipation,

allows the LM2750 to operate under very demanding power dissipation conditions.